Exploring Physical Geography. Part 2/2

321 Pages·145.155 MB·English

by  Reynolds S. et al.

CHAPTER
11
Volcanoes, Deformation,
and Earthquakes
EARTH’S SURFACE IS A REMARKABLE DIARY of the events and processes that shaped it. Magma generated at
depth erupts to the surface in volcanoes, either as hot, glowing lava, or as spectacularly explosive eruptions of gas
and volcanic ash. Tectonic forces break and bend solid rocks, frequently generating earthquakes that uplift moun-
tains, lower valleys, and inflict destruction across entire regions. All landscapes contain clues — whether in the
shape of a volcano or as small fractures exposed in a cliff — that record the processes and events that sculpt that
landscape. What comes to mind when you think about a volcano, earthquake, or mountain?
11.00.a2 Mount Everest 11.00.a3 Tibet
◀ Mount Everest is the world’s highest mountain, rising
8,848 m (29,029 ft) above sea level. It sits atop the Himalaya,
the highest mountain range on Earth. On the northern side
of the Himalaya is the Tibetan Plateau, the highest plateau on
Earth. The photograph to the left was taken from the Tibet
(north) side from one of the base camps where mountaineers
begin their arduous and dangerous climb to the top.
Traveling to the base camp, the climbers drive through
mountain ranges composed of wildly folded rock layers (▶).
What processes produced such an incredibly high region, and are
the folded layers somehow related to the same processes and
events that uplifted the mountains and plateau?
Mount Fuji on Honshu Island in
11.00.a1
Japan is a beautiful and symmetric
mountain (▼). It is also an active
volcano, which last erupted in 1708.
From the shape of the volcano and
the types of materials the volcano
contains, we can tell that it is a type
of volcano that has extremely explo-
sive, dangerous eruptions. The
Cascade Mountains of the Pacific
Large earthquakes are common along the deep ocean trenches of the Pacific Northwest contain the same type of
and Indian Oceans. The 2004 Indian Ocean earthquake unleashed a large wave dangerous volcano with similar, if less
called a tsunami that swept onto surrounding coastlines. These satellite images elegant, shapes.
(▼) show the city of Banda Aceh, Sumatra, before (left image) and after (right
image) the wave. Most of this city of 320,000 people was stripped bare by the What is a volcano, how do we recognize
wave, and nearly a third of its inhabitants were killed. one, and what makes some volcanoes
more dangerous than others?
What types of earthquakes are most likely to produce a tsunami?
11.00.a4–5
11.00.a6
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Volcanoes, Deformation, and Earthquakes 345
TOPICS IN THIS CHAPTER
11.1 What Is a Volcano? 346 11.11 How Are Folds Expressed in Landscapes? 366
11.2 How Do Volcanoes Erupt? 348 11.12 How Do Local Mountains and Basins Form? 368
11.3 What Volcanic Features Consist of Basalt? 350 11.13 What Is an Earthquake? 370
11.4 What Are Composite Volcanoes 11.14 Where Do Most Earthquakes Occur? 372
and Volcanic Domes? 352 11.15 What Causes Earthquakes Along Plate
11.5 What Are Calderas? 354 Boundaries and Within Plates? 374
11.6 What Hazards Are Associated with Volcanoes? 356 11.16 How Do Earthquakes Cause Damage? 376
11.7 What Areas Have the Highest Potential 11.17 What Were Some Recent Large Earthquakes? 378
for Volcanic Hazards? 358 11.18 CONNECTIONS: What Happened During
11.8 How Are Magmatic Conduits Exposed? 360 the Great Alaskan Earthquake of 1964? 380
11.9 What Is Deformation and How Is It 11.19 INVESTIGATION: Where Is the Safest Place
Expressed in Landscapes? 362 to Live in This Area? 382
11.10 How Are Fractures Expressed in Landscapes? 364
11.00.a7
Bangladesh Our Dynamic Planet
India (2) Myanmar
(18,045) (61) V
olcanoes, earthquakes, and moun-
Thailand
(8,212) tain ranges are vivid demonstra-
tions of the dynamic nature of our
Maldives planet. All are related to internal processes,
Somalia (180) those that originate deep beneath the sur-
(289)
Kenya Seychelles Sri Lanka Ma(7la5y)sia face and are powered mostly by Earth’s
(1) (2) (35,322) 1 internal heat. Many other features — at the
regional scale of a mountain range or at
Tanzania 2 Indonesia the local scale of a single exposure of
(13) (167,736)
3 rock — are related to these dynamic inter-
nal processes.
4
Volcanoes are a clear expression of this
5
dynamism, bringing hot, molten rock from
6
deep in Earth’s crust or mantle and erupting
7 it as magma on the surface. Different types
8 of volcanoes erupt in characteristic ways,
9 and often we can recognize the difference
10 from a distance, just from the shape of the
11 N volcano or the color of the volcanic materi-
1000 km
als. The type of volcano determines how
dangerous it is and types of hazards it poses.
The world’s strongest earthquake in 40 years struck Indonesia on December 26, 2004. The
Most earthquakes occur when forces
magnitude 9.1 earthquake occurred west of Sumatra and was caused by movement on a fault,
cause two parts of Earth’s crust to shift
shown by the red line on this map. Yellow dots nearby show the locations of smaller, related
abruptly relative to one another. Such
earthquakes. The earthquake occurred beneath the ocean, abruptly uplifting a large region of
seafloor and displacing overlying seawater that formed a massive wave — a tsunami — that spread movement can uplift a mountain range
across the Indian Ocean as a low wave, traveling at speeds approaching 800 km/hr (500 mi/hr)! (e.g., the Himalaya), downdrop a valley, or
The curved dark gray lines show a model of the wave’s position by hour (numbers in small white horizontally slide one crustal block past
circles). The tsunami increased in height as it approached the coasts of Indonesia, Thailand, Sri another, as occurs along the San Andreas
Lanka, India, east Africa, and various islands. Low coastal areas fault of California. If rock layers are soft
were inundated by as much as 20 to 30 m (65 to 100 ft) of enough, such as when they are warm and
water. Cities and villages, like Banda Aceh (◀), were completely
buried deeply, they may respond to the
demolished along hundreds of kilometers of coastline, leaving
imposed forces by bending rather than
more than 220,000 people dead or missing. The numbers in 1
breaking, forming folds, like those in Tibet.
ovals on the map above show casualties by location. 1
Many regions have it all — volcanoes, .
0
What causes earthquakes, where are they most likely to strike, and earthquakes, fractures, and folds. Here, we
which ones are most hazardous? explore why.
11.00.a8 Banda Aceh, Indonesia
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346
What Is a Volcano?
11.1
AN ERUPTING VOLCANO IS UNMISTAKABLE — glowing orange lava cascading down a hillside, molten fragments
blasting into the air, or an ominous, billowing plume of gray volcanic ash rising into the atmosphere. But what if
a volcano is not erupting? How do we tell if a mountain is a volcano?
What Are the Characteristics of a Volcano?
How would you describe a volcano to someone who had never seen one? Examine the two volcano photographs
below and look for common characteristics. Volcanoes are formed by magma, so the resulting rocks are igneous.
11.01.a1 Kilauea, HI
A volcano is a vent where magma and other volcanic products erupt onto the surface. The volcano on the
left (the Pu`u `O¯ `o¯ volcano in Hawaii Volcanoes National Park) produces large volumes of molten lava,
whereas the one on the right 11.01.a2 Kanaga volcano, AK
(Kanaga volcano, Alaska) is erupting
volcanic ash. Many physical geogra-
phers reserve the term volcano for
hills or mountains that have been
constructed by volcanic eruptions.
Some eruptions do not produce hills
or mountains, but we consider them
to be volcanoes, too.
Most volcanoes have a crater, a
roughly circular depression usually
located near the top of the volcano.
Other volcanoes have no obvious
crater or are nothing but a crater.
Volcanoes consist of volcanic rocks,
which form from lava, pumice,
volcanic ash, and other volcanic Many volcanoes display evidence of having been active during
materials. the last several hundred to several million years, or even during
the last several days. Such evidence can include a layer of
Besides erupting from volcanoes that volcanic ash on hillslopes (left side of volcano shown above) or
have the classic shape of a cone, lava and ash flows that are relatively unweathered and that lack a
magma erupts from fairly linear cracks well-developed soil. Over time, erosion degrades and disguises
called fissures and from huge circular volcanoes and volcanic craters, making them less obvious.
depressions called calderas.
Is Every Hill Composed of Volcanic Rocks a Volcano?
If a landscape lacks most of the diagnostic features described above, it is probably not a volcano. Many mountains and
hills are not volcanoes, and some volcanoes do not make mountains or hills.
▼ The flat-topped hill below, called a mesa, 1. Lava erupts from a central volcanic vent or from a
has a cap of volcanic rocks, but it is not a linear vent called a fissure. Once erupted, the lava
volcano. It did not form over a volcanic spreads outward and cools into a solid rock.
vent. Instead, it is an eroded remnant of a 2. Erosion removes the
once extensive lava flow that covered the edges of the lava flow and
region, as shown in the figures to the right. works inward toward a
central remnant.
11.01.b1 Hopi Buttes, AZ
3. The past location of the
fissure is marked by a small,
linear ridge of dark rocks
that cross the landscape.
4. The lava flow is more resistant to erosion than underlying rocks
and so forms a steep-sided, flat-topped mesa. It is a hill composed 11.01.b2–4
of volcanic rocks, but it is not over the vent and is not a volcano.
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Volcanoes, Deformation, and Earthquakes 347
What Are Some Diff erent Types of Volcanoes?
Volcanoes have different sizes and shapes and contain different types of volcanic materials. These variations are due to
differences in the composition of the magmas and the style of the eruptions. There are four common types of
volcanoes that are shaped like hills and mountains: cinder cones, shield volcanoes, composite volcanoes, and volcanic
domes. Later in this chapter, we describe other types of volcanoes that are not hills or mountains.
Cinder Cone
◀ Cinder cones are cone-shaped hills several hundred meters high, or higher, usually with a small
crater at their summit. They are also called scoria cones. They contain loose black or red, pebble-
sized volcanic cinders, along with larger volcanic bombs. Most cinder is a type of basalt. Cinder
cones generally occur as isolated features or in an area with numerous cones, but some form
next to, or on the flanks of, composite and shield volcanoes.
11.01.c1
Shield Volcano
▶ Shield volcanoes have broad, gently curved slopes (shaped like an ancient battle shield) and
can be relatively small (less than a kilometer across) or can form huge mountains tens of kilome-
ters wide and thousands of meters high. They commonly contain a crater, or line of craters, and
have fissures along their summit. Shield volcanoes consist mostly of lava flows of basalt, with
smaller amounts of cinder and volcanic ash. 11.01.c2
Composite Volcano
◀ Composite volcanoes are typically fairly symmetrical mountains thousands of meters high, with
moderately steep slopes and a crater at the top. They may be large, but are, on average, much
smaller than shield volcanoes. Their name derives from the interlayering of various types of
volcanic rocks and volcanic mudflows. They consist mostly of andesite, a fine-grained volcanic
rock, but they can also contain other compositions of rocks. Composite volcanoes are a very
dangerous type of volcano.
1 km
11.01.c3
Volcanic Dome
▶ Volcanic domes are dome-shaped features that may be hundreds of meters high. They consist
of solidified lava, which can be highly fractured or mostly intact. Domes include some volcanic
ash intermixed with rock fragments derived from solidified lava in the dome. They form where
magma cannot easily flow and so it piles up around a vent. Many domes are within craters of
composite volcanoes or in larger volcanic depressions, called calderas, which are described later
in this chapter. 11.01.c4
The Relative Sizes of Different Types of Volcanoes
V
olcanoes vary from small hills less the same drawing with small cinder cones.
Before You Leave This Page
than a hundred meters high to broad The figure does accurately show which vol-
mountains tens of kilometers across. canoes are the largest and which ones are Be Able To
Although sizes vary quite a bit, we can make the smallest.
some generalizations about the relative sizes Cinder cones and domes, which typically
Sketch or describe the diagnostic
of the different volcano types. form during a single eruptive episode, are the characteristics of a volcano.
The figure below illustrates that some smallest volcanoes. Shield volcanoes and
Describe or sketch why not every hill
types of volcanoes are larger than others. composite volcanoes are much larger because
composed of volcanic rocks is a
The volcanoes on this figure cannot be they are constructed, layer by layer, by mul-
volcano.
drawn to their true scale relative to one tiple eruptions. Shield volcanoes have more
another because the largest shield volcanoes gentle slopes than cinder cones, domes, or Sketch and describe the four main
are so large that we cannot show them on composite volcanoes. types of volcanoes that construct
hills and mountains. 1
1
Sketch or describe the relative sizes .
1
of different types of volcanoes.
11.01.t1
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348
How Do Volcanoes Erupt?
11.2
THE DIFFERENT SHAPES OF VOLCANOES indicate differences in the style of eruption. Some eruptions are explosive,
whereas others are comparatively calm. What causes these differences? The answer involves magma chemistry and
gas content, both of which control how magma behaves near the surface. A key aspect of this behavior is the magma’s
resistance to flow, a parameter called viscosity. Rocks with high viscosity strongly resist flowing.
What Are Ways that Magma Erupts?
Magma may behave in several different ways once it reaches Earth’s surface. Pyroclastic eruptions throw bits of lava,
volcanic ash, and other particles into the atmosphere. During less explosive eruptions, lava issues from a vent and
flows onto the surface. Both types of eruptions can occur from the same volcano.
Eruptions of Lava Pyroclastic Eruptions
▶ When magma erupts onto ▶ Some explosive eruptions
the surface and flows away send molten lava into the air.
from a vent, it creates a lava A lava fountain, such as that
flow. Erupted lava can be fairly shown here, can accompany
fluid, flowing downhill like a basaltic volcanism and results
fast river of molten rock. Some from a high initial gas content
lava flows are not so fluid and in a less viscous lava. The gas
travel only a short distance propels the lava and separates
before solidifying. it into discrete pieces.
11.02.a1 Kilauea, HI 11.02.a3 Kilauea, HI
▶ A lava dome forms from the ▶ Other explosive eruptions
eruption of highly viscous lava. eject a mixture of volcanic ash,
The high viscosity of the lava pumice, and rock fragments
causes the lava to pile up in a into the air. Airborne particles
dome-shaped mass around the that are sand sized or smaller
vent, instead of flowing away. are volcanic ash. Ash mostly
Domes are often accompanied forms when bubbles blow
by several types of explosive apart bits of magma. Larger
eruptions. fragments are typically pumice,
fragmented volcanic glass, and
other shattered rocks.
11.02.a2 Mount St. Helens, WA 11.02.a4 Redoubt volcano, AK
Two Diff erent Types of Pyroclastic Eruptions from the Same Volcano
Augustine volcano in Alaska produces ash and pumice in two eruptive styles — an eruption column and a pyroclastic flow.
11.02.a5 Augustine volcano, AK
▼ Pyroclastic Flow — Some ash does not jet The two kinds of eruptions differ primarily
straight up but collapses down the side of the because of the gas content of the magma.
volcano as a dense, hot cloud of ash particles An eruption column forms when large
and gas. This eruption style is a pyro clastic flow volumes of volcanic gas come
or simply an ash flow. A pyroclastic flow can out of the magma and
be devastating be cause of its high speed overcome gravity to carry
(more than 100 km/hr) and high temperature the cloud of ash and
(exceeding 500°C). pumice up into the
11.02.a6 Augustine volcano, AK atmosphere.
A pyroclastic flow forms
when the amount of gas
is less and cannot support
▲ Eruption Column — Volcanic ash and the eruption column, so
pumice form when magma is blown apart by the column
volcanic gases and can erupt high into the rapidly
atmosphere, forming an eruption column. The collapses and
ash and pumice fall back to Earth as solidified flows
and cooled pieces of rock. Finer particles of downhill under
ash drift many kilometers away from the the force of 11.02.a7
volcano and slowly settle to the ground. gravity.
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Volcanoes, Deformation, and Earthquakes 349
How Do Gases Aff ect Magma?
1. To envision dissolved 2. Magma, like the soda, 4. As the magma
gas in magma, think what 11.02.b1 contains some dissolved approaches the
happens when you open gases, including HO (water surface, pressure
2
a bottle or can of soda. vapor), CO (carbon dioxide), decreases and the
2
The liquid may have no and SO (sulfur dioxide). gases cannot
2
bubbles until it is opened, These gases have a critical remain in solution.
at which time bubbles effect on eruption style and Bubbles of gas form
appear in the liquid, rise help the magma rise toward in the magma. If
to the top, and perhaps the surface. enough bubbles
cause the soda to spill form quickly, the
out. The dissolved gas was expanding bubbles
always in the liquid, but it 3. As in this enlargement cause the magma
only became visible when of the magma, confining to be more buoyant
you opened the top and pressure at depth keeps and help it rise
released the pressure that most of the gases in toward the surface
held the gas in solution. solution and keeps and erupt out of
bubbles from forming. the volcano.
11.02.b2
How Does Viscosity Aff ect Gases in Magma?
Viscosity, the resistance to flow, dictates how fast a magma can flow and how fast crystals and gas can move through
the magma. When gas in a magma comes out of solution, movement of the resulting bubbles is resisted by the
magma’s viscosity. If the bubbles cannot escape, the magma is potentially more explosive.
11.02.c1 Mount St. Helens, WA More Viscous 11.02.c2 Kilauea, HI
◀ Felsic magmas — with abundant quartz and feldspar — contain a lot
of silica, which forms in long, complicated chains, and so are relatively
viscous. The high viscosity prevents gas from escaping easily. Gas
builds up in the magma and, when it expands, greatly increases the
pressure on the surrounding rock. This can cause explosive eruptions.
Less Viscous
▶ Mafic magmas — with abundant iron and magnesium — are less
viscous, so gas bubbles can escape relatively easily. This can lead to a
fairly nonexplosive eruption, such as this basaltic lava flow that flows
smoothly downhill from the vent.
Composition, Viscosity, and Eruptive Style
C
omposition of magma is the main Other magmas have more silicate chains,
control on a volcano’s eruptive style, and the chains interact, restricting the flow of
shape, types of materials, and poten- the magma and making it more viscous. Magma
tial hazards. This is because composition, with a high viscosity does not flow easily across Before You Leave This Page
especially the amount, length, and linkage of the surface, and so the erupted lava piles up
Be Able To
silicate chains in the melt, controls viscosity close to the volcanic vent. As a result, this type
and whether gas builds up in the magma. of lava produces steep volcanic domes and
Some magma has fewer linked silicate steep composite volcanoes. High-viscosity Describe four ways that magma erupts.
chains and so is relatively less viscous (flows magma can trap gas, leading to explosive Describe the difference between
more easily). Lower viscosity allows magma pyroclastic eruptions of gas-propelled volcanic an eruption column and a pyroclastic
to flow from the volcano in a more fluid lava ash, pumice, and rock fragments. Such erup- flow, and the role that gas plays in
flow. If the magma is relatively fluid, it tends tions are associated with volcanic domes, com- eruptive style.
to spread out and form relatively gentle posite volcanoes, and large volcanic calderas.
Explain how gas behaves at different
slopes, as in shield volcanoes. Explosive gases Therefore, these volcanoes produce a mix of
depths in a magma and how it
can build up in low-viscosity magma, as dem- pyroclastic rocks and lava flows, mostly of felsic
influences eruptive style. 1
onstrated by lava fountains, but the resulting and intermediate composition. Composition
1
explosive eruptions are rather small and controls viscosity, eruptive style, the shape of Describe how viscosity influences .
2
localized, scattering cinder and ash near the the volcano, and the rock types that compose how explosive an eruption is.
cinder cone. that volcano.
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350
What Volcanic Features Consist of Basalt?
11.3
BASALT IS THE MOST ABUNDANT VOLCANIC ROCK on Earth’s surface. Dark-colored basalt covers large areas on
every continent and forms the upper part of oceanic crust. When magma with a composition of basalt erupts, it can
produce a variety of landforms. It forms cinder cones and associated dark lava flows, and also produces shield vol-
canoes and lava flows that cover huge areas, called flood basalts. Which type of volcanic feature forms is largely
controlled by the gas in the magma, the total volume of magma, and how fast the magma erupts.
How Are Cinder Cones and Basalt Flows Expressed in Landscapes?
Basaltic magma has a relatively low viscosity compared to other magmas, and it erupts in characteristic ways.
A basaltic eruption can form a lava flow or throw pieces of molten rock into the air. The lava can flow smoothly, like a
hot, glowing stream, or it can partially solidify and break apart. Lava erupted into water forms distinctive features.
Basaltic Eruptions — At the beginning Cinder Cones — Pieces of cinder from the lava fountain gradually
of many basaltic eruptions, gases create a cone-shaped hill called a cinder cone. Ejected frag-
carry bits of lava into the air, forming ments can be as small as sand grains or as large as huge
a lava fountain. The airborne bits of boulders. Cinder cones typically form in a short amount of time,
lava cool and then fall around the from a few months to a few years, and generally are no more
vent as loose pieces of cinder. The than 300 m (about 1,000 ft) high.
lava fountain may be followed by or
Basaltic Lava Flows — Fluid basaltic lava pours from the vent and
accompanied by eruption of a
flows downhill. Sometimes, as shown here, the lava fills up and
basaltic lava flow.
overtops the crater in the cinder cone. At other times, a lava
flow issues from cracks near the base of the cinder cone after
most of the cone has been constructed.
11.03.a1 Hawaii
Cinder Cones Types of Lava Flows Tubes and Pillows
AZ aii aii
Crater, a4 Haw a6 Haw
unset 11.03. 11.03.
S
2
a
3.
0
1.
1
Most cinder cones begin with a conical Aa lava (pronounced “ah-ah”) is a type of Lava tubes form when the surface of a lava
shape (▲) and a central crater at the top of rough-surfaced lava flow, formed when the flow solidifies to form an insulating roof over
the cone. Young cinder cones have little soil lava breaks apart into a mass of jumbled, the hot, still-moving interior of the flow (▲).
or vegetation on them, and commonly are angular blocks of hardened lava that tumble Lava flows insulated by lava tubes flow farther
associated with dark, fresh-looking lava flows. down the front of the flow as it moves (▲). than lava flows on the surface because the
Aa forms a very rough, jagged surface. lava stays hotter longer. If the tube drains, it
Z becomes a curving, tube-shaped cave.
A
ern Pahoehoe (pronounced “pa-hoy-hoy”) is a type 11.03.a7 San Juan Islands, WA
orth of lava flow that has an upper surface with When fluid lava
N small billowing folds that form a “ropy” texture. erupts into water,
3
3.a A pahoehoe lava flow is usually fed by a lava the lava grows
0
1. tube and grows as a series of lobes (▼). As forward as small,
1
the front of the flow solidifies, the lava breaks individual lumps
out and forms a new lobe, as shown here. that form rounded
Pahoehoe shapes called
lava moves pillows (▶). Pillows
relatively are reliable
smoothly evidence that lava
Over time, erosion wears away the summit of and easily erupted into water.
a cinder cone, making the cone into a compared to Basaltic lava flows
rounded hill (▲) without a central crater. aa flows. on the seafloor
Erosion cuts into the slopes, and the slopes contain countless
gradually build up a veneer of soil and plants. pillows.
11.03.a5 Hawaii
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Volcanoes, Deformation, and Earthquakes 351
What Is a Shield Volcano?
Shield volcanoes have a broad, shield-shaped form and fairly gentle slopes when compared to other volcanoes, because
their eruptions are dominated by relatively nonexplosive outpourings of low-viscosity lava from fissures and vents.
Shield volcanoes form in various settings, but the largest ones, including those in Hawaii, formed at oceanic hot spots.
1. This image shows satellite 2. Mauna Loa, the central mountain, 3. Kilauea volcano,
data superimposed on is the world’s largest volcano. It rises probably the most active
topography of the Big Island 9,000 m (29,500 ft) above the seafloor volcano in the world, is
of Hawaii. The island consists and is 4,170 m (13,680 ft) above sea on the southeastern side HI
mainly of three large volca- level. From seafloor to peak, Mauna of the island. Recent lava a,
o
L
noes. Green areas are heavily Loa is Earth’s tallest mountain. Nearby flows (shown in dark a
n
vegetated, and recent lava Mauna Kea is an inactive shield grayish brown) flowed au
M
flows are brown or dark gray. volcano and the site of astronomical eastward, destroying 2
b
observatories. roads and housing 03.
subdivisions. 11.
11.03.b1 10 km
4. In shield volcanoes, magma rises
through a fracture and erupts onto the
Mauna Kea surface from a long fissure (▲). Large
volumes of lava can flow out of the
fissure, and escaping gas throws smaller
Kilauea
amounts of molten rock into the air as
Mauna Loa a fiery curtain.
5. Eruptions also occur in
more centralized vents, such as
Fissure on Kilauea (▼). These vents are
Eruption interpreted to overlie fissures.
6. The spine of Mauna Loa is a fissure from which lava flows erupted as recently as 1984.
The fissure is the surface expression of one or more magma-filled fissures at depth.
11.03.b3 Kilauea, HI
What Are Flood Basalts and How Are They Expressed in Landscapes?
Flood basalts are basaltic lava flows covering vast areas and are commonly several kilometers thick. They generally
involve multiple eruption events, but individual lava flows can cover thousands of square kilometers and contain more
than 1,000 cubic kilometers of magma! Flood basalts are fed by a series of long fissures.
A fissure forms Some of the
Before You Leave
when pressure most famous
from the flood basalts are This Page Be Able To
magma pushes in the Columbia
outward Plateau of
Summarize the features
against the wall Washington,
of cinder cones and
rocks, holding Oregon, and
them apart western Idaho. basalt flows.
while magma Cut into the
Describe the general
passes through. plateau are
characteristics of a
A wide fissure 11.03.c2 canyons that
11.03.c1 allows faster expose multiple basalt flows, each forming a ledge or step shield volcano and how
the eruptions occur.
eruption rates, which result in a large volume of in the canyon walls. Each flow represents a single eruption, 1
erupted magma that can remain hot and travel separated by thousands of years in which not much Describe flood basalts 1
long distances. Narrower fissures restrict the happened. One basalt flow on the Columbia Plateau and explain why they .3
eruption rates, leading to lower volume lava flows covers more than 130,000 km2 (50,000 mi2) and probably can cover huge areas.
that cool off before they can travel as far. erupted very quickly, perhaps in only several decades.
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352
What Are Composite Volcanoes
11.4
and Volcanic Domes?
COMPOSITE VOLCANOES FORM STEEP, CONICAL MOUNTAINS that are hard to mistake for anything other
than a volcano. They are c ommon above subduction zones, especially along the Pacific Ring of Fire. Composite
volcanoes are extremely dangerous. Volcanic domes consist of smaller, dome-shaped masses of highly viscous lava
that accumulated over and close to the associated volcanic vent — they are also very dangerous.
What Are Some Characteristics of a Composite Volcano?
Composite volcanoes are constructed of interlayered volcanic material formed by lava flows, pyroclastic flows, falling
ash, and volcano-related mudflows and landslides. Composite volcanoes, also called stratovolcanoes, erupt over long
time periods, which explains their large size and complex internal structure.
11.04.a2 Mount St. Helens, WA
◀ 1. Eruption Column — Composite
volcanoes produce a distinctive ▶ 2. Pyroclastic Flows—
column of pumice, ash, and gas These are the most
that rises upward many tens violent eruptions from
of kilometers into the the volcano. They form
atmosphere. Coarser when the eruption
pieces settle around the column collapses
volcano, but finer downward as a dense,
particles (volcanic ash) swirling cloud of hot
can drift hundreds of gases, volcanic ash, and
kilometers in the angular rocks. Pyroclastic
prevailing winds. flows are one of the
main mechanisms
by which these
volcanoes are
6. Shape — Composite volcanoes display the classic volcano
constructed.
shape because most material erupts out of a central vent
and then settles nearby. They have steep slopes because
they form from small eruptions of viscous lava
flows that pile up on the flanks of the
volcano and help protect pyroclastic 11.04.a3 Mount Mayon, Philippines
material from erosion. The shape
represents one snapshot in a series of
3. Landslides and
stacked volcanic mountains that have
Mudflows — Composite
been built over time.
volcanoes can be large
5. Lava Domes and Flows — Lava domes and flows mountains that collect rain or
can erupt from any level of a composite volcano. snow. Rain and snowmelt mix with
Lava may erupt from the summit crater or escape loose ash and rocks on the volcano’s
through vents on the volcano’s sides or base. Lavas flanks, causing a volcano-derived mudflow called
associated with composite volcanoes are moderately to a lahar. There is a high hazard for landslides and
highly viscous, and so they move slowly and with difficulty. The lahars (▼) because of the steep slopes, loose rocks,
11.04.a1
lava may break into blocks that fall, slide, or roll downhill, forming a and abundant slippery clay minerals produced when
tongue or apron (▼) of jumbled pieces from the lava. hot water interacts with the volcanic rocks.
11.04.a4 Augustine volcano, AK 11.04.a5 Mount St. Helens, WA
4. Rocks — Composite volcanoes consist
of alternating layers of pyroclastic flows,
lava flows, and deposits from landslides
and mudflows. Most rocks are interme-
diate to felsic in composition. The
volcanoes we see today, formed during
eruptions from long-lived vents, are
built on and around earlier versions of
the volcanoes. A large composite
volcano can be constructed in tens of
thousands of years, or a few hundred
thousand years.
rey95166_ch11_344-383.indd 352 10/9/13 12:28 PM
Volcanoes, Deformation, and Earthquakes 353
What Are Some Characteristics of a Volcanic Dome?
Volcanic domes form when viscous lava mounds around a vent. Some volcanic domes have a nearly symmetrical
dome shape, but most have an irregular shape because some parts of the dome have grown more than other parts or
because one side of the dome has collapsed. Domes may be hundreds of meters high and one or several kilometers
across, but they can be much smaller, too.
Growth of a Dome
2. Domes mostly grow from
the inside as magma injects
into the interior of the dome.
This new material causes the
dome to expand upward
Dome
and outward, fracturing the
partially solidified outer crust
of the dome. This process
creates the blocks of rubbly,
solidified lava that coat the 11.04.b2
11.04.b1 outside of the dome. The
1. This rubble-covered dome (▲) formed blocks and other pieces,
near the end of the 1912 eruption in the 3. Domes also grow as magma breaks through to the surface
which are as small as several
Valley of Ten Thousand Smokes in Alaska. and flows outward as thick, slow-moving lava. As the magma
centimeters across to as large
Volcanic domes commonly have this type advances, the front of the flow cools, solidifies, and can collapse
as houses, commonly tumble
of rubbly appearance because their outer into angular blocks and ash. Most domes display both modes
or slide down the steep
surface consists of angular, broken pieces of growth.
slopes on the side of a dome.
of the dome.
Collapse or Destruction of a Dome
5. Domes can also be
4. Domes can be partially
destroyed by explo-
destroyed when steep
sions originating
flanks of the dome
within the dome
collapse and break
(◀). These typically
into a jumble of
occur when
blocks and ash that
magma solidifies in
flow downhill as
the conduit and
small-scale
traps gases that
pyroclastic flows (▶).
build up until the
11.04.b3
11.04.b4 pressure can no
longer be held.
Deadly Collapse of a Dome at Mount Unzen, Japan
M
ount Unzen towers above a n
a
small city in southern Japan. pa Before You Leave This
The top of the mountain con- J ,ne Page Be Able To
tains a steep volcanic dome that formed znU
and collapsed repeatedly between 1990 tnu
amnodr e1 9t9h5a.n T h1e0 ,c0o0l0la psmsinagll dpoymroecsl ausntilce afslohweds oM 1t.40.1 Dccohemasrcaprciobtseeirt ieos rtv icsoksl ceoatfcn hao tahned
(top photograph) toward the city below. 1
how such a volcano forms.
In 1991, the opportunity to observe and
film these small pyroclastic flows attracted Sketch and explain the
volcanologists and other onlookers to the characteristics of a
mountain. Unfortunately, partial collapse volcanic dome and the
of the dome caused a pyroclastic flow n two ways by which a
a
larger than had occurred previously. This paJ ,n volcanic dome can grow.
larger flow killed 43 journalists and volca- ezn Explain or sketch how a 1
nologists and left a path of destruction U tn volcanic dome can 1
through the valley (lower photograph). uo collapse or be destroyed .
M 4
Note that damage was concentrated along 2 by an explosion.
valleys that drain the mountain. t.40
.1
1
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