4 Igneous Processes and Volcanoes

Learning Objectives

By the end of this chapter, students should be able to:

  • Explain the origin of as it relates to .
  • Describe how the relates and melting temperatures.
  • Explain how cooling of leads to rock compositions and textures, and how these are used to classify rocks.
  • Analyze the features of common landforms and how they relate to their origin.
  • Describe how silica content affects and eruptive style of .
  • Describe types, eruptive styles, , and their settings.
  • Describe hazards.

 rock is formed when liquid rock freezes into a solid rock. This molten material is called  when it is in the ground and  when it is on the surface. Only the Earth’s is liquid; the Earth’s and is naturally solid. However, there are a few minor pockets of that form near the surface where geologic processes cause melting. It is this that becomes the source for and . This chapter will describe the classification of rocks, the unique processes that form , types of and processes, hazards, and  landforms. 

Pahoehoe lava flow in Hawaii
Figure 4.1: Lava flow in Hawai’i.

cools quickly on the surface of the earth and forms tiny microscopic crystals. These are known as fine-grained , or , rocks. rocks are often , filled with holes from escaping gas bubbles. is the process in which is erupted. Depending on the properties of the that is erupted, the can be drastically different, from smooth and gentle to dangerous and explosive. This leads to different types of and different hazards.

An intrusive igneous mass now exposed at the surface by erosion
Figure 4.2: Half Dome, an intrusive igneous batholith in Yosemite National Park.

In contrast, that cools slowly below the earth’s surface forms larger crystals which can be seen with the naked eye. These are known as coarse-grained , or , rocks. This relationship between cooling rates and grain sizes of the solidified in rocks is important for interpreting the rock’s geologic history.

4.1 Classification of Igneous Rocks

rocks are classified based on and . describes the physical characteristics of the , such as grain size. This relates to the cooling history of the molten from which it came. refers to the rock’s specific mineralogy and chemical . Cooling history is also related to changes that can occur to the of rocks.

4.1.1 Texture

Image showing three or four distinct colors of clearly visible minerals.
Figure 4.3: Granite is a classic coarse-grained (phaneritic) intrusive igneous rock. The different colors are unique minerals. The black colors are likely two or three different minerals.

If cools slowly, deep within the , the resulting rock is called or . The slow cooling process allows crystals to grow large, giving a coarse-grained or . The individual crystals in are readily visible to the unaided eye.

Show dark rock with no visible minerals
Figure 4.4: Basalt is a classic fine-grained extrusive igneous rock.

When is extruded onto the surface, or intruded into shallow fissures near the surface and cools, the resulting is called or . rocks have a fine-grained or , in which the grains are too small to see with the unaided eye. The fine-grained indicates the quickly cooling did not have time to grow large crystals. These tiny crystals can be viewed under a petrographic microscope. In some cases, cools so rapidly it does not develop crystals at all. This non-crystalline material is not classified as , but as glass. This is a common component of and rocks like .

Porphyritic teture with large crystals in a finer grained groundmass
Figure 4.5: Porphyritic texture.

Some rocks have a mix of coarse-grained surrounded by a matrix of fine-grained material in a called . The large crystals are called and the fine-grained matrix is called the or matrix. indicates the body underwent a multi-stage cooling history, cooling slowly while deep under the surface and later rising to a shallower depth or the surface where it cooled more quickly.

Pegmatic texture with large grains of minerals, mostly of felsic composition
Figure 4.6: Pegmatitic texture.

Residual molten material expelled from intrusions may form veins or masses containing very large crystals of like , , beryl, tourmaline, and . This , which indicates a very slow , is called . A rock that chiefly consists of is known as a . To give an example of how large these crystals can get, transparent sheets of were used as windows during the Middle Ages.

A lava rock full of bubbles called scoria
Figure 4.7: Scoria.

All contain gases in solution called . As the rises to the surface, the drop in pressure causes the to come bubbling out of , like the fizz in an opened bottle of soda. The gas bubbles become trapped in the solidifying to create a , with the holes specifically called vesicles. The type of with common vesicles is called scoria.

A pumice stone, a hardened froth of volcanic glass
Figure 4.8: Pumice.

An extreme version of scoria occurs when volatile-rich  is very quickly quenched and becomes a meringue-like froth of glass called . Some is so full of vesicles that the density of the rock drops low enough that it will float.

Photo of obsidian, a volcanic glass
Figure 4.9: Obsidian (volcanic glass). Note conchoidal fracture.

that cools extremely quickly may not form crystals at all, even microscopic ones. The resulting rock is called  glass. Obsidian is a rock consisting of glass.  as a glassy rock shows an excellent example of similar to the (see chapter 3).

Tuff showing various size fragments of minerals and ash blown out of a volcano
Figure 4.10: Welded tuff.

When erupt explosively, vast amounts of , rock, , and gases are thrown into the . The solid parts, called , settle back to earth and cool into rocks with textures. Pyro, meaning fire, refers to the source of the and refers to the rock fragments. fragments are named based on size— (<2 mm), (2-64 mm), and or blocks (>64 mm). is usually recognized by the chaotic mix of crystals, angular glass shards, and rock fragments. Rock formed from large deposits of fragments is called . If the fragments accumulate while still hot, the heat may deform the crystals and weld the mass together, forming a welded .

4.1.2 Composition

refers to a rock’s chemical and make-up . For , is divided into four groups: , , , and . These groups refer to differing amounts of silica, iron, and magnesium found in the that make up the rocks. It is important to realize these groups do not have sharp boundaries in nature, but rather lie on a continuous spectrum with many transitional compositions and names that refer to specific quantities of As an example, is a commonly-used term, but has a very specific definition which includes exact quantities of like and . Rocks labeled as ‘‘ in laymen applications can be several other rocks, including syenite, tonalite, and monzonite. To avoid these complications, the following figure presents a simplified version of nomenclature focusing on the four main groups, which is adequate for an introductory student.

Diagram showing the mineral composition of the four classes of igneous rocks, ultramafic, mafic, intermediate, and felsic.
Figure 4.11: Mineral composition of common igneous rocks. Percentage of minerals is shown on the vertical axis. Percentage of silica is shown on the horizontal axis. Rock names at the top include a continuous spectrum of compositions grading from one into another.

Felsic refers to a predominance of the light-colored ()  feldspar and silica in the form of . These light-colored have more silica as a proportion of their overall chemical formula. Minor amounts of dark-colored () like and biotite may be present as well. rocks are rich in silica (in the 65-75% range, meaning the rock would be 65-75% weight percent SiO2) and poor in iron and magnesium.

is a between and . It usually contains roughly-equal amounts of light and dark , including light grains of and dark like amphibole. It is in silica in the 55-60% range.

Mafic refers to a abundance of ferromagnesian (with magnesium and iron, chemical symbols Mg and Fe) plus . It is mostly made of dark like and , which are rich in iron and magnesium and relatively poor in silica. rocks are low in silica, in the 45-50% range.

refers to the extremely rocks of mostly and some which have even more magnesium and iron and even less silica. These rocks are rare on the surface, but make up , the rock of the upper . It is poor in silica, in the 40% or less range.

On the figure above, the top row has both and rocks arranged in a continuous spectrum from on the left to , , and toward the right.  thus refers to the and rocks, and thus refer to and rocks. and  likewise refer to and rocks (with dacite and granodiorite applying to those rocks with between and ).  and are the and names for rocks, and is , with komatiite as the fine-grained equivalent. Komatiite is a rare rock because material that comes direct from the is not common, although some examples can be found in ancient rocks. Nature rarely has sharp boundaries and the classification and naming of rocks often imposes what appear to be sharp boundary names onto a continuous spectrum.

Classification table of igneous rock.
Figure 4.12: Igneous rock classification table with composition as vertical columns and texture as horizontal rows.
Aphanitic/phaneritic rock types
Felsic composition
Solid, opaque, square shaped, smooth sided rock with specs of pink, black, gray, and cream particles
Granite
Solid, opaque, square shaped, smooth sided rock made up of fine grained pink particles
Rhyolite
Granite is a course-crystalline felsic intrusive rock. The presence of quartz is a good indicator of granite. Granite commonly has large amounts of salmon pink potassium feldspar and white plagioclase crystals that have visible cleavage planes. Granite is a good approximation for the continental crust, both in density and composition. Rhyolite is a fine-crystalline felsic extrusive rock. Rhyolite is commonly pink and will often have glassy quartz phenocrysts. Because felsic lavas are less mobile, it is less common than granite. Examples of rhyolite include several lava flows in Yellowstone National Park and the altered rhyolite that makes up the Grand Canyon of the Yellowstone.
Intermediate composition
Solid, opaque, roughly rectangular, rough sided rock with specs of black and gray particles
Diorite
Solid, opaque rock with a rough triangle shape that is made of mostly fine grained cream particles with few larger gray and pink particles
Andesite
Diorite is a coarse-crystalline intermediate intrusive igneous rock. Diorite is identifiable by it’s Dalmatian-like appearance of black hornblende and biotite and white plagioclase feldspar. It is found in its namesake, the Andes Mountains as well as the Henry and Abajo mountains of Utah. Andesite is a fine crystalline intermediate extrusive rock. It is commonly grey and porphyritic. It can be found in the Andes Mountains and in some island arcs (see Chapter 2). It is the fine grained compositional equivalent of diorite.
Mafic composition
Solid, opaque, roughly triangular, fine-grained rock made of small brown and black particles
Gabbro
Solid, opaque, porous rock with smooth edges that is bluish gray
Vesicular basalt
Gabbro is a coarse-grained mafic igneous rock, made with mainly mafic minerals like pyroxene and only minor plagioclase. Because mafic lava is more mobile, it is less common than basalt. Gabbro is a major component of the lower oceanic crust. Basalt is a fine-grained mafic igneous rock. It is commonly vesicular and aphanitic. When porphyritic, it often has either olivine or plagioclase phenocrysts. Basalt is the main rock which is formed at mid-ocean ridges, and is therefore the most common rock on the Earth’s surface, making up the entirety of the ocean floor (except where covered by sediment).

Table 4.1: Aphanitic and phaneritic rock types with images.

4.1.3 Igneous Rock Bodies

rocks are common in the geologic record, but surprisingly, it is the rocks that are more common. rocks, because of their small crystals and glass, are less durable. Plus, they are, by definition, exposed to the of immediately. rocks, forming underground with larger, stronger crystals, are more likely to last. Therefore, most landforms and rock groups that owe their origin to rocks are bodies. A significant exception to this is active , which are discussed in a later section on volcanism. This section will on the common bodies which are found in many places within the of Earth.

Igneous dike cuts across Baffin Island in the Canadian Arctic.
Figure 4.13: Dike of olivine gabbro cuts across Baffin Island in the Canadian Arctic.

When intrudes into a weakness like a crack or fissure and solidifies, the resulting cross-cutting feature is called a  (sometimes spelled ). Because of this, are often vertical or at an angle relative to the pre-existing rock layers that they intersect. are therefore discordant intrusions, not following any layering that was present. are important to geologists, not only for the study of rocks themselves but also for dating rock sequences and interpreting the geologic history of an area. The is younger than the rocks it cuts across and, as discussed in the chapter on Geologic Time (chapter 7), may be used to assign actual numeric ages to sedimentary sequences, which are notoriously difficult to age date. 

Igneous sill intruding in between Paleozoic strata in Nova Scotia
Figure 4.14: Igneous sill intruding between Paleozoic strata in Nova Scotia.

are another type of structure. A is a concordant intrusion that runs parallel to the sedimentary layers in the . They are formed when exploits a weakness between these layers, shouldering them apart and squeezing between them. As with , are younger than the surrounding layers and may be radioactively dated to study the age of sedimentary .

Exposure of white rock between tres
Figure 4.15: Quartz monzonite in the Cretaceous of Montana, USA.

A is a large underground of molten rock. The path of rising is called a . The processes by which a intrudes into the surrounding or are not well understood and are the subject of ongoing geological inquiry. For example, it is not known what happens to the pre-existing as the intrudes. One is the overriding rock gets shouldered aside, displaced by the increased volume of . Another is the rock is melted and consumed into the rising or broken into pieces that settle into the , a process known as . It has also been proposed that diapirs are not a real phenomenon, but just a series of that blend into each other. The may be intruding over millions of years, but since they may be made of similar material, they would be appearing to be formed at the same time. Regardless, when a cools, it forms an mass of rock called a . can have irregular shapes, but can often be somewhat round.

View showing an expansive area of a mountain range with exposed white granite in many places.
Figure 4.16: Half Dome in Yosemite National Park, California, is a part of the Sierra Nevada batholith which is mostly made of granite.

When many merge together in an extensive single feature, it is called a . are found in the cores of many mountain ranges, including the of Yosemite National Park in the Sierra Nevada of California. They are typically more than 100 km2 in area, associated with zones, and mostly in . A stock is a type of with less surface exposure than a , and may represent a narrower neck of material emerging from the top of a . and stocks are discordant intrusions that cut across and through surrounding .

Henry Mountains, Utah, interpreted to be a laccolith.
Figure 4.17: The Henry Mountains in Utah are interpreted to be a laccolith, exposed by erosion of the overlying layers.
Laccolith forms as a blister in between sedimentary layers
Figure 4.18: Laccolith forms as a blister in between sedimentary strata.

are blister-like, concordant intrusions of that form between sedimentary layers. The Henry Mountains of Utah are a famous topographic landform formed by this process. bulge upwards; a similar downward-bulging intrusion is called a .


Complete this interactive activity to check your understanding.
Click on the plus signs on the illustration for descriptions of several igneous features.

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4.2 Bowen’s Reaction Series

Diagram of Bowen's Reaction Series, Y-shpaed with 8 minerals and a temperature scale
Figure 4.19: Bowen’s Reaction Series. Higher temperature minerals shown at top (olivine) and lower temperature minerals shown at bottom (quartz).
The crystal is light green.
Figure 4.20: Olivine, the first mineral to crystallize in a melt.

describes the at which crystallize when cooled, or melt when heated. The low end of the scale where all crystallize into solid rock, is approximately 700°C (1292°F). The upper end of the range where all exist in a molten state, is approximately 1,250°C (2,282°F). These numbers reference that crystallize at standard sea-level pressure, 1 bar. The values will be different for located deep below the Earth’s surface due to the increased pressure, which affects and melting temperatures. However, the order and relationships are maintained.

In the figure, the righthand column lists the four groups of from top to bottom: , , , and . The down-pointing arrow on the far right shows increasing amounts of silica, sodium, aluminum, and potassium as the goes from to . The up-pointing arrow shows increasing ferromagnesian components, specifically iron, magnesium, and calcium. To the far left of the diagram is a scale. near the top of diagram, such as olivine and anorthite (a type of plagioclase), crystallize at higher temperatures. Minerals near the bottom, such as and , crystalize at lower temperatures.

Photo of Normal L. Bowen in 1909.
Figure 4.21: Norman L. Bowen.

The most important aspect of is to notice the relationships between and . Norman L. Bowen (1887-1956) was an early 20th Century geologist who studied igneous rocks. He noticed that in rocks, certain always occur together and these assemblages exclude other . Curious as to why, and with the  in mind that it had to do with the at which the rocks cooled, he set about conducting experiments on rocks in the early 1900s. He conducted experiments on —grinding combinations of rocks into powder, sealing the powders into metal capsules, heating them to various temperatures, and then cooling them.

Photo of Bowen and Tuttle sitting down looking at samples
Figure 4.22: Norman L. Bowen and his colleague working at the Carnegie Institution of Washington Geophysical Laboratory.

When he opened the quenched capsules, he found a glass surrounding crystals that he could identify under his petrographic microscope. The results of many of these experiments, conducted at different temperatures over a of several years, showed that the common crystallize from at different temperatures. He also saw that occur together in rocks with others that crystallize within similar ranges, and never crystallize with other . This relationship can explain the main difference between and rocks. rocks contain more , and therefore, crystallize at higher temperatures than rocks. This is even seen in flows, with lavas erupting hundreds of degrees cooler than their counterparts. Bowen’s work laid the foundation for understanding (the study of rocks) and resulted in his book, The Evolution of the Igneous Rocks in 1928.


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4.3 Magma Generation

and contain three components: melt, solids, and . The melt is made of ions from that have liquefied. The solids are made of crystallized floating in the liquid melt. These may be that have already cooled  are gaseous components—such as water vapor, carbon dioxide, sulfur, and chlorine— in the . The presence and amount of these three components affect the physical behavior of the and will be discussed more below.

4.3.1 Geothermal Gradient

Diagram showing temperature increase with depth in the Earth
Figure 4.23: Geothermal gradient.

Below the surface, the of the Earth rises. This heat is caused by residual heat left from the of Earth and ongoing decay. The rate at which increases with depth is called the . The average in the upper 100 km (62 mi) of the is about 25°C per kilometer of depth. So for every kilometer of depth, the increases by about 25°C.

Diagram showing pressures and temperatures of the geothermal gradient increasing deeper in the earth. The solidus line shows that temperatures need to be much higher or pressure needs to be lower in order for rocks to start to melt.
Figure 4.24: Pressure-temperature diagram showing temperature in degrees Celsius on the x-axis and depth below the surface in kilometers (km) on the y-axis. The red line is the geothermal gradient and the green solidus line represents the temperature and pressure regime at which melting begins. Rocks at pressures and temperatures left of the green line are solid. If pressure/temperature conditions change so that rocks pass to the right of the green line, then they will start to melt.

The depth- graph (see figure 4.23) illustrates the relationship between the (geotherm, red line) and the start of rock melting (solidus, green line). The changes with depth (which has a direct relationship to pressure) through the into upper . The area to the left of the green line includes solid components; to the right is where liquid components start to form. The increasing with depth makes the depth of about 125 kilometers (78 miles) where the natural is closest to the solidus.

The at 100 km (62 mi) deep is about 1,200°C (2,192°F). At bottom of the , 35 km (22 mi) deep, the pressure is about 10,000 bars. A bar is a measure of pressure, with 1 bar being normal atmospheric pressure at sea level. At these pressures and temperatures, the and are solid. To a depth of 150 km (93 mi), the line stays to the left of the solidus line. This relationship continues through the to the boundary, at 2,880 km (1,790 mi).

The solidus line slopes to the right because the melting of any substance depends on pressure. The higher pressure created at greater depth increases the needed to melt rock. In another example, at sea level with an atmospheric pressure close to 1 bar, water boils at 100°C. But if the pressure is lowered, as shown on the video below, water boils at a much lower .


Video 4.1: Boiling water at room temperature.

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There are three principal ways rock behavior crosses to the right of the green solidus line to create molten : 1) caused by lowering the pressure, 2) caused by adding (see more below), and 3) heat-induced melting caused by increasing the . The shows that melt at different temperatures. Since is a mixture of different , the solidus boundary is more of a fuzzy zone rather than a well-defined line; some are melted and some remain solid. This type of rock behavior is called and represents real-world , which typically contain solid, liquid, and volatile components.

4.4 Volcanism

When emerges onto the Earth’s surface, the molten rock is called . A is a type of land created when solidifies into rock. have been an important part of human society for centuries, though their understanding has greatly increased as our understanding of has made them less mysterious. This section describes location, type, hazards, and monitoring.

4.4.1. Distribution and Tectonics

Diagram showing how volcanoes are associated with plate boundaries
Figure 4.25: Association of volcanoes with plate boundaries.

Most are . are located at active boundaries created by at , zones, and . The prefix “inter-“ means between. Some volcanoes are . The prefix “intra-“ means within, and intraplate volcanoes are located within , far removed from plate boundaries. Many are formed by .

Volcanoes at Mid-ocean Ridges

Map showing spreading ridges throughout the world. These ridges are all over the world.
Figure 4.26: Map of spreading ridges throughout the world.

Most on Earth occurs on the along , a type of (see chapter 2). These are also the least observed and famous, since most of them are located under 3,000-4,500 m (10,000-15,000 ft) of ocean and the eruptions are slow, gentle, and oozing. One exception is the of Iceland. The diverging and thinning allow hot rock to rise, releasing pressure and causing . rock, consisting largely of , partially melts and generates that is basaltic. Because of this, almost all on the are basaltic. In fact, most is basaltic near the surface, with and underneath.

Pillow basalt on sea floor near Hawaii.
Figure 4.27: Pillow basalt on sea floor near Hawai’i.

When basaltic erupts underwater it emerges in small explosions and/or forms pillow-shaped structures called pillow basalts. These seafloor eruptions enable entire underwater ecosystems to thrive in the deep ocean around . This ecosystem exists around tall vents emitting black, hot -rich water called deep-sea vents, also known as .

There is a large build up of minerals around the vent
Figure 4.28: Black smoker hydrothermal vent with a colony of giant (6’+) tube worms.
Map showing worldwide distgrbution of hydrothermal vent fields;
Figure 4.29: Distribution of hydrothermal vent fields.

Without sunlight to support photosynthesis, these organisms instead utilize a process called . Certain bacteria are able to turn hydrogen (H2S), a gas that smells like rotten eggs, into life-supporting nutrients and water. Larger organisms may eat these bacteria or absorb nutrients and water produced by bacteria living symbiotically inside their bodies. The videos show some of the ecosystems found around deep-sea vents.


Video 4.2: Updating the deep–diving submarine at 50-years-old.

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Video 4.3: Incredible views on board the deep-sea vessel.

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Volcanoes at Subduction Zones

Map showing volcanoes follow the edges of tectonic plates.
Figure 4.30: Distribution of volcanoes on the planet. Click here for an interactive map of volcano distributions.

The second most commonly found location for is adjacent to zones, a type of (see chapter 2). The process of expels water from hydrated in the descending , which causes in the overlying rock. Because occurs in a , the thickened promotes and differentiation. These evolve the from the mantle into more silica-rich . The Ring of Fire surrounding the Pacific Ocean, for example, is dominated by -generated eruptions of mostly silica-rich ; the and consist largely of -to- rock such as , , , and .

Volcanoes at Continental Rifts

A barren landscape of lava flows in central Utah.
Figure 4.31: Basaltic cinder cones of the Black Rock Desert near Beaver, Utah.

Some are created at , where crustal thinning is caused by diverging lithospheric , such as the East African Rift Basin in Africa. caused by crustal thinning without is found in the Province in North America. In this location, activity is produced by rising that stretches the overlying . Lower or upper material rises through the thinned , releases pressure, and undergoes decompression-induced . This is less dense than the surrounding rock and continues to rise through the to the surface, erupting as basaltic . These eruptions usually result in , cones, and basaltic flows (see video). Relatively young cones of basaltic can be found in south-central Utah, in the Black Rock Desert Volcanic Field, which is part of the zone of crustal . These Utah cones and flows started erupting around 6 million years ago, with the last eruption occurring 720 years ago.


Video 4.4: Basin and range volcanic processes.

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Hotspots

The plate is moving to the left, the magma stays in the center am makes a chain of volcanoes.
Figure 4.32: Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate.

are the main source of . occur when lithospheric glide over a hot , an ascending column of solid heated rock originating from deep within the . The generates melts as material rises, with the rising even more. When the ascending reaches the lithospheric crust, it spreads out into a mushroom-shaped head that is tens to hundreds of kilometers across.

The hotspot started near the Idaho-Oregon-Nevada boarder, then moved toward its present location neat the Wyoming-Idaho-Montana boarder.
Figure 4.33: The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago.

Since most plumes are located beneath the , the early stages of typically take place underwater. Over time, basaltic may build up from the sea floor into islands, such as the Hawaiian Islands. Where a is found under a , contact with the hot magma may cause the overlying rock to melt and mix with the mafic material below, forming . Or the may continue to rise, and cool into a granitic or erupt as a . The Yellowstone caldera is an example of that resulted in an explosive eruption.

There are a series of island and seamounts in the Pacific Ocean, with a bend in the middle.
Figure 4.34: The Hawaiian–Emperor seamount and island chain.

A zone of actively erupting connected to a chain of indicates located over a . These chains are created by the overriding slowly moving over a . These chains are seen on the seafloor and continents and include that have been inactive for millions of years. The Hawaiian Islands on the Pacific Oceanic plate are the active end of a long chain that extends from the northwest Pacific Ocean to the Emperor Seamounts, all the way to the to the zone beneath the Kamchatka Peninsula. The overriding North American moved across a for several million years, creating a chain of calderas that extends from Southwestern Idaho to the presently active Yellowstone caldera in Wyoming.

The three-minute video (below) illustrates .


Video 4.5: What is a volcanic hotspot?

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4.4.2 Volcano Features and Types

There are several different types of based on their shape, eruption style, magmatic , and other aspects.


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A smaller parasitic cone called Shastina on the flanks of Mt. Shasta in Washington
Figure 4.35: Mt. Shasta in Washington state with Shastina, its parasitic cone.

The figure shows the main features of a typical : 1) , 2) upper layers of , 3) the  or narrow pipe through which the  erupts, 4) the base or edge of the , 5) a of between layers of the , 6) a or feeder tube to the , 7) layers of () from previous eruptions, 8 & 9) layers of erupting from the and flowing down the sides of the , 10) the crater at the top of the , 11) layers of and on (12), a . A  is a small located on the flank of a larger volcano such as Shastina on Mount Shasta. Kilauea sitting on the flank of Mauna Loa is not considered a because it has its own separate , 13) the vents of the parasite and the main , 14) the rim of the crater, 15) clouds of blown into the sky by the eruption; this settles back onto the and surrounding land.

The mountain has a large hole in the center that is filled with the lake.
Figure 4.36: Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.

The largest craters are called , such as the Crater Lake Caldera in Oregon. Many features are produced by , a basic property of a . is the resistance to flowing by a fluid. Low flows easily more like syrup, the basaltic that occurs in Hawai’i on . High means a thick and sticky , typically or , that flows slowly, similar to toothpaste.

Shield Volcano

The mountain has low-angle flanks
Figure 4.37: Kilauea in Hawai’i.

The largest are . They are characterized by broad low-angle flanks, small vents at the top, and chambers. The name comes from the side view, which resembles a medieval warrior’s . They are typically associated with , , or with rising upper material. The low-angle flanks are built up slowly from numerous low- basaltic flows that spread out over long distances. The basaltic erupts effusively, meaning the eruptions are small, localized, and predictable.

Lava from Kiluea destroying road in Hawaii.
Figure 4.38: Eruption of Kiluea in 2018 produced high viscosity lava shown here crossing a road. This eruption caused much property damage.

Typically, eruptions are not much of a hazard to human life—although non-explosive eruptions of Kilauea (Hawai’i) in 2018 produced uncharacteristically large lavas that damaged roads and structures. Mauna Loa (see USGS page) and Kilauea (see USGS page) in Hawai’i are examples of . are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African Rift.

Ariel view of volcano. Symmetrical and round with the dark part focused in the center
Figure 4.39: Olympus Mons, an enormous shield volcano on Mars, the largest volcano in the solar system, standing about two and a half times higher than Everest is above sea level.

The largest edifice in the Solar System is Olympus Mons on Mars. This (possibly ) covers an area the size of the state of Arizona. This may indicate the erupted over a for millions of years, which means Mars had little, if any, activity.

The lava is ropey
Figure 4.40: Ropey pahoehoe lava.

Basaltic forms special landforms based on , , and content of gases and water vapor. The two main types of basaltic have Hawaiian names— and . might come from low- lava that flows easily into ropey strands.

The lava is sharp and jagged
Figure 4.41: Blocky a’a lava.

(sometimes spelled a’a or and pronounced “ah-ah”) is more and has a crumbly blocky appearance. The exact details of what forms the two types of flows are still up for debate. lavas have lower temperatures and more silica, and thus are higher . These also form -style flows.

The magma is sputtering outward
Figure 4.42: Volcanic fissure and flow, which could eventually form a lava tube.

Low-, fast-flowing basaltic tends to harden on the outside into a tube and continue to flow internally. Once flow subsides, the empty outer shell may be left as a tube. tubes, with or without collapsed roofs, make famous caves in Hawai’i, Northern California, the Columbia River Basalt Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monument in Idaho.

Fissures are cracks that commonly originate from -style eruptions. emerging from fissures is typically and very fluid. The 2018 Kiluaea eruption included fissures associated with the flows. Some fissures are caused by the activity rather than flows. Some fissures are influenced by , such as the common fissures located parallel to the boundary in Iceland.

The rock is full of columns
Figure 4.43: Devils Tower in Wyoming has columnar jointing.
Columnar jointing on Giant's Causeway in Ireland.
Figure 4.44: Columnar jointing on Giant’s Causeway in Ireland.

Cooling can contract into columns with semi-hexagonal cross sections called columnar jointing. This feature forms the famous Devils Tower in Wyoming, possibly an ancient from which the surrounding layers of and have been removed by . Another well-known exposed example of columnar jointing is the Giant’s Causeway in Ireland.

Stratovolcano

The mountain is very tall, and looms over the city
Figure 4.45: Mount Rainier towers over Tacoma, Washington.

A , also called a , has steep flanks, a symmetrical cone shape, distinct crater, and rises prominently above the surrounding landscape. The term composite refers to the alternating layers of fragments like and , and solidified flows of varying . Examples include Mount Rainier in Washington state and Mount Fuji in Japan.

Volcano from side view. symmetrical volcano, increasing slope, visible crater at the top. White at the top
Figure 4.46: Mt. Fuji in Japan, a typical stratovolcano, symmetrical, increasing slope, visible crater at the top.

Stratovolcanoes usually have to chambers, but can even produce lavas. Stratovolcanoes have   flows and , punctuated by explosive eruptions. This produces with steep flanks.

Lava Domes

The mountain has a hole, but the hole has filled in somewhat
Figure 4.47: Lava domes have started the rebuilding process at Mount St. Helens, Washington.

are accumulations of silica-rich , such as and . Too to flow easily, the tends to pile up near the in blocky masses. often form in a within the collapsed crater of a , and grow by internal expansion. As the expands, the outer surface cools, hardens, and shatters, and spills loose fragments down the sides. Mount Saint Helens has a good example of a inside of a collapsed crater. Examples of stand-alone are Chaiten in Chile and Mammoth Mountain in California.

Caldera

It shows the eruption forming a caldera.
Figure 4.48: Timeline of events at Mount Mazama.
The island is forested, as are the flanks
Figure 4.49: Wizard Island sits in the caldera at Crater Lake.

are steep-walled, -shaped depressions formed by the collapse of a edifice into an empty . Calderas are generally very large, with diameters of up to 25 km (15.5 mi). The term specifically refers to a ; however, it is frequently used to describe a type. are typically formed by eruptions of high- having high content.

Crater Lake, Yellowstone, and the Long Valley Caldera are good examples of this type of . The at Crater Lake National Park in Oregon was created about 6,800 years ago when Mount Mazama, a , erupted in a huge explosive blast. The ejected large amounts of and rapidly drained the , causing the top to collapse into a large depression that later filled with water. Wizard Island in the middle of the lake is a later resurgent that formed within the .

The map shows locations of calderas and rocks within Yellowstone
Figure 4.50: Map of calderas and related rocks around Yellowstone.

The Yellowstone erupted three times in the recent geologic past—2.1, 1.3, and 0.64 million years ago—leaving behind three basins. Each eruption created large flows as well as flows that solidified into . These extra-large eruptions rapidly emptied the , causing the roof to collapse and form a . The youngest of the three calderas contains most of Yellowstone National Park, as well as two resurgent . The calderas are difficult to see today due to the amount of time since their eruptions and subsequent and .

Yellowstone started about 17-million years ago as a under the North American lithospheric near the Oregon/Nevada border. As the moved to the southwest over the stationary , it left behind a track of past activities. Idaho’s Snake Plain was created from that produced a series of calderas and flows. The eventually arrived at its current location in northwestern Wyoming, where formed the Yellowstone calderas.

The eruptions trend eastward due to prevailing winds.
Figure 4.51: Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall (yellow).

The Long Valley Caldera near Mammoth, California, is the result of a large eruption that occurred 760,000 years ago. The explosive eruption dumped enormous amounts of across the United States, in a manner similar to the Yellowstone eruptions. The Bishop Tuff deposit near Bishop, California, is made of from this eruption. The current is 17 km by 32 km (10 mi by 20 mi), large enough to contain the town of Mammoth Lakes, major ski resort, airport, major highway, resurgent , and several hot springs.

Cinder Cone

The cone is relatively small and red
Figure 4.52: Sunset Crater, Arizona is a cinder cone.

cones are small with steep sides, and made of fragments that have been ejected from a pronounced central . The small fragments are called and the largest are . The eruptions are usually short-lived events, typically consisting of lavas with a high content of . Hot is ejected into the air, cooling and solidifying into fragments that accumulate on the flank of the . cones are found throughout western North America.

A person looks at the eruption of ash
Figure 4.53: Soon after the birth of Parícutin in 1943.
Church that is covered in lava
Figure 4.54: Lava from Parícutin covered the local church and destroyed the town of San Juan, Mexico.

A recent and striking example of a is the eruption near the village of Parícutin, Mexico that started in 1943. The started explosively shooting out of the in the middle of a farmer’s field. The quickly built up the cone to a height of over 90 m (300 ft) within a week, and 365 m (1,200 ft) within the first 8 months. After the initial explosive eruption of gases and , basaltic poured out from the base of the cone. This is a common order of events for cones: violent eruption, cone and crater , low- flow from the base. The is not strong enough to support a column of rising to the top of the crater, so the breaks through and emerges near the bottom of the . During nine years of eruption activity, the ashfall covered about 260 km2 (100 mi2) and destroyed the nearby town of San Juan.

Flood Basalts

World map of flood basalts. Note the largest is the Siberian Traps
Figure 4.55: World map of flood basalts. Note the largest is the Siberian Traps.

A rare eruption type, unobserved in modern times, is the . are some of the largest and lowest types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption are still mysterious. Some famous examples include the Columbia River Flood Basalts in Washington, Oregon, and Idaho, the Deccan Traps, which cover about 1/3 of the country of India, and the Siberian Traps, which may have been involved in the Earth’s largest (see chapter 8).

Table of igneous rocks and related volcano types. Horizontal axis is arranged from low to high silica content (i.e. from ultramafic to felsic). First row shows the extrusive (surface) igneous rocks basalt, andesite, and rhyolite. Second row shows volcano types: mid-ocean ridge, shield, cinder cone, and strato (composite). Third row shows examples of each volcano: mid-atlantic ridge, Mauna Kea (Hawaii), Paricutin, and Mt. St. Helens. Forth row shows intrusive rocks from mafic to felsic: Dunite, gabbro, diorige, granite. Fifth row shows common plate-tectonic settings: divergent oceanic hot spot, and convergent boundaries. Sixth row is typical composition: ultramafic, mafic, intermediate, and felsic.
Figure 4.56: Igneous rock types and related volcano types. Mid-ocean ridges and shield volcanoes represent more mafic compositions, and strato (composite) volcanoes generally represent a more intermediate or felsic composition and a convergent plate tectonic boundary. Note that there are exceptions to this generalized layout of volcano types and igneous rock composition.

4.4.3 Volcanic Hazards and Monitoring

It shows many hazards
Figure 4.57: General diagram of volcanic hazards.

While the most obvious hazard is , the dangers posed by go far beyond flows. For example, on May 18, 1980, Mount Saint Helens (Washington, United States) erupted with an explosion and that removed the upper 400 m (1,300 ft) of the mountain. The initial explosion was immediately followed by a lateral blast, which produced a that covered nearly 600 km2 (230 mi2) of forest with hot and debris. The pyroclastic flow moved at speeds of 80-130 kph (50-80 mph), flattening trees and ejecting clouds of ash into the air. The USGS video provides an account of this explosive eruption that killed 57 people.


Video 4.6: Mout St. Helens.

If you are using an offline version of this text, access this YouTube video via the QR code.

The body is outlined with a cast, and the bones are seen.
Figure 4.58: Human remains from the 79 CE eruption of Vesuvius.

In 79 AD, Mount Vesuvius, located near Naples, Italy, violently erupted sending a over the Roman countryside, including the cities of Herculaneum and Pompeii. The buried towns were discovered in an archeological expedition in the 18th century. Pompeii famously contains the remains () of people suffocated by ash and covered by 10 feet (3 m) of , , and collapsed roofs.

Left: The volcano is conical and forested. Right: The top of the mountain is gone.
Figure 4.59: Mount St. Helens, the day before the May 18th, 1980 eruption (left) and 4 months after the major eruption (right).
Volcano erupting with billowing smoke
Figure 4.60: Image from the May 18, 1980, eruption of Mt. Saint Helens, Washington.

Pyroclastic Flows

Most of the material is heading up, but small portions of the eruption column head downward.
Figure 4.61: The material coming down from the eruption column is a pyroclastic flow.

The most dangerous hazard are flows (video). These flows are a mix of blocks, , , and hot gases between 200°C-700°C (400°F-1,300°F). The turbulent cloud of and gas races down the steep flanks at high speeds up to 193 kph (120 mph) into the valleys around composite . Most explosive, silica-rich, high such as composite cones usually have flows. The rock and welded is often formed from these flows.

A man is seen overlooking the destroyed city
Figure 4.62: The remains of St. Pierre.

There are numerous examples of deadly flows. In 2014, the Mount Ontake in Japan killed 47 people. The flow was caused by heating into steam, which then rapidly ejected with and . Some were killed by inhalation of toxic gases and hot ash, while others were struck by volcanic bombs. Two short videos below document eye-witness video of flows. In the early 1990s, Mount Unzen erupted several times with flows. The  shown in this famous short video killed 41 people. In 1902, on the Caribbean Island Martinique, Mount Pelee erupted with a violent that destroyed the entire town of St. Pierre and killing 28,000 people in moments.


Video 4.7: Dome collapse and pyroclastic flow at Unzen Volcano.

If you are using an offline version of this text, access this YouTube video via the QR code.

Landslides and Landslide–Generated Tsunamis

The landslide opened an area for the eruption
Figure 4.63: Sequence of events for Mount St. Helens, May 18th, 1980. Note that an earthquake caused a landslide, which caused the “uncorking” of the mountain and started the eruption.

The steep and unstable flanks of a can lead to slope failure and dangerous . These can be triggered by movement, explosive eruptions, large earthquakes, and/or heavy rainfall. During the 1980 Mount St. Helens eruption, the entire north flank of the collapsed and released a huge that moved at speeds of 160-290 kph (100-180 mph).

If enough material reaches the ocean, it may cause a . In 1792, a caused by the Mount Unzen eruption reached the Ariaka Sea, generating a that killed 15,000 people (see USGS page). When Mount Krakatau in Indonesia erupted in 1883, it generated ocean waves that towered 40 m (131 ft) above sea level. The killed 36,000 people and destroyed 165 villages.

Tephra

The man is wearing a mask to prevent pneumonoultramicroscopicsilicovolvanoconiosis.
Figure 4.64: Aman sweeps ash from an eruption of Kelud, Indonesia.

, especially composite , eject large amounts of (ejected rock materials), most notably  ( fragments less than 0.08 inches [2 mm]). Larger is heavier and closer to the . Larger blocks and pose hazards to those close to the eruption such as at the 2014 Mount Ontake disaster in Japan discussed earlier.

Micrograph of silica particle in volcanic ash. A cloud of these is capable of destroying an aircraft or automobile engine.
Figure 4.65: Micrograph of silica particle in volcanic ash. A cloud of these is capable of destroying an aircraft or automobile engine.

Hot poses an immediate danger to people, animals, plants, machines, roads, and buildings located close to the eruption. is fine grained (< 2mm) and can travel airborne long distances away from the eruption site. Heavy accumulations of can cause buildings to collapse. In people, it may cause respiratory issues like silicosis. is destructive to aircraft and automobile engines, which can disrupt transportation and shipping services. In 2010, the Eyjafjallajökull volcano in Iceland emitted a large cloud into the upper , causing the largest air-travel disruption in northern Europe since World War II. No one was injured, but the service disruption was estimated to have cost the world economy billions of dollars.

Volcanic Gases

As rises to the surface the pressure decreases, and allows gases to escape into the . Even that are not actively erupting may emit hazardous gases, such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen (H2S), and hydrogen (HF, HCl, or HBr).

Carbon dioxide tends to sink and accumulate in depressions and basins. In areas known to emit carbon dioxide, low-lying areas may hazardous concentrations of this colorless and odorless gas. The Mammoth Mountain Ski Resort in California, is located within the Long Valley Caldera, is one such area of carbon dioxide-producing . In 2006, three ski patrol members died of suffocation caused by carbon dioxide after falling into a snow depression near a fumarole (info).

In rare cases, may create a sudden emission of gases without warning. Limnic eruptions (limne is Greek for lake), occur in crater lakes associated with active . The water in these lakes is supercharged with high concentrations of gases. If the water is physically jolted by a or earthquake, it may an immediate and release of gases out of . An analogous example would be what happens to vigorously shaken bottle of carbonated soda when the cap is opened. An infamous limnic eruption occurred in 1986 at Lake Nyos, Cameroon. Almost 2,000 people were killed by a release of carbon dioxide.

Lahars

The mud line is far up on the trees
Figure 4.66: Mud line shows the extent of lahars around Mount St. Helens.

is an Indonesian word and is used to describe a mudflow that forms from rapidly melting snow or . are slurries resembling wet concrete, and consist of water, , rock fragments, and other debris. These mudflows flow down the flanks of or mountains covered with freshly-erupted and on steep slopes can reach speeds of up to 80 kph (50 mph).

The cities are on top of old lahar deposits
Figure 4.67: Old lahars around Tacoma, Washington.

Several major cities, including Tacoma, are located on prehistoric flows that extend for many kilometers across the flood plains surrounding Mount Rainier in Washington (see map). A map of Mount Baker in Oregon shows a similar potential hazard for flows (see map). A tragic scenario played out recently, in 1985, when a from the Nevado del Ruiz in Colombia buried the town of Armero and killed an estimated 23,000 people.

Monitoring

Geologists use various instruments to detect changes or indications that an eruption is imminent. The three videos show different types of monitoring used to predict eruptions 1) earthquake activity; 2) increases in gas emission; and 3) changes in land surface orientation and elevation.

One video shows how monitoring earthquake frequency, especially special vibrational earthquakes called harmonic tremors, can detect movement and possible eruption. Another video shows how gas monitoring may be used to predict an eruption. A rapid increase of gas emission may indicate that is actively rising to surface and releasing gases out of , and that an eruption is imminent. The last video shows how a GPS unit and tiltmeter can detect land surface changes, indicating the is moving underneath it.


Video 4.8: Earthquake signals.

If you are using an offline version of this text, access this YouTube video via the QR code.


Video 4.9: Measuring gas emissions.

If you are using an offline version of this text, access this YouTube video via the QR code.


Video 4.10: Using tiltmeters and GPS to monitor a volcano.

If you are using an offline version of this text, access this YouTube video via the QR code.


Take this quiz to check your comprehension of this section.

If you are using an offline version of this text, access the quiz for section 4.4 via the QR code.

 

Summary

is divided into two major groups: rock that solidifies from underground , and rock formed from that erupts and cools on the surface. is generated from material at several situations by three types of melting: , , or heat-induced melting. is determined by differences in the melting temperatures of the components (). The processes affecting include , , , and . come in a wide variety of shapes and sizes, and are classified by a multiple factors, including , and activity. Because presents serious hazards to human civilization, geologists carefully monitor activity to mitigate or avoid the dangers it presents.


Take this quiz to check your comprehension of this chapter.

If you are using an offline version of this text, access the quiz for chapter 4 via the QR code.

 

URLs Listed Within This Chapter

USGS page of Mauna Loa: https://www.usgs.gov/volcanoes/mauna-loa 

USGS page on Kilauea: https://www.usgs.gov/volcanoes/kilauea

Pyroclastic flows video: https://volcanoes.usgs.gov/vsc/movies/movie_101/PF_Animation.mp4

USGS page on volcano landslides that trigger waves and tsunamis: https://volcanoes.usgs.gov/Imgs/Jpg/Unzen/MayuyamaSlide_caption.html

Ski patrol’s fatal fall into a volcanic fumarole: https://pubmed.ncbi.nlm.nih.gov/19364170/

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Figure References

Figure 4.1: Lava flow in Hawai’i. Brocken Inaglory. 2007. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:P%C4%81hoehoe_and_Aa_flows_at_Hawaii.jpg

Figure 4.2: Half Dome, an intrusive igneous batholith in Yosemite National Park. Jon Sullivan. 2004. Public domain. https://commons.wikimedia.org/wiki/File:Yosemite_20_bg_090404.jpg

Figure 4.3: Granite is a classic coarse-grained (phaneritic) intrusive igneous rock. James St. John. 2019. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Granite_47_(49201189712).jpg

Figure 4.4: Basalt is a classic fine-grained extrusive igneous rock. James St. John. 2019. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Basalt_3_(48674276863).jpg

Figure 4.5: Porphyritic texture. Jstuby. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Olearyandesite.jpg

Figure 4.6: Pegmatitic texture. Jstuby. 2007. Public domain. https://commons.wikimedia.org/wiki/File:We-pegmatite.jpg

Figure 4.7: Scoria. Jonathan Zander (Digon3). 2008. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Scoria_Macro_Digon3.jpg

Figure 4.8: Pumice. deltalimatrieste. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Pomice_di_veglia.jpg

Figure 4.9: Obsidian (volcanic glass). Note conchoidal fracture. Ji-Elle. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Lipari-Obsidienne_(5).jpg

Figure 4.10: Welded tuff. Wilson44691. 2010. Public domain. https://commons.wikimedia.org/wiki/File:HoleInTheWallTuff.JPG

Figure 4.11: Mineral composition of common igneous rocks. Woudloper. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Mineralogy_igneous_rocks_EN.svg

Figure 4.12: Igneous rock classification table with composition as vertical columns and texture as horizontal rows. Kindred Grey. 2022. Adapted from Belinda Madsen, An Introduction to Geology. OpenStax. Salt Lake Community College. CC BY-NC-SA 4.0.

Table 4.1: Aphanitic and phaneritic rock types with images. Quartz monzonite 36mw1037 by B.W. Hallett, V. F. Paskevich, L.J. Poppe, S.G. Brand, and D.S. Blackwood via USGS (Public domain, https://commons.wikimedia.org/wiki/File:Quartz_monzonite_36mw1037.jpg). PinkRhyolite by Michael C. Rygel, 2014 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:PinkRhyolite.tif). Diorite MA by Amcyrus2012, 2015 (CC BY 4.0, https://commons.wikimedia.org/wiki/File:Diorite_MA.JPG). Andesite by James St. John, 2014 (CC BY 2.0, https://flic.kr/p/oBkKSy). GabbroRockCreek1 by Mark A. Wilson, 2008 (Public domain, https://commons.wikimedia.org/wiki/File:GabbroRockCreek1.jpg). VesicularBasalt1 by Jstuby, 2008 (Public domain, https://commons.wikimedia.org/wiki/File:VesicularBasalt1.jpg).

Figure 4.13: Dike of olivine gabbro cuts across Baffin Island in the Canadian Arctic. Mike Beauregard. 2012. CC BY 2.0. https://en.wikipedia.org/wiki/File:Franklin_dike_on_northwestern_Baffin_Island..jpg

Figure 4.14: Igneous sill intruding between Paleozoic strata in Nova Scotia. Mikenorton. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Horton_Bluff_mid-Carboniferous_sill.JPG

Figure 4.15: Quartz monzonite in the Cretaceous of Montana, USA. James St. John. 2010. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Butte_Quartz_Monzonite_(Late_Cretaceous,_76_Ma;_Rampart_Mountain,_northeast_of_Butte,_Montana,_USA)_1.jpg

Figure 4.16: Half Dome in Yosemite National Park, California, is a part of the Sierra Nevada batholith which is mostly made of granite. Jon Sullivan. 2004. Public domain. https://commons.wikimedia.org/wiki/File:Yosemite_20_bg_090404.jpg

Figure 4.17: The Henry Mountains in Utah are interpreted to be a laccolith, exposed by erosion of the overlying layers. Steven Mahoney. 2005. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Henry_Mountains,_Utah,_2005-06-01.jpg

Figure 4.18: Laccolith forms as a blister in between sedimentary strata. Erimus and Stannered. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Laccolith.svg

Figure 4.19: Bowen’s Reaction Series. Colivine. 2011. Public domain. https://commons.wikimedia.org/wiki/File:Bowen%27s_Reaction_Series.png

Figure 4.20: Olivine, the first mineral to crystallize in a melt. S kitahashi. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Peridot2.jpg

Figure 4.21: Norman L. Bowen. Unknown author. 1909. Public domain. https://commons.wikimedia.org/wiki/File:NormanLBowen_1909.jpg

Figure 4.22: Norman L. Bowen and his colleague working at the Carnegie Institution of Washington Geophysical Laboratory. Smithsonian Institution. 2010. Public domain. https://commons.wikimedia.org/wiki/File:(left_to_right)-_Norman_Levi_Bowen_(1887-1956)_and_Orville_Frank_Tuttle_(1916-1983)_(4730112454)_(cropped).jpg

Figure 4.23: Geothermal gradient. Bkilli1. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Temperature_schematic_of_inner_Earth.jpg

Figure 4.24: Pressure-temperature diagram showing temperature in degrees Celsius on the x-axis and depth below the surface in kilometers (km) on the y-axis. Kindred Grey. 2022. CC BY-SA 3.0. Adapted from Partial melting asthenosphere EN by Woudloper, 2010 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File%3APartial_melting_asthenosphere_EN.svg).

Figure 4.25: Association of volcanoes with plate boundaries. Jose F. Vigil via USGS. 1997. Public domain. https://commons.wikimedia.org/wiki/File:Tectonic_plate_boundaries.png

Figure 4.26: Map of spreading ridges throughout the world. Eric Gaba. 2006. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Spreading_ridges_volcanoes_map-fr.svg

Figure 4.27: Pillow basalt on sea floor near Hawai’i. NOAA. 1988. Public domain. https://commons.wikimedia.org/wiki/File:Nur05018-Pillow_lavas_off_Hawaii.jpg

Figure 4.28: Black smoker hydrothermal vent with a colony of giant (6’+) tube worms. NOAA. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Main_Endeavour_black_smoker.jpg

Figure 4.29: Distribution of hydrothermal vent fields. DeDuijn. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Distribution_of_hydrothermal_vent_fields.png

Figure 4.30: Distribution of volcanoes on the planet. USGS. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Map_plate_tectonics_world.gif

Figure 4.31: Basaltic cinder cones of the Black Rock Desert near Beaver, Utah. Lee Siebert via Smithsonian Institution. 1996. Public domain. https://commons.wikimedia.org/wiki/File:Black_Rock_Desert_volcanic_field.jpg

Figure 4.32: Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate. Los688. 2008. Public domain. https://en.wikipedia.org/wiki/File:Hotspot(geology)-1.svg

Figure 4.33: The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago. Kelvin Case. 2013. CC BY 3.0. https://commons.wikimedia.org/wiki/File:HotspotsSRP_update2013.JPG

Figure 4.34: The Hawaiian–Emperor seamount and island chain. Ingo Wölbern. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Hawaii-Emperor_engl.png

Figure 4.35: Mt. Shasta in Washington state with Shastina, its parasitic cone. Don Graham. 2013. CC BY-SA 2.0. https://commons.wikimedia.org/wiki/File:Mt._Shasta_and_Mt._Shastina,_CA_9-13_(26491330883).jpg

Figure 4.36: Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama. Zainubrazvi. 2006. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Crater_lake_oregon.jpg

Figure 4.37: Kilauea in Hawai’i. Quinn Dombrowski. 2007. CC BY-SA 2.0. https://commons.wikimedia.org/wiki/File:Kilauea_Shield_Volcano_Hawaii_20071209A.jpg

Figure 4.38: Eruption of Kiluea in 2018 produced high viscosity lava shown here crossing a road. USGS. 2018. Public domain. https://commons.wikimedia.org/wiki/File:USGS_K%C4%ABlauea_multimediaFile-1955.jpg

Figure 4.39: Olympus Mons, an enormous shield volcano on Mars, the largest volcano in the solar system, standing about two and a half times higher than Everest is above sea level. NASA. 1978. Public domain. https://en.wikipedia.org/wiki/File:Olympus_Mons_alt.jpg

Figure 4.40: Ropey pahoehoe lava. Bbb. 2010. GNU Free Documentation License 1.2. https://de.wikivoyage.org/wiki/Datei:ReU_PtFournaise_Lavastr%C3%B6me.jpg

Figure 4.41: Blocky a’a lava. Librex. 2009. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Lava_del_Volcan_Pacaya_2009-11-28.jpg

Figure 4.42: Volcanic fissure and flow, which could eventually form a lava tube. NPS. 2004. Public domain. https://commons.wikimedia.org/wiki/File:Volcano_q.jpg

Figure 4.43: Devils Tower in Wyoming has columnar jointing. Colin.faulkingham. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Devils_Tower_CROP.jpg

Figure 4.44: Columnar jointing on Giant’s Causeway in Ireland. Udri. 2014. CC BY-NC-SA 2.0. https://flic.kr/p/2j1mgwE

Figure 4.45: Mount Rainier towers over Tacoma, Washington. Lyn Topinka via USGS. 1984. Public domain. https://commons.wikimedia.org/wiki/File:Mount_Rainier_over_Tacoma.jpg

Figure 4.46: Mt. Fuji in Japan, a typical stratovolcano, symmetrical, increasing slope, visible crater at the top. Alpsdake. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Numazu_and_Mount_Fuji.jpg

Figure 4.47: Lava domes have started the rebuilding process at Mount St. Helens, Washington. Willie Scott via USGS. 2006. Public domain. https://commons.wikimedia.org/wiki/File:MSH06_aerial_crater_from_north_high_angle_09-12-06.jpg

Figure 4.48: Timeline of events at Mount Mazama. USGS and NPS. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Mount_Mazama_eruption_timeline.PNG

Figure 4.49: Wizard Island sits in the caldera at Crater Lake. Don Graham. 2006. CC BY-SA 2.0. https://commons.wikimedia.org/wiki/File:Crater_Lake_National_Park,_OR_2006_(6539577313).jpg

Figure 4.50: Map of calderas and related rocks around Yellowstone. USGS. 1905. Public domain. https://www.usgs.gov/media/images/simplified-map-yellowstone-caldera

Figure 4.51: Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall (yellow). USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Yellowstone_volcano_-_ash_beds.svg

Figure 4.52: Sunset Crater, Arizona is a cinder cone. NPS. Unknown date. Public domain. https://commons.wikimedia.org/wiki/File:Sunset_Crater10.jpg

Figure 4.53: Soon after the birth of Parícutin in 1943. K. Segerstrom via USGS. 1943. Public domain. https://commons.wikimedia.org/wiki/File:Paricutin_30_612.jpg

Figure 4.54: Lava from Parícutin covered the local church and destroyed the town of San Juan, Mexico. Sparksmex. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Paricutin2.jpg

Figure 4.55: World map of flood basalts. Williamborg. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Flood_Basalt_Map.jpg

Figure 4.56: Igneous rock types and related volcano types. Unknown author. Unknown date. “Personal use.” https://www.seekpng.com/ipng/u2t4y3r5o0r5r5w7_table-of-igneous-rocks-and-related-volcano-types/

Figure 4.57: General diagram of volcanic hazards. USGS. 2008. Public domain. https://pubs.usgs.gov/fs/fs002-97/old%20html%20files/

Figure 4.58: Human remains from the 79 CE eruption of Vesuvius. Gary Todd. 2019. Public domain. https://commons.wikimedia.org/wiki/File:Pompeii_Ruins_Cast_of_Human_Victim_at_Villa_of_the_Mysteries_(48445486616).jpg

Figure 4.59: Mount St. Helens, the day before the May 18th, 1980 eruption (left) and 4 months after the major eruption (right). Mount St. Helens, one day before the devastating eruption by Harry Glicken, USGS/CVO, 1980 (Public domain, https://commons.wikimedia.org/wiki/File:Mount_St._Helens,_one_day_before_the_devastating_eruption.jpg). MSH80 st helens from johnston ridge 09-10-80 by Harry Glicken via USGS (Public domain, https://commons.wikimedia.org/wiki/File:MSH80_st_helens_from_johnston_ridge_09-10-80.jpg).

Figure 4.60: Image from the May 18, 1980, eruption of Mt. Saint Helens, Washington. Austin Post via USGS. 1980. Public domain. https://commons.wikimedia.org/wiki/File:MSH80_eruption_mount_st_helens_05-18-80-dramatic-edit.jpg

Figure 4.61: The material coming down from the eruption column is a pyroclastic flow. C.G. Newhall via USGS. 1984. Public domain. https://en.wikipedia.org/wiki/File:Pyroclastic_flows_at_Mayon_Volcano.jpg#:~:text=English%3A%20Pyroclastic%20flows%20at%20Mayon,50%20km%20toward%20the%20west.

Figure 4.62: The remains of St. Pierre. Angelo Heilprin. 1902. Public domain. https://commons.wikimedia.org/wiki/File:Pelee_1902_3.jpg

Figure 4.63: Sequence of events for Mount St. Helens, May 18th, 1980. Lyn Topinka via USGS. 1998. Public domain. https://commons.wikimedia.org/wiki/File:Msh_may18_sequence.gif

Figure 4.64: Aman sweeps ash from an eruption of Kelud, Indonesia. Crisco 1492. 2014. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01.jpg

Figure 4.65: Micrograph of silica particle in volcanic ash. USGS. 1980. Public domain. https://volcanoes.usgs.gov/volcanic_ash/components_ash.html

Figure 4.66: Mud line shows the extent of lahars around Mount St. Helens. USGS. 1980. Public domain. https://www.usgs.gov/media/images/lahars-resulting-may-18-1980-eruption-mount-st-helens

Figure 4.67: Old lahars around Tacoma, Washington. USGS. 1905. Public domain. https://www.usgs.gov/media/images/lahar-pathways-events-heading-mount-rainier-map-showing-t

 

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