Geologic Column of Missouri Summer 2008 PDF
Preview Geologic Column of Missouri Summer 2008
O F M I S S O U R I VOLUME 3, ISSUE 1 Summer 2008 n a m off H d avi D d n a d ar ey n Vi D. erry by J o ot h p R N D Aerial view of Elephant Rocks State Park. Igneous Rocks The Volcanic Stone of Missouri The St. Francois Mountains The igneous rocks are core and granite in the St. Francois of southeast Missouri are known features in some of Missouri’s Mountains led to the develop- for their exposures of 1.5 to 1.2 most spectacular scenery and ment of settlements and towns, billion year old igneous rocks. popular state parks, including and was often the impetus behind These exposures are remarkably Johnson’s Shut-Ins, Elephant the development of infrastructure fresh and unaltered, and are Rocks and Taum Sauk Mountain. in the region. Today, this region is unique within the mid-continent Each of these parks owes its still a major producer of mineral region of the United States. The unique character to the igne- products. area also gives a rare glimpse ous rocks and to the geologic into rocks that are formed by the processes that have formed and most violent volcanic eruptions shaped them. known on the earth. Consequently, The rocks have also provided MISSOURI geologists come from around the a livelihood for generations of DEPARTMENT OF NATURAL RESOURCES world to study them. Missourians. Early mining of iron Division of Geology and Land Survey Missouri’s Volcanic Past Granites and Approximately 1.4 billion years the more viscous, or thick, will be the Rhyolites ago, southeast Missouri was a land- lava. Viscous magmas tend to hold scape dominated by volcanic calderas, more volcanic gas and therefore erupt some of which were up to 15 miles in explosively. Rhyolite calderas produce Granite is the most common diameter. These calderas were different the most violent eruptions of any type of igneous rock type at depth in Mis- from volcanoes active today in places volcano on earth. souri, but it makes up very little of like Hawaii. Instead, they more closely the exposed rock in the St. Francois Formation of a Caldera Volcano resembled the Yellowstone caldera, Mountains. Most of the outcrops are which today, is the largest dormant Volcanic Eruption Pre-existing rock is a volcanic rock called rhyolite. While volcano in the United States. Unlike RRRRRRRRRRRRRhhhhhhhhhhhhhyyyyyyyyyyyyyooooooooooooo llllllllllllliiiiiiiiiiiiittttttttttttteeeeeeeeeeeee pushed upward and the two rocks look very different, they “typical” volcanoes, calderas do not fractured by magma have essentially the same chemical in the chamber below. erupt from one main vent or develop a Rhyolite ash and lava composition and are made up of the large cone-shaped mountain. Instead, MAGMA erupt to the surface. same minerals. Granite was formed magma (molten rock) from an underlying from magma that never Volcanic Collapse chamber moves upward under pressure reached the surface. After eruptions, the and erupts lava from multiple vents in a overlying rocks collapse Because it cooled series of smaller eruptions. Over time, into the mostly emptied slowly under- the pressure in the magma chamber MAGMA magma chamber. ground, it had builds to the point that the overlying Erosion time to de- rock cannot withstand the pressure and As the caldera becomes vveelloopp llaarrggee a giant eruption occurs, forming a crater extinct, remaining magma crystals miles in diameter. Layers of volcanic cools into granite. of GRANITE Overlying igneous rocks rock form thousands of feet thick. Close-up of Missouri red granite slowly begin to erode. Large calderas such as these gener- ally erupt rhyolite (see Granites and Today the minerals quartz, feldspar and Rhyolites), a volcanic rock that is very mica. Rhyolite, on the other hand, high in silica, a compound comprised The rhyolite has eroded, erupted from a volcano, cooling so exposing the large granite of the elements silicon and oxygen. GRANITE ffaasstt tthhaatt tthhee llaavvaa ddiidd nnoott body at the surface. The The higher the silica content in a magma, harder rhyolite forms knobs. hhaavvee ttiimmee ttoo ggrrooww llaarrggee ccrryyssttaallss.. Lava vs. Ash MMoosstt ooff tthhee ccrryyssttaallss tthhaatt mmaakkee uupp When many people think of volcanic eruptions, they picture lava fl ows. rrhhyyoolliittee While some of the volcanic rocks in Missouri are lava fl ows, most of them are a are type called ash fl ow tuffs, similar to those formed by the Mt. St. Helens erup- so Close-up of Missouri rhyolite tions in the early 1980s. Ash-fl ows occur when the lava explodes so forcibly that it comes out in small droplets that immediately harden into fi ne, gritty pieces small that oneneeds a microscope of rock, called volcanic ash. The ash is mixed with hot volcanic gases that are to see them. also part of the eruption. This mixture of ash and hot gases fl ows like a liquid Because the mineral grains are down the side of the volcano, and is called a pyroclastic fl ow. Pyroclastic fl ows larger, granite in the St. Francois can move at speeds of up to 100 miles per hour, and may be near 1,800 degrees Mountains weathers and erodes Fahrenheit. The ash is so hot that it welds together when it falls to the ground, faster than rhyolite. Therefore, areas forming a solid rock. This is the type of eruption that destroyed the Roman with granite bedrock are often very cities of Pompeii and Herculaneum, except that the eruptions from Missouri’s low knobs or lowlands. Rhyolite is volcanoes are estimated to have been more than 100 times larger. much more resistant to weathering, Other volcanic rocks in Missouri are called ash-fall tuffs. These igneous rocks and makes up most of the steep, form when smaller eruptions occur. The volcanic ash is blown up into the air, but high knobs that form the St. Francois does not contain as much volcanic gas as when a pyroclastic fl ow is unleashed. Mountains. The ash falls to the ground, or is blown by winds before being deposited. Volcanic Other igneous rocks found in ash that is blown high into the atmosphere can go around the Earth several the St. Francois Mountains include times before it settles to the ground. trachyte, diorite and syenite. These Pyroclastic Flow Pyroclastic Flow look very much like rhyolite, except they are black or near black in color. I They contain less quartz and feldspar than rhyolite, and more minerals that are dark in color that contain high amounts of iron and magnesium. They are exposed in only a few places, but weather (and form knobs) like rhyolite. 2 A Moment in Time Iron Mountain is located in St. Fran- the M.A. Hanna Company made the de- date, the company began processing cois County and was originally given in cision to cease iron mining operations piles of unmineralized igneous rock, 1797 to James Pratt, a settler of great at the site. Several years before this called waste rock, which was left over infl uence in the Upper Louisiana from the iron mining for use as territory, as part of a land grant aggregate, or crushed stone. by the Spanish government. Today, the site still produces At the time, the mountain was igneous rock once considered believed to be entirely iron ore. In waste from the mining opera- 1836, the property was conveyed tion as aggregate, making it to the Missouri Iron Company, nk the oldest continually mined a which began mining the site. The br property in the United States. e H fi rst mining was of ore exposed at Art The hoist for the underground the surface, but operations soon by workings and many of the origi- o changed to underground mining. hot nal buildings remain on the p Iron Mountain was mined almost NR site, preserved by the current D continuously by various compa- Headframe, buildings and water tanks from the Iron Mountain mine, which and past operators as a living nies from 1836 until 1966, when began mining in 1836. piece of Missouri’s history. St. Francois Mountains Young and Old The history of the St. Francois Mountains began when When rocks are removed by erosion and the erosional repeated rhyolite ash-fl ows, ash-falls and lava eruptions surface is then covered by younger rocks, unconformities cooled and solidifi ed on the surface. This process lasted for occur. Unconformities represent gaps in the geologic record at least several hundred million years. When the magma similar to pages missing from the middle of a book. chambers emptied after large eruptions, the overlying rock In the St. Francois Mountains, there are numerous places would fracture and collapse. Large blocks of rhyolite would where a person’s hand can span rocks that bracket 900 tilt and slip against one another. Magma chambers refi lled million years of missing geology — a period of time that rep- and the calderas erupted many times. At the same time, resents more than one-fi fth of the geologic history of the Earth. granites intruded into the pre-existing rock and solidifi ed Fossils are often useful indicators of the age of rocks. below the surface. The total thickness of the granite and Igneous rocks, however, do not contain fossils, because they rhyolite sequence is not known, but is easily tens of thou- either formed deep Sea with knobs forming islands sands of feet. underground or Eventually, the underlying magma cooled and the calde- were too hot for ras became inactive. The igneous rocks eroded, forming a any life to survive mountainous topography. Geologists estimate that as much their placement on as 6,000 feet of igneous rock may have been eroded to form the earth’s surface. After eruptions stopped, igneous rocks and calderas the range of knobs and hills that we call the St. Francois eroded into a series of knobs or mountains. Instead, radioactive Mountains. The calderas themselves were also eroded, so Sedimentary rock isotopes are used that today we see only the roots of the once giant calderas. to date igneous Roughly 520 million years ago, the Ozark region was rocks. Radioactive lowered due to continental movement. Seas inundated the Around 530 million years ago elements, such as area and began to deposit sedimentary rocks — fi rst beach- uranium, naturally The seas inundated the area, depositing conglomer- like deposits of sandstone and then limestone and dolomite. ates and sandstone, then limestone and dolomite. decay. This decay The gap in time between the igneous rocks forming The seas advanced and retreated many times, burying the and the conglomerate is our great unconformity. occurs at a known St. Francois Mountains under layers of sediment that lith- rate. Geologists Sedimentary rock ifi ed, or hardened, into sedimentary rocks. fi nd the percent- Recently, in geologic terms, the Ozark region has been ).'""f,\]1"~
