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SUBCOURSE EDITION EN5341 A US ARMY ENGINEER CENTER AND SCHOOL GEOLOGIC ANALYSIS GEOLOGIC ANALYSIS Subcourse EN5341 EDITION A United States Army Engineer School Fort Leonard Wood, Missouri 65473-6500 9 Credit Hours Edition Date: July 1993 SUBCOURSE OVERVIEW We designed this subcourse to provide you with a basic knowledge of the physical properties required to identify geologic materials, an understanding of the natural processes that act upon these materials, and the ability to classify the resulting soils. There are no prerequisites for this subcourse. This subcourse reflects the doctrine that was current at the time it was prepared. In your own work situation, always refer to the latest official publications. Unless otherwise stated, the masculine gender of singular pronouns is used to refer to both men and women. TERMINAL LEARNING OBJECTIVE ACTION: You will identify and interpret characteristics and properties of basic geologic materials, as well as landforms derived from alteration and transportation of rock material due to external forces. In addition, you will also characterize and classify soils. CONDITION: You will be given the information required to identify, interpret, characterize, and classify basic elements of geology and surface materials. You will need a Number 2 pencil and an Army Correspondence Course Program (ACCP) Examination Response Sheet (Army Training Support Center (ATSC) Form 328). A calculator is recommended. STANDARD: To demonstrate competency of this task, you must achieve a minimum of 70 percent on the subcourse examination. i EN5341 TABLE OF CONTENTS Subcourse Overview.................................................................................................................................................... i Terminal Learning Objective.......................................................................................................................................i Table of Contents........................................................................................................................................................ii Lesson 1: Analysis of Consolidated Material (Rock)............................................................................................. 1-1 Overview........................................................................................................................................1-1 Introduction................................................................................................................................... 1-1 Part A: Sources of Information..................................................................................................1-2 Maps..............................................................................................................................1-2 Topographic Maps..................................................................................................1-2 Surface Configuration Maps...................................................................................1-2 Landform Distribution Maps..................................................................................1-2 Remotely Sensed Imagery.............................................................................................1-3 Aerial Photography.................................................................................................1-3 Multispectral Imagery.............................................................................................1-3 Geologic/Geographic Literature................................................................................... 1-3 Part B: Rock Type Identification...............................................................................................1-3 Igneous Rocks............................................................................................................... 1-4 Intrusive Igneous Rocks..........................................................................................1-6 Characteristic Intrusive Rock Bodies...............................................................1-6 Batholiths...................................................................................................1-7 Dikes..........................................................................................................1-7 Volcanic Necks or Volcanic Pipes............................................................ 1-7 Sills............................................................................................................ 1-7 Laccoliths...................................................................................................1-7 Intrusive Rock Types....................................................................................... 1-7 Granite....................................................................................................... 1-7 Gabbro-Diorite...........................................................................................1-7 Relationship of Topography to Intrusive Igneous Rocks.................................1-8 Landforms Developed on Intrusive Igneous Rocks...................................1-8 Drainage Developed on Intrusive Igneous Rocks..................................... 1-8 Vegetation in Areas of Intrusive Igneous Rocks.......................................1-8 Engineering Properties of Intrusive Igneous Rocks.........................................1-8 Extrusive Igneous Rocks........................................................................................ 1-8 Characteristic Extrusive Rock Bodies............................................................. 1-8 EN5341 ii Volcanoes................................................................................................ 1-12 Lava Flows...............................................................................................1-12 Extrusive Rock Types.................................................................................... 1-12 Stony Texture.......................................................................................... 1-12 Glassy Texture.........................................................................................1-12 Frothy Texture.........................................................................................1-12 Pyroclastic Materials............................................................................... 1-13 Relationship of Topography to Extrusive Igneous Rocks............................. 1-13 Landforms Developed in Areas of Extrusive Rocks...............................1-13 Drainage Developed on Extrusive Rocks................................................1-15 Vegetation in Areas of Extrusive Rocks................................................. 1-15 Engineering Properties of Extrusive Igneous Rocks......................................1-15 Sedimentary Rocks......................................................................................................1-18 Clastic Sedimentary Rocks...................................................................................1-19 Conglomerate................................................................................................. 1-19 Breccia............................................................................................................1-19 Sandstone....................................................................................................... 1-19 Relationship of Topography to Sandstone Rocks................................... 1-19 Engineering Properties of Sandstone...................................................... 1-20 Shale...............................................................................................................1-20 Relationship of Topography to Shale......................................................1-20 Engineering Properties of Shale..............................................................1-21 Chemical or Biochemical Sedimentary Rocks............................................................1-21 Limestone.......................................................................................................1-21 Relationship of Topography to Limestone Rocks...................................1-21 Engineering Properties of Limestone......................................................1-22 Coquina.......................................................................................................... 1-22 Dolomite.........................................................................................................1-22 Chert...............................................................................................................1-26 Coal................................................................................................................ 1-25 Interbedded Sedimentary Rocks...........................................................................1-25 Flat-Lying Interbedded Sedimentary Rocks.................................................. 1-25 Relationship of Topography to Flat-Lying Interbedded Sedimentary Rocks..............................................................1-25 Engineering Properties of Flat-Lying Interbedded Sedimentary Rocks..............................................................1-27 Tilted or Folded, Interbedded Sedimentary Rocks........................................ 1-27 Relationship of Topography to Tilted or Folded Interbedded Sedimentary Rocks..............................................................1-29 Engineering Properties of Tilted and Folded Interbedded Sedimentary Rocks..............................................................1-30 Metamorphic Rocks.................................................................................................... 1-30 Foliated Metamorphic Rocks................................................................................1-30 Slate................................................................................................................1-30 Relationship of Topography to Slate.......................................................1-30 Engineering Properties of Slate...............................................................1-31 Schist..............................................................................................................1-31 Relationship of Topography to Schist.....................................................1-32 Engineering Properties of Schist............................................................. 1-32 iii EN5341 Gneiss..........................................................................................................................1-32 Relationship of Topography to Gneiss.................................................................1-32 Engineering Properties of Gneiss......................................................................... 1-33 Nonfoliated Metamorphic Rocks................................................................................1-33 Quartizte............................................................................................................... 1-33 Marble...................................................................................................................1-33 Part C: Geologic Overlays.......................................................................................................1-33 Steps Involved in the Creation of a Geologic Overlay............................................... 1-33 Example of a Geologic Overlay..................................................................................1-34 Practice Exercise......................................................................................................................... 1-37 Answer Key and Feedback..........................................................................................................1-38 Lesson 2: Weathering, Erosion, and Deposition..................................................................................................... 2-1 Overview........................................................................................................................................2-1 Introduction................................................................................................................................... 2-1 Part A: Weathering.................................................................................................................... 2-2 Mechanical Weathering................................................................................................ 2-2 Freeze/Thaw Water Cycles.....................................................................................2-2 Temperature Changes.............................................................................................2-2 Exfoliation.............................................................................................................. 2-2 Abrasion..................................................................................................................2-2 Organic Destruction................................................................................................2-2 Chemical Weathering....................................................................................................2-3 Oxidation................................................................................................................ 2-3 Hydration................................................................................................................ 2-3 Hydrolysis...............................................................................................................2-3 Carbonation.............................................................................................................2-3 Solution...................................................................................................................2-3 Part B: Erosion and Deposition.................................................................................................2-3 Water............................................................................................................................. 2-3 Running Water........................................................................................................2-4 Erosional Features of Running Water..............................................................2-4 Gullies........................................................................................................2-4 Stream and River Valleys..........................................................................2-4 Other Erosional Features of Fluvial Systems............................................2-8 The Fluvial Cycle of Erosion.................................................................... 2-8 Depositional Features of Running Water.........................................................2-9 Alluvial Fans............................................................................................2-13 Channel Deposits.....................................................................................2-13 Overbank Deposits.................................................................................. 2-13 Deltas.......................................................................................................2-16 EN5341 iv Fresh Water...........................................................................................................2-16 Waves................................................................................................................... 2-17 Erosional Features Created By Wave Action................................................ 2-17 Wave-Cut Cliffs.......................................................................................2-19 Wave-Cut Benches.................................................................................. 2-19 Stacks.......................................................................................................2-20 Depositional Features Created by Wave Action............................................2-20 Bars..........................................................................................................2-20 Tombolos.................................................................................................2-20 Spits......................................................................................................... 2-20 Hooks.......................................................................................................2-21 Wind............................................................................................................................2-21 Erosional Features Created by Wind Action........................................................2-21 Desert Pavement.............................................................................................2-21 Ventifacts....................................................................................................... 2-21 Other Erosional Eolian Features....................................................................2-21 Depositional Features Created by Wind Action...................................................2-21 Dunes..............................................................................................................2-21 Loess.............................................................................................................. 2-22 Ice................................................................................................................................ 2-23 Alpine or Valley Glaciers.....................................................................................2-23 Erosional Features Created by Alpine Glaciation......................................... 2-23 Depositional Features Created by Alpine Glaciation.................................... 2-23 Continental Glaciers............................................................................................. 2-25 Erosional Features of Continentally Glaciated Regions...........................................................................................................2-25 Depositional Features of Continentally Glaciated Regions...........................................................................................................2-28 Gravity and Mass Wasting..........................................................................................2-28 Erosion by Mass Wasting.....................................................................................2-31 Slow Erosion of Slopes..................................................................................2-31 Creep........................................................................................................2-31 Solifluction.............................................................................................. 2-31 Rock Glaciers.......................................................................................... 2-31 Rapid Erosion of Slopes.................................................................................2-31 Debris Flow............................................................................................. 2-31 Avalanche................................................................................................ 2-32 Debris Slide............................................................................................. 2-32 Rockslide................................................................................................. 2-32 Slump.......................................................................................................2-32 Rockfall................................................................................................... 2-32 Deposition by Mass Wasting................................................................................2-32 Talus...............................................................................................................2-32 Boulder Field..................................................................................................2-32 Practice Exercise......................................................................................................................... 2-33 Answer Key and Feedback .........................................................................................................2-36 v EN5341 Lesson 3: Analysis of Unconsolidated Material (Soils)..........................................................................................3-1 Overview........................................................................................................................................3-1 Introduction................................................................................................................................... 3-1 Part A: Sources of Information..................................................................................................3-2 Field Investigations....................................................................................................... 3-2 Maps..............................................................................................................................3-2 Soil Maps................................................................................................................3-2 Landform Distribution Maps..................................................................................3-2 Geologic Maps........................................................................................................3-3 Topographic Maps..................................................................................................3-3 Aerial Photography....................................................................................................... 3-3 Soil Reports...................................................................................................................3-3 Part B: Classification of Soils................................................................................................... 3-3 General Soil Categories.................................................................................................3-3 Gravel......................................................................................................................3-3 Sand........................................................................................................................ 3-3 Silt...........................................................................................................................3-4 Clay.........................................................................................................................3-4 Organic Matter........................................................................................................3-4 Classification of Soils Based on Grain Size................................................................. 3-4 Sieve Analysis........................................................................................................ 3-4 Settling Tube (Wet Mechanical) Analysis............................................................. 3-6 Classification of Soils Based on Consistency...............................................................3-8 Atterberg Limits......................................................................................................3-8 Plastic Limit..................................................................................................... 3-8 Liquid Limit..................................................................................................... 3-8 Plasticity Index.................................................................................................3-8 Plasticity Chart....................................................................................................... 3-8 Unified Soil Classification System............................................................................... 3-8 Coarse-Grained Soils..............................................................................................3-8 Gravels............................................................................................................. 3-8 Gravels Containing Little or No Fine Material....................................... 3-10 Gravels Containing Appreciable Amounts of Fine Material....................................................................................................3-10 Gravels Containing Moderate Amounts of Fine Material....................................................................................................3-10 Sands.............................................................................................................. 3-10 Sands Containing Little or No Fine Material..........................................3-14 Sands Containing Appreciable Amounts of Fine Material....................................................................................................3-14 Sands Containing Moderate Amounts of Fine Material....................................................................................................3-14 Fine-Grained Soils................................................................................................3-14 Liquid Limit Less Than 50.............................................................................3-14 Silts With Low Plasticity.........................................................................3-14 EN5341 vi Clays With Low Plasticity.......................................................................3-15 Low Plasticity Clay/Silt Mixtures...........................................................3-15 Organics With Low Plasticity................................................................. 3-15 Liquid Limit Greater Than 50........................................................................3-15 Silts With High Plasticity........................................................................3-15 Clays With High Plasticity...................................................................... 3-15 Organics With High Plasticity.................................................................3-15 Highly Organic Soils............................................................................................ 3-15 Examples of Soil Classification..................................................................................3-16 Example 1............................................................................................................. 3-16 Example 2............................................................................................................. 3-16 Example 3............................................................................................................. 3-16 Conversion of Soils Classified Under Alternate Systems to the USCS.................................................................................................................3-16 Part C: Remote Interpretation of Aerial Photography.............................................................3-16 Analysis of Tone......................................................................................................... 3-17 Analysis of the Effects of Erosion.............................................................................. 3-17 Analysis of Drainage...................................................................................................3-17 Analysis of Topography..............................................................................................3-17 Analysis of Vegetation................................................................................................3-18 Analysis of Land Use..................................................................................................3-18 Analysis of Pattern......................................................................................................3-18 Analysis of Geography................................................................................................3-18 Part D: Recording Remotely Sensed Soil Information............................................................3-18 Collection of Information............................................................................................3-20 Examination of Materials............................................................................................3-20 Analysis of Data..........................................................................................................3-20 Delineation of Boundaries.......................................................................................... 3-20 Labeling of Polygons.................................................................................................. 3-20 Creation of Legend......................................................................................................3-20 Recording of Tabular Data..........................................................................................3-20 Storage and Retrieval of Information..........................................................................3-20 Part E: Application of Soil Information..................................................................................3-21 General Engineering Properties.................................................................................. 3-21 Well-Graded Gravel..............................................................................................3-21 Poorly Graded Gravel...........................................................................................3-21 Silty Gravel...........................................................................................................3-21 Clayey Gravel....................................................................................................... 3-21 Well-Graded Sand................................................................................................ 3-22 Poorly Graded Sand..............................................................................................3-22 Silty Sand..............................................................................................................3-22 Clayey Sand..........................................................................................................3-22 Silt With Low Plasticity....................................................................................... 3-22 Clay With Low Plasticity..................................................................................... 3-23 Organics With Low Plasticity.............................................................................. 3-23 Silt With High Plasticity.......................................................................................3-23 vii EN5341 Clay With High Plasticity.....................................................................................3-23 Organics With High Plasticity..............................................................................3-23 Peat....................................................................................................................... 3-24 Trafficability............................................................................................................... 3-24 Qualitative Estimations of Soil Trafficability......................................................3-24 Gradational Characteristics............................................................................3-24 Soil Type........................................................................................................ 3-24 Gravel...................................................................................................... 3-24 Sand......................................................................................................... 3-24 Silt............................................................................................................3-24 Clay..........................................................................................................3-25 Organic Material......................................................................................3-25 Quantitative Determination of Soil Trafficability................................................3-25 Preliminary Information.................................................................................3-26 Conditions for Sampling..........................................................................3-26 Identification of Soil Type.......................................................................3-26 Identification of Critical Layer................................................................3-26 Sampling Density.....................................................................................3-27 Field/Laboratory Procedures..........................................................................3-27 Cone Index (CI)....................................................................................... 3-27 Remolding Index (RI)..............................................................................3-31 Rating Cone Index...................................................................................3-36 Vehicle Cone Index (VCI).......................................................................3-36 Comparison of Cone Index or Rating Index to Vehicle Cone Index................................................................................. 3-36 Presentation of Information........................................................................... 3-39 Additional Factors Affecting Soil Trafficability..................................................3-39 Stickiness........................................................................................................3-39 Slipperiness.................................................................................................... 3-39 Other Factors Influencing General Trafficability (Cross-Country Movement)........................................................................... 3-39 Practice Exercise......................................................................................................................... 3-47 Answer Key and Feedback..........................................................................................................3-48 Examination.............................................................................................................................................................E-1 Appendix A: List of Common Acronyms.......................................................................................................... A-1 Appendix B: Recommended Reading List.........................................................................................................B-1 These publications provide additional information about the material in this subcourse. You do not need these materials to complete this subcourse. EN5341 viii LESSON 1 ANALYSIS OF CONSOLIDATED MATERIAL (ROCKS) OVERVIEW LESSON DESCRIPTION: In this lesson, you will learn to analyze terrain to determine basic rock types. TERMINAL LEARNING OBJECTIVE: ACTION: Identify the basic concepts of consolidated geologic materials, to include available information sources, rock classification and identification, engineering resource potential, and importance to military operations and planning. CONDITION: You will be given information on the analysis of rock material. STANDARD: Demonstrate the competency of the skills and knowledge involved in identifying and interpreting elements of basic rock identification according to FM 5-33 and TMs 5-545 and 5-330. REFERENCES: The material contained in this lesson was derived from the following publications: FM 5-33, TM 5-545, and TM 5-330. INTRODUCTION The earth is made up of both consolidated material (rock) and unconsolidated material (soil). This lesson deals with the formation, mineral composition, classification, and identification of rocks. (Similar soil parameters are discussed in Lesson 3.) An understanding of the composition and properties of various rock types is important in assessing the location, quality, and quantity of construction resources; in siting roads and airfields; in rating the suitability of a site for construction of buildings and underground installations; in estimating the ease of excavation of shallow defensive positions as well as obstacles and barriers; in locating groundwater supplies; and in predicting soil types in areas where no direct data is available. Because it is impossible to predict the ultimate military value of geologic information, all available information concerning the area of interest should be collected as standing operating procedure (SOP). Then, during operations, the actual geologic conditions encountered should be noted in order to update, verify, or modify the existing data bases and also to update intelligence estimates in cases where the new information has an impact on current operations. This type of information may have an important bearing on ongoing or future projects. 1-1 EN5341 PART A - SOURCES OF INFORMATION In performing a geologic evaluation, the terrain analyst utilizes several types of available source materials, including maps, imagery, and literature. In many areas, geologic maps of various scales already exist. If these maps do exis, the terrain analyst should exhaust all efforts to procure copies for the data base. Other useful materials required to update existing studies or to construct new studies include current 1:25,000 and 1:50,000 scale topographic maps, aerial photography at scales ranging from 1:20,000 to 1:40,000, land satellite (LANDSAT) or Systeme Probatoire d' Observation de la Terre (SPOT) imagery, and regional studies of landforms, geology, and geomorphology. These materials may be obtained from various governmental agencies (such as the United States Geological Survey, the Defense Mapping Agency, the National Aeronautics and Space Administration, the United States Soil Conservation Service, the National Forest Service, and state geological agencies) and private organizations as well as from books and periodicals. It is often necessary to use materials prepared by foreign governments or companies, such as reports available from numerous mineral and gas exploration efforts. The adequacy of source materials varies from one area to another; therefore, the analyst should use as many different sources as possible to achieve maximum accuracy and completeness of the data base. If the information sources are questionable, the analyst should annotate the uncertain reliability of the data. In some cases, there may be no source materials readily available, in which case it may be necessary to plan and request collection support. The collection of geologic information is an ongoing process whereby the analyst continually updates the data base with more current and increasingly accurate data. Once a sufficient amount of data has been collected, the analysis process begins with a critical review of the data base materials on hand. 1. Maps. In addition to geologic maps, there are three other basic map types available that can be used to construct a geologic overlay: topographic maps, surface configuration maps, and landform distribution maps. a. Topographic Maps. Standard topographic maps displaying elevation and planimetric data are available at scales ranging from large (1:25,000) to small (1:1,000,000). Maps produced at large scales allow for the extraction of more information than those produced at smaller scales. However, geologic overlays are normally produced at scales of 1:50,000, so information derived from maps with scales other than 1:50,000 need to be reduced or enlarged to be used in conjunction with the overlays. In addition to the use of topographic maps for the extraction of relief and areal data, the analyst also uses them for the delineation of drainage patterns, which is a primary interpretation key in the determination of rock type and structure. b. Surface Configuration Maps. These maps are usually produced at small scales (1:1,000,000 or smaller) and can be found in virtually any geography text or world atlas. They depict very broad categories of surface configurations. In general, these maps and associated descriptions will not provide the detail required for specific landform identification. However, they should be reviewed by the analyst for extraction of data in support of large-scale country studies and for general familiarization with a specific area of interest. c. Landform Distribution Maps. Landform distribution maps, often called physiographic maps, are characterized by their large scales and detailed information; they EN5341 1-2 provide an excellent technical source of data. Because these maps are produced for a limited function and area, they may be difficult to locate, obtain, and reproduce. 2. Remotely Sensed Imagery. A remotely sensed image is any image of the earth's surface that has been recorded by a device not in physical or intimate contact with the object or phenomenon under study. These images can provide a large portion of the information required to analyze geologic data and construct a geologic overlay. Individual rock formations and structures, such as faults and folds, are often identifiable on remotely sensed imagery. The accuracy and detail that can be discerned from the image depend largely on the season and scale of the imagery as well as on the skill and knowledge of the analyst. There are two broad categories of imagery frequently used by terrain analysts in evaluating geologic conditions-aerial photography and multispectral imagery. a. Aerial Photography. Aerial photographs are images of the earth's surface that have been recorded using airborne sensors which detect the reflectance of light in the visible or near-visible spectrum. The images may be either black and white (panchromatic) or colored. One especially useful type of aerial photograph is known as a stereopair (also stereoscopic pair or stereogram). A stereopair is made up of two photographs taken of adjacent areas in such a way that a portion appears on both photographs. For optimum stereoscopic (or three-dimensional) viewing, the photography should have a 60 percent overlap along the flight line and a 30 percent overlap along two adjacent flight lines. The best aerial photographs available are 9-inch by 9-inch prints with a scale of 1:20,000 or larger. b. Multispectral Imagery. Multispectral imagery is imagery that has been obtained simultaneously in a number of discrete bands in the electromagnetic spectrum (not just in the visible or infrared part of the spectrum, as in aerial photography). LANDSAT and SPOT are two examples of multispectral imagery that can be used as an excellent source of information supporting regional analysis. The most common scale for this type of imagery is 1:250,000, but almost any scale can be obtained. It has been found that electromagnetic wavelengths of 1.55 to 1.75 microns and 2.00 to 2.35 microns (midinfrared range) are best for geologic interpretation. These wavelengths correspond to LANDSAT Bands 5 and 7. A course on multispectral imagery and interpretation will significantly improve the analyst's ability to request and interpret this imagery. 3. Geologic/Geographic Literature. This source of information is nearly unlimited in quantity, scope of subject matter, and coverage of geographic regions. The most useful literature contains information about a specific geographic area or a specific geologic feature and provides an understanding of the region physiographic divisions and major topographic landforms. Unfortunately, these sources are often too general to be of any real value to the terrain analyst. For this reason, area-specific literature should be reviewed by the terrain analyst as background source material only. PART B - ROCK TYPE IDENTIFICATION A rock is an aggregate of one or more minerals. (A mineral is a naturally occurring, inorganic compound that has a definite chemical composition.) Based on the principal mode of formation, rocks are grouped into three broad categories: igneous, sedimentary, and metamorphic. 1-3 EN5341 Although each category is separate and distinct, natural processes exist that transform the rock types of one category to those of another, resulting in a constant recycling of earth materials without a net increase or decrease in the total amount of material on the earth. This phenomenon is referred to as the rock cycle, and it is diagrammatically represented in figure 1-1. Figure 1-1. The rock cycle Because an understanding of the composition and formation of igneous, sedimentary, and metamorphic rocks is essential for effective terrain analysis, each of these broad categories is discussed in further detail. 1. Igneous Rocks. Igneous rocks form from the solidification, or crystallization, of molten rock material. This molten material originates a magma deep within the earth where temperatures and pressures are extremely high. Because magma is generally less dense than the surrounding solid rock, and because it is somewhat fluid, it has a tendency to migrate upward along cracks and weaknesses in the overlying rock material. As the magma rises, it encounters progressively cooler temperatures, and at some point, it begins to crystallize. If crystallization takes place before the magma reaches the earth's surface, the resulting rocks EN5341 1-4 are termed intrusive, or plutonic rocks. However, if the magma does not solidify before reaching the earth's surface, the molten material that pours out onto the surface is called lava, and the rocks that crystallize from the lava are termed extrusive or volcanic rocks. Because intrusive igneous rocks crystallize at depths where conditions of temperature and pressure are not drastically different from those under which the original magma formed, the cooling rate is relatively slow and coarse-grained, or phaneritic, textures develop. On the other hand, lava cools very quickly upon exposure to atmospheric temperatures that are considerably lower than the initial temperatures of the molten material. As a result, there is not sufficient time for extrusive igneous rocks to develop large crystals; rather, a fine-grained, or aphanitic texture is formed. Three types of aphanitic textures occur: stony (the rock consists of individual mineral grains that cannot be discerned with the unaided eye), glassy (the rock cooled very rapidly, prohibiting the formation of any crystals), and frothy (the rock formed while gases were escaping from the lava so that small cavities, or vesicles, have been preserved). The texture of an igneous rock, coupled with its mineral content (roughly how light or dark the mineral is), is used to determine the rock type (see figure 1-2). Figure 1-2. Classification of igneous rocks 1-5 EN5341 Occasionally, igneous rocks of intermediate texture occur. Such rocks began to cool at depth so that a number of large crystals were formed. However, before the magma had a chance to completely solidify, it was forced further upward, allowing for more rapid cooling and the formation of correspondingly smaller crystals. The resulting rocks, which have several large crystals, or phenocrysts, embedded in a finer matrix, or ground mass, are said to exhibit porphyritic texture. A final category of igneous rock, which is classified based on the mode of formation rather than on texture, is that of pyroclastic rocks. They are composed of individual volcanic rock fragments that were explosively or aerially ejected from a volcano and subsequently lithified, or cemented. Because these types of rocks are formed above the earth's surface, they are considered to be a type of extrusive igneous rock. Characteristic rock bodies occur for both intrusive and extrusive igneous rocks. The most predominant of these is illustrated in figure 1-3. a. Intrusive Igneous Rocks. (1) Characteristic Intrusive Rock Bodies. Several different types of intrusive igneous rock bodies can be distinguished based on their relationships with the structure of the intruded, or country rock (see figure 1-3). Figure 1-3. Intrusive and extrusive rock bodies EN5341 1-6

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Preview US Army Engineer Course - Geologic Analysis EN5341

SUBCOURSE EDITION EN5341 A US ARMY ENGINEER CENTER AND SCHOOL GEOLOGIC ANALYSIS GEOLOGIC ANALYSIS Subcourse EN5341 EDITION A United States Army Engineer School Fort Leonard Wood, Missouri 65473-6500 9 Credit Hours Edition Date: July 1993 SUBCOURSE OVERVIEW We designed this subcourse to provide you with a basic knowledge of the physical properties required to identify geologic materials, an understanding of the natural processes that act upon these materials, and the ability to classify the resulting soils. There are no prerequisites for this subcourse. This subcourse reflects the doctrine that was current at the time it was prepared. In your own work situation, always refer to the latest official publications. Unless otherwise stated, the masculine gender of singular pronouns is used to refer to both men and women. TERMINAL LEARNING OBJECTIVE ACTION: You will identify and interpret characteristics and properties of basic geologic materials, as well as landforms derived from alteration and transportation of rock material due to external forces. In addition, you will also characterize and classify soils. CONDITION: You will be given the information required to identify, interpret, characterize, and classify basic elements of geology and surface materials. You will need a Number 2 pencil and an Army Correspondence Course Program (ACCP) Examination Response Sheet (Army Training Support Center (ATSC) Form 328). A calculator is recommended. STANDARD: To demonstrate competency of this task, you must achieve a minimum of 70 percent on the subcourse examination. i EN5341 TABLE OF CONTENTS Subcourse Overview.................................................................................................................................................... i Terminal Learning Objective.......................................................................................................................................i Table of Contents........................................................................................................................................................ii Lesson 1: Analysis of Consolidated Material (Rock)............................................................................................. 1-1 Overview........................................................................................................................................1-1 Introduction................................................................................................................................... 1-1 Part A: Sources of Information..................................................................................................1-2 Maps..............................................................................................................................1-2 Topographic Maps..................................................................................................1-2 Surface Configuration Maps...................................................................................1-2 Landform Distribution Maps..................................................................................1-2 Remotely Sensed Imagery.............................................................................................1-3 Aerial Photography.................................................................................................1-3 Multispectral Imagery.............................................................................................1-3 Geologic/Geographic Literature................................................................................... 1-3 Part B: Rock Type Identification...............................................................................................1-3 Igneous Rocks............................................................................................................... 1-4 Intrusive Igneous Rocks..........................................................................................1-6 Characteristic Intrusive Rock Bodies...............................................................1-6 Batholiths...................................................................................................1-7 Dikes..........................................................................................................1-7 Volcanic Necks or Volcanic Pipes............................................................ 1-7 Sills............................................................................................................ 1-7 Laccoliths...................................................................................................1-7 Intrusive Rock Types....................................................................................... 1-7 Granite....................................................................................................... 1-7 Gabbro-Diorite...........................................................................................1-7 Relationship of Topography to Intrusive Igneous Rocks.................................1-8 Landforms Developed on Intrusive Igneous Rocks...................................1-8 Drainage Developed on Intrusive Igneous Rocks..................................... 1-8 Vegetation in Areas of Intrusive Igneous Rocks.......................................1-8 Engineering Properties of Intrusive Igneous Rocks.........................................1-8 Extrusive Igneous Rocks........................................................................................ 1-8 Characteristic Extrusive Rock Bodies............................................................. 1-8 EN5341 ii Volcanoes................................................................................................ 1-12 Lava Flows...............................................................................................1-12 Extrusive Rock Types.................................................................................... 1-12 Stony Texture.......................................................................................... 1-12 Glassy Texture.........................................................................................1-12 Frothy Texture.........................................................................................1-12 Pyroclastic Materials............................................................................... 1-13 Relationship of Topography to Extrusive Igneous Rocks............................. 1-13 Landforms Developed in Areas of Extrusive Rocks...............................1-13 Drainage Developed on Extrusive Rocks................................................1-15 Vegetation in Areas of Extrusive Rocks................................................. 1-15 Engineering Properties of Extrusive Igneous Rocks......................................1-15 Sedimentary Rocks......................................................................................................1-18 Clastic Sedimentary Rocks...................................................................................1-19 Conglomerate................................................................................................. 1-19 Breccia............................................................................................................1-19 Sandstone....................................................................................................... 1-19 Relationship of Topography to Sandstone Rocks................................... 1-19 Engineering Properties of Sandstone...................................................... 1-20 Shale...............................................................................................................1-20 Relationship of Topography to Shale......................................................1-20 Engineering Properties of Shale..............................................................1-21 Chemical or Biochemical Sedimentary Rocks............................................................1-21 Limestone.......................................................................................................1-21 Relationship of Topography to Limestone Rocks...................................1-21 Engineering Properties of Limestone......................................................1-22 Coquina.......................................................................................................... 1-22 Dolomite.........................................................................................................1-22 Chert...............................................................................................................1-26 Coal................................................................................................................ 1-25 Interbedded Sedimentary Rocks...........................................................................1-25 Flat-Lying Interbedded Sedimentary Rocks.................................................. 1-25 Relationship of Topography to Flat-Lying Interbedded Sedimentary Rocks..............................................................1-25 Engineering Properties of Flat-Lying Interbedded Sedimentary Rocks..............................................................1-27 Tilted or Folded, Interbedded Sedimentary Rocks........................................ 1-27 Relationship of Topography to Tilted or Folded Interbedded Sedimentary Rocks..............................................................1-29 Engineering Properties of Tilted and Folded Interbedded Sedimentary Rocks..............................................................1-30 Metamorphic Rocks.................................................................................................... 1-30 Foliated Metamorphic Rocks................................................................................1-30 Slate................................................................................................................1-30 Relationship of Topography to Slate.......................................................1-30 Engineering Properties of Slate...............................................................1-31 Schist..............................................................................................................1-31 Relationship of Topography to Schist.....................................................1-32 Engineering Properties of Schist............................................................. 1-32 iii EN5341 Gneiss..........................................................................................................................1-32 Relationship of Topography to Gneiss.................................................................1-32 Engineering Properties of Gneiss......................................................................... 1-33 Nonfoliated Metamorphic Rocks................................................................................1-33 Quartizte............................................................................................................... 1-33 Marble...................................................................................................................1-33 Part C: Geologic Overlays.......................................................................................................1-33 Steps Involved in the Creation of a Geologic Overlay............................................... 1-33 Example of a Geologic Overlay..................................................................................1-34 Practice Exercise......................................................................................................................... 1-37 Answer Key and Feedback..........................................................................................................1-38 Lesson 2: Weathering, Erosion, and Deposition..................................................................................................... 2-1 Overview........................................................................................................................................2-1 Introduction................................................................................................................................... 2-1 Part A: Weathering.................................................................................................................... 2-2 Mechanical Weathering................................................................................................ 2-2 Freeze/Thaw Water Cycles.....................................................................................2-2 Temperature Changes.............................................................................................2-2 Exfoliation.............................................................................................................. 2-2 Abrasion..................................................................................................................2-2 Organic Destruction................................................................................................2-2 Chemical Weathering....................................................................................................2-3 Oxidation................................................................................................................ 2-3 Hydration................................................................................................................ 2-3 Hydrolysis...............................................................................................................2-3 Carbonation.............................................................................................................2-3 Solution...................................................................................................................2-3 Part B: Erosion and Deposition.................................................................................................2-3 Water............................................................................................................................. 2-3 Running Water........................................................................................................2-4 Erosional Features of Running Water..............................................................2-4 Gullies........................................................................................................2-4 Stream and River Valleys..........................................................................2-4 Other Erosional Features of Fluvial Systems............................................2-8 The Fluvial Cycle of Erosion.................................................................... 2-8 Depositional Features of Running Water.........................................................2-9 Alluvial Fans............................................................................................2-13 Channel Deposits.....................................................................................2-13 Overbank Deposits.................................................................................. 2-13 Deltas.......................................................................................................2-16 EN5341 iv Fresh Water...........................................................................................................2-16 Waves................................................................................................................... 2-17 Erosional Features Created By Wave Action................................................ 2-17 Wave-Cut Cliffs.......................................................................................2-19 Wave-Cut Benches.................................................................................. 2-19 Stacks.......................................................................................................2-20 Depositional Features Created by Wave Action............................................2-20 Bars..........................................................................................................2-20 Tombolos.................................................................................................2-20 Spits......................................................................................................... 2-20 Hooks.......................................................................................................2-21 Wind............................................................................................................................2-21 Erosional Features Created by Wind Action........................................................2-21 Desert Pavement.............................................................................................2-21 Ventifacts....................................................................................................... 2-21 Other Erosional Eolian Features....................................................................2-21 Depositional Features Created by Wind Action...................................................2-21 Dunes..............................................................................................................2-21 Loess.............................................................................................................. 2-22 Ice................................................................................................................................ 2-23 Alpine or Valley Glaciers.....................................................................................2-23 Erosional Features Created by Alpine Glaciation......................................... 2-23 Depositional Features Created by Alpine Glaciation.................................... 2-23 Continental Glaciers............................................................................................. 2-25 Erosional Features of Continentally Glaciated Regions...........................................................................................................2-25 Depositional Features of Continentally Glaciated Regions...........................................................................................................2-28 Gravity and Mass Wasting..........................................................................................2-28 Erosion by Mass Wasting.....................................................................................2-31 Slow Erosion of Slopes..................................................................................2-31 Creep........................................................................................................2-31 Solifluction.............................................................................................. 2-31 Rock Glaciers.......................................................................................... 2-31 Rapid Erosion of Slopes.................................................................................2-31 Debris Flow............................................................................................. 2-31 Avalanche................................................................................................ 2-32 Debris Slide............................................................................................. 2-32 Rockslide................................................................................................. 2-32 Slump.......................................................................................................2-32 Rockfall................................................................................................... 2-32 Deposition by Mass Wasting................................................................................2-32 Talus...............................................................................................................2-32 Boulder Field..................................................................................................2-32 Practice Exercise......................................................................................................................... 2-33 Answer Key and Feedback .........................................................................................................2-36 v EN5341 Lesson 3: Analysis of Unconsolidated Material (Soils)..........................................................................................3-1 Overview........................................................................................................................................3-1 Introduction................................................................................................................................... 3-1 Part A: Sources of Information..................................................................................................3-2 Field Investigations....................................................................................................... 3-2 Maps..............................................................................................................................3-2 Soil Maps................................................................................................................3-2 Landform Distribution Maps..................................................................................3-2 Geologic Maps........................................................................................................3-3 Topographic Maps..................................................................................................3-3 Aerial Photography....................................................................................................... 3-3 Soil Reports...................................................................................................................3-3 Part B: Classification of Soils................................................................................................... 3-3 General Soil Categories.................................................................................................3-3 Gravel......................................................................................................................3-3 Sand........................................................................................................................ 3-3 Silt...........................................................................................................................3-4 Clay.........................................................................................................................3-4 Organic Matter........................................................................................................3-4 Classification of Soils Based on Grain Size................................................................. 3-4 Sieve Analysis........................................................................................................ 3-4 Settling Tube (Wet Mechanical) Analysis............................................................. 3-6 Classification of Soils Based on Consistency...............................................................3-8 Atterberg Limits......................................................................................................3-8 Plastic Limit..................................................................................................... 3-8 Liquid Limit..................................................................................................... 3-8 Plasticity Index.................................................................................................3-8 Plasticity Chart....................................................................................................... 3-8 Unified Soil Classification System............................................................................... 3-8 Coarse-Grained Soils..............................................................................................3-8 Gravels............................................................................................................. 3-8 Gravels Containing Little or No Fine Material....................................... 3-10 Gravels Containing Appreciable Amounts of Fine Material....................................................................................................3-10 Gravels Containing Moderate Amounts of Fine Material....................................................................................................3-10 Sands.............................................................................................................. 3-10 Sands Containing Little or No Fine Material..........................................3-14 Sands Containing Appreciable Amounts of Fine Material....................................................................................................3-14 Sands Containing Moderate Amounts of Fine Material....................................................................................................3-14 Fine-Grained Soils................................................................................................3-14 Liquid Limit Less Than 50.............................................................................3-14 Silts With Low Plasticity.........................................................................3-14 EN5341 vi Clays With Low Plasticity.......................................................................3-15 Low Plasticity Clay/Silt Mixtures...........................................................3-15 Organics With Low Plasticity................................................................. 3-15 Liquid Limit Greater Than 50........................................................................3-15 Silts With High Plasticity........................................................................3-15 Clays With High Plasticity...................................................................... 3-15 Organics With High Plasticity.................................................................3-15 Highly Organic Soils............................................................................................ 3-15 Examples of Soil Classification..................................................................................3-16 Example 1............................................................................................................. 3-16 Example 2............................................................................................................. 3-16 Example 3............................................................................................................. 3-16 Conversion of Soils Classified Under Alternate Systems to the USCS.................................................................................................................3-16 Part C: Remote Interpretation of Aerial Photography.............................................................3-16 Analysis of Tone......................................................................................................... 3-17 Analysis of the Effects of Erosion.............................................................................. 3-17 Analysis of Drainage...................................................................................................3-17 Analysis of Topography..............................................................................................3-17 Analysis of Vegetation................................................................................................3-18 Analysis of Land Use..................................................................................................3-18 Analysis of Pattern......................................................................................................3-18 Analysis of Geography................................................................................................3-18 Part D: Recording Remotely Sensed Soil Information............................................................3-18 Collection of Information............................................................................................3-20 Examination of Materials............................................................................................3-20 Analysis of Data..........................................................................................................3-20 Delineation of Boundaries.......................................................................................... 3-20 Labeling of Polygons.................................................................................................. 3-20 Creation of Legend......................................................................................................3-20 Recording of Tabular Data..........................................................................................3-20 Storage and Retrieval of Information..........................................................................3-20 Part E: Application of Soil Information..................................................................................3-21 General Engineering Properties.................................................................................. 3-21 Well-Graded Gravel..............................................................................................3-21 Poorly Graded Gravel...........................................................................................3-21 Silty Gravel...........................................................................................................3-21 Clayey Gravel....................................................................................................... 3-21 Well-Graded Sand................................................................................................ 3-22 Poorly Graded Sand..............................................................................................3-22 Silty Sand..............................................................................................................3-22 Clayey Sand..........................................................................................................3-22 Silt With Low Plasticity....................................................................................... 3-22 Clay With Low Plasticity..................................................................................... 3-23 Organics With Low Plasticity.............................................................................. 3-23 Silt With High Plasticity.......................................................................................3-23 vii EN5341 Clay With High Plasticity.....................................................................................3-23 Organics With High Plasticity..............................................................................3-23 Peat....................................................................................................................... 3-24 Trafficability............................................................................................................... 3-24 Qualitative Estimations of Soil Trafficability......................................................3-24 Gradational Characteristics............................................................................3-24 Soil Type........................................................................................................ 3-24 Gravel...................................................................................................... 3-24 Sand......................................................................................................... 3-24 Silt............................................................................................................3-24 Clay..........................................................................................................3-25 Organic Material......................................................................................3-25 Quantitative Determination of Soil Trafficability................................................3-25 Preliminary Information.................................................................................3-26 Conditions for Sampling..........................................................................3-26 Identification of Soil Type.......................................................................3-26 Identification of Critical Layer................................................................3-26 Sampling Density.....................................................................................3-27 Field/Laboratory Procedures..........................................................................3-27 Cone Index (CI)....................................................................................... 3-27 Remolding Index (RI)..............................................................................3-31 Rating Cone Index...................................................................................3-36 Vehicle Cone Index (VCI).......................................................................3-36 Comparison of Cone Index or Rating Index to Vehicle Cone Index................................................................................. 3-36 Presentation of Information........................................................................... 3-39 Additional Factors Affecting Soil Trafficability..................................................3-39 Stickiness........................................................................................................3-39 Slipperiness.................................................................................................... 3-39 Other Factors Influencing General Trafficability (Cross-Country Movement)........................................................................... 3-39 Practice Exercise......................................................................................................................... 3-47 Answer Key and Feedback..........................................................................................................3-48 Examination.............................................................................................................................................................E-1 Appendix A: List of Common Acronyms.......................................................................................................... A-1 Appendix B: Recommended Reading List.........................................................................................................B-1 These publications provide additional information about the material in this subcourse. You do not need these materials to complete this subcourse. EN5341 viii LESSON 1 ANALYSIS OF CONSOLIDATED MATERIAL (ROCKS) OVERVIEW LESSON DESCRIPTION: In this lesson, you will learn to analyze terrain to determine basic rock types. TERMINAL LEARNING OBJECTIVE: ACTION: Identify the basic concepts of consolidated geologic materials, to include available information sources, rock classification and identification, engineering resource potential, and importance to military operations and planning. CONDITION: You will be given information on the analysis of rock material. STANDARD: Demonstrate the competency of the skills and knowledge involved in identifying and interpreting elements of basic rock identification according to FM 5-33 and TMs 5-545 and 5-330. REFERENCES: The material contained in this lesson was derived from the following publications: FM 5-33, TM 5-545, and TM 5-330. INTRODUCTION The earth is made up of both consolidated material (rock) and unconsolidated material (soil). This lesson deals with the formation, mineral composition, classification, and identification of rocks. (Similar soil parameters are discussed in Lesson 3.) An understanding of the composition and properties of various rock types is important in assessing the location, quality, and quantity of construction resources; in siting roads and airfields; in rating the suitability of a site for construction of buildings and underground installations; in estimating the ease of excavation of shallow defensive positions as well as obstacles and barriers; in locating groundwater supplies; and in predicting soil types in areas where no direct data is available. Because it is impossible to predict the ultimate military value of geologic information, all available information concerning the area of interest should be collected as standing operating procedure (SOP). Then, during operations, the actual geologic conditions encountered should be noted in order to update, verify, or modify the existing data bases and also to update intelligence estimates in cases where the new information has an impact on current operations. This type of information may have an important bearing on ongoing or future projects. 1-1 EN5341 PART A - SOURCES OF INFORMATION In performing a geologic evaluation, the terrain analyst utilizes several types of available source materials, including maps, imagery, and literature. In many areas, geologic maps of various scales already exist. If these maps do exis, the terrain analyst should exhaust all efforts to procure copies for the data base. Other useful materials required to update existing studies or to construct new studies include current 1:25,000 and 1:50,000 scale topographic maps, aerial photography at scales ranging from 1:20,000 to 1:40,000, land satellite (LANDSAT) or Systeme Probatoire d' Observation de la Terre (SPOT) imagery, and regional studies of landforms, geology, and geomorphology. These materials may be obtained from various governmental agencies (such as the United States Geological Survey, the Defense Mapping Agency, the National Aeronautics and Space Administration, the United States Soil Conservation Service, the National Forest Service, and state geological agencies) and private organizations as well as from books and periodicals. It is often necessary to use materials prepared by foreign governments or companies, such as reports available from numerous mineral and gas exploration efforts. The adequacy of source materials varies from one area to another; therefore, the analyst should use as many different sources as possible to achieve maximum accuracy and completeness of the data base. If the information sources are questionable, the analyst should annotate the uncertain reliability of the data. In some cases, there may be no source materials readily available, in which case it may be necessary to plan and request collection support. The collection of geologic information is an ongoing process whereby the analyst continually updates the data base with more current and increasingly accurate data. Once a sufficient amount of data has been collected, the analysis process begins with a critical review of the data base materials on hand. 1. Maps. In addition to geologic maps, there are three other basic map types available that can be used to construct a geologic overlay: topographic maps, surface configuration maps, and landform distribution maps. a. Topographic Maps. Standard topographic maps displaying elevation and planimetric data are available at scales ranging from large (1:25,000) to small (1:1,000,000). Maps produced at large scales allow for the extraction of more information than those produced at smaller scales. However, geologic overlays are normally produced at scales of 1:50,000, so information derived from maps with scales other than 1:50,000 need to be reduced or enlarged to be used in conjunction with the overlays. In addition to the use of topographic maps for the extraction of relief and areal data, the analyst also uses them for the delineation of drainage patterns, which is a primary interpretation key in the determination of rock type and structure. b. Surface Configuration Maps. These maps are usually produced at small scales (1:1,000,000 or smaller) and can be found in virtually any geography text or world atlas. They depict very broad categories of surface configurations. In general, these maps and associated descriptions will not provide the detail required for specific landform identification. However, they should be reviewed by the analyst for extraction of data in support of large-scale country studies and for general familiarization with a specific area of interest. c. Landform Distribution Maps. Landform distribution maps, often called physiographic maps, are characterized by their large scales and detailed information; they EN5341 1-2 provide an excellent technical source of data. Because these maps are produced for a limited function and area, they may be difficult to locate, obtain, and reproduce. 2. Remotely Sensed Imagery. A remotely sensed image is any image of the earth's surface that has been recorded by a device not in physical or intimate contact with the object or phenomenon under study. These images can provide a large portion of the information required to analyze geologic data and construct a geologic overlay. Individual rock formations and structures, such as faults and folds, are often identifiable on remotely sensed imagery. The accuracy and detail that can be discerned from the image depend largely on the season and scale of the imagery as well as on the skill and knowledge of the analyst. There are two broad categories of imagery frequently used by terrain analysts in evaluating geologic conditions-aerial photography and multispectral imagery. a. Aerial Photography. Aerial photographs are images of the earth's surface that have been recorded using airborne sensors which detect the reflectance of light in the visible or near-visible spectrum. The images may be either black and white (panchromatic) or colored. One especially useful type of aerial photograph is known as a stereopair (also stereoscopic pair or stereogram). A stereopair is made up of two photographs taken of adjacent areas in such a way that a portion appears on both photographs. For optimum stereoscopic (or three-dimensional) viewing, the photography should have a 60 percent overlap along the flight line and a 30 percent overlap along two adjacent flight lines. The best aerial photographs available are 9-inch by 9-inch prints with a scale of 1:20,000 or larger. b. Multispectral Imagery. Multispectral imagery is imagery that has been obtained simultaneously in a number of discrete bands in the electromagnetic spectrum (not just in the visible or infrared part of the spectrum, as in aerial photography). LANDSAT and SPOT are two examples of multispectral imagery that can be used as an excellent source of information supporting regional analysis. The most common scale for this type of imagery is 1:250,000, but almost any scale can be obtained. It has been found that electromagnetic wavelengths of 1.55 to 1.75 microns and 2.00 to 2.35 microns (midinfrared range) are best for geologic interpretation. These wavelengths correspond to LANDSAT Bands 5 and 7. A course on multispectral imagery and interpretation will significantly improve the analyst's ability to request and interpret this imagery. 3. Geologic/Geographic Literature. This source of information is nearly unlimited in quantity, scope of subject matter, and coverage of geographic regions. The most useful literature contains information about a specific geographic area or a specific geologic feature and provides an understanding of the region physiographic divisions and major topographic landforms. Unfortunately, these sources are often too general to be of any real value to the terrain analyst. For this reason, area-specific literature should be reviewed by the terrain analyst as background source material only. PART B - ROCK TYPE IDENTIFICATION A rock is an aggregate of one or more minerals. (A mineral is a naturally occurring, inorganic compound that has a definite chemical composition.) Based on the principal mode of formation, rocks are grouped into three broad categories: igneous, sedimentary, and metamorphic. 1-3 EN5341 Although each category is separate and distinct, natural processes exist that transform the rock types of one category to those of another, resulting in a constant recycling of earth materials without a net increase or decrease in the total amount of material on the earth. This phenomenon is referred to as the rock cycle, and it is diagrammatically represented in figure 1-1. Figure 1-1. The rock cycle Because an understanding of the composition and formation of igneous, sedimentary, and metamorphic rocks is essential for effective terrain analysis, each of these broad categories is discussed in further detail. 1. Igneous Rocks. Igneous rocks form from the solidification, or crystallization, of molten rock material. This molten material originates a magma deep within the earth where temperatures and pressures are extremely high. Because magma is generally less dense than the surrounding solid rock, and because it is somewhat fluid, it has a tendency to migrate upward along cracks and weaknesses in the overlying rock material. As the magma rises, it encounters progressively cooler temperatures, and at some point, it begins to crystallize. If crystallization takes place before the magma reaches the earth's surface, the resulting rocks EN5341 1-4 are termed intrusive, or plutonic rocks. However, if the magma does not solidify before reaching the earth's surface, the molten material that pours out onto the surface is called lava, and the rocks that crystallize from the lava are termed extrusive or volcanic rocks. Because intrusive igneous rocks crystallize at depths where conditions of temperature and pressure are not drastically different from those under which the original magma formed, the cooling rate is relatively slow and coarse-grained, or phaneritic, textures develop. On the other hand, lava cools very quickly upon exposure to atmospheric temperatures that are considerably lower than the initial temperatures of the molten material. As a result, there is not sufficient time for extrusive igneous rocks to develop large crystals; rather, a fine-grained, or aphanitic texture is formed. Three types of aphanitic textures occur: stony (the rock consists of individual mineral grains that cannot be discerned with the unaided eye), glassy (the rock cooled very rapidly, prohibiting the formation of any crystals), and frothy (the rock formed while gases were escaping from the lava so that small cavities, or vesicles, have been preserved). The texture of an igneous rock, coupled with its mineral content (roughly how light or dark the mineral is), is used to determine the rock type (see figure 1-2). Figure 1-2. Classification of igneous rocks 1-5 EN5341 Occasionally, igneous rocks of intermediate texture occur. Such rocks began to cool at depth so that a number of large crystals were formed. However, before the magma had a chance to completely solidify, it was forced further upward, allowing for more rapid cooling and the formation of correspondingly smaller crystals. The resulting rocks, which have several large crystals, or phenocrysts, embedded in a finer matrix, or ground mass, are said to exhibit porphyritic texture. A final category of igneous rock, which is classified based on the mode of formation rather than on texture, is that of pyroclastic rocks. They are composed of individual volcanic rock fragments that were explosively or aerially ejected from a volcano and subsequently lithified, or cemented. Because these types of rocks are formed above the earth's surface, they are considered to be a type of extrusive igneous rock. Characteristic rock bodies occur for both intrusive and extrusive igneous rocks. The most predominant of these is illustrated in figure 1-3. a. Intrusive Igneous Rocks. (1) Characteristic Intrusive Rock Bodies. Several different types of intrusive igneous rock bodies can be distinguished based on their relationships with the structure of the intruded, or country rock (see figure 1-3). Figure 1-3. Intrusive and extrusive rock bodies EN5341 1-6

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