Chapter 8 - Volcanoes and Plutons
vesicles
Holes in lava rock that formed when the lava solidified before bubbles of gas or water could escape.
pluton
A body of intrusive igneous rock. After a pluton forms, tectonic forces may push that part of the crust upward, and erosion may expose parts of the pluton at Earth's surface.
crater
A bowl-like depression at the summit of a volcano, created by volcanic activity.
lava plateau
A broad plateau covering thousands of square kilometers, formed by the accumulation of many individual lava flows that occur over a short period of geologic time. The Columbia River plateau in eastern Washington, northern Oregon, and western Idaho is a lava plateau containing 350,000 cubic kilometers of basalt. The lava is up to 3,000 meters thick and covers 200,000 square kilometers. The Columbia River basalts formed as a series of eruptions that began as early as 17.4 million years ago, peaked about 16 million years ago, and essentially ended about 12 million years ago. The individual flows are between 15 and 100 meters thick and some flowed for hundreds of kilometers. Much of the basalt erupted from a series of dikes where the states of Washington, Oregon, and Idaho all share a border.
volcano
A hill or mountain formed from lava and rock fragments ejected from a volcanic vent. The material erupted from volcanoes creates a wide variety of rocks and landforms. Many islands, including the Hawaiian Islands, Iceland, and most islands of the southwestern Pacific Ocean, were built entirely through volcanic eruptions. If a volcano erupts explosively, it may eject a combination of hot gas, liquid magma, and solid rock fragments.
Calderas
A large circular depression created by the collapse of the magma chamber after an explosive volcanic eruption. A large caldera may be 40 kilometers in diameter and have walls as much as a kilometer high. Some calderas fill up with volcanic debris; others maintain the circular depression and steep walls. Each caldera has a roughly circular outline and steep walls. We usually think of volcanic landforms as mountain peaks, but the topographic depression of a caldera is an exception.
batholith
A large pluton, exposed across more than 100 square kilometers of Earth's surface. An average batholith is about 10 kilometers thick, although a large one may be 20 kilometers thick. A batholith is commonly composed of numerous smaller plutons intruded sequentially over millions of years. For example, the Sierra Nevada batholith contains about 100 individual plutons, most of which were emplaced over a period of 50 million years. \ The formation of this complex batholith ended about 80 million years ago.
Shield Volcanoes
A large, gently sloping volcanic mountain formed by successive flows of basaltic magma. The sides of a shield volcano generally slope away from the vent at angles between 6 and 12 degrees from horizontal. Although their slopes are gentle, shield volcanoes can be enormous. Although shield volcanoes, such as those of Hawaii and Iceland, erupt regularly, the eruptions are normally gentle and rarely life threatening. Lava flows occasionally overrun homes and villages, but the flows advance slowly enough to give people time to evacuate.
stock
A pluton exposed over less than 100 square kilometers of Earth's surface; similar to a batholith, but smaller.
Effects of Water on Magma Behavior
A second difference between the two magmas is that granitic magma contains more water than basaltic magma does. Water lowers the temperature at which magma solidifies. If dry granitic magma solidifies at 700 degrees Celsius, the same magma with 10 percent water may not become solid until the temperature drops below 600 degrees Celsius. Water tends to escape as steam from hot magma. But deep in the crust where granitic magma forms, high pressure prevents the water from escaping. As the magma rises, pressure decreases and water escapes. Because the magma loses water, its solidification temperature rises, causing it to crystallize. Water loss causes rising granitic magma to solidify within the crust. Because basaltic magmas have only 1 to 2 percent water to begin with, water loss is relatively unimportant. As a result, rising basaltic magma usually remains liquid all the way to Earth's surface, and basalt volcanoes are common.
dike
A sheetlike igneous rock, cutting through layers of country rock, that forms when magma is injected into a fracture. Dikes cut across sedimentary layers or other features in country rock and range from less than a centimeter to more than a kilometer thick. A dike is commonly more resistant to weathering than surrounding rock. As the country rock erodes, the dike is left standing on the surface.
sill
A sheetlike igneous rock, parallel to the grain or layering of country rock, that forms when magma is injected between layers. Like dikes, sills vary in thickness from less than a centimeter to more than a kilometer and may extend for tens of kilometers in length and width.
Cinder Cones
A small volcano, typically less than 300 meters high, made up of loose pyroclastic fragments blasted out of a central vent; usually active for only a short time. A cinder cone forms when large amounts of gas accumulate in rising magma. When the gas pressure builds sufficiently, the entire mass erupts explosively, hurling cinders, ash, and molten magma into the air. The particles then fall back around the vent, to accumulate as a small mountain of pyroclastic debris. A cinder cone is usually active for only a short time, because once the gas escapes, the driving force behind the eruption is gone. Cinder cones usually are symmetrical and can be steep (about ), especially near the vent, where ash and cinders pile up (Figure 8.25). Most are less than 300 meters high, although a large one can be up to 700 meters high. A cinder cone erodes easily and quickly because the pyroclastic fragments are not cemented together.
stratovolcano
A steep-sided volcano formed by an alternating series of lava flows and pyroclastic deposits and marked by repeated eruption.
Composite Cones
A steep-sided volcano formed by an alternating series of lava flows and pyroclastic eruptions and marked by repeated eruption. Many of the highest mountains of the Andes and some of the most spectacular mountains of western North America are composite cones. Repeated eruptions are a trademark of a composite volcano.
air-fall tuff
A tuff formed during an eruption by fallout of ash from the atmosphere. In contrast, the hot pyroclastic flows that developed as the eruption proceeded left behind an ash-flow tuff. Ash-flow tuffs accumulate so rapidly and contain enough heat that they partially melt after being deposited. As the eruption continues and more flows are added to the top of the deposit, their weight causes tuff near the base to compact and fuse together, forming a welded tuff.
Volcanic Eruptions and Global Climate
A volcanic eruption can profoundly affect the atmosphere, the climate, and living organisms, thereby providing an excellent example of systems interactions. For instance, the 1991 eruptions of Mount Pinatubo in the Philippines produced the greatest ash and sulfur clouds in the latter half of the 20th century. Satellite measurements show that the total solar radiation reaching Earth's surface declined by 2 to 4 percent after the Pinatubo eruptions. The following two years, 1992 and 1993, were a few tenths of a degree Celsius cooler than the temperatures of the previous decade. Temperatures rose again in 1994, after the ash and sulfur settled out. Shows a compilation of five geologically recent eruptions and the temperature either observed or calculated from isotopic information preserved in ice cores. Another example of a volcanic eruption that affected climate occurred in 1783 in Iceland, when the largely nonexplosive eruption of the Laki crater occurred during June of that year. The eruption lasted nearly 9 months and produced a bluish haze of sulfur aerosols across Iceland that subsequently spread across Europe. This haze obscured the Sun, significantly reducing the solar energy reaching the surface. The 1783 eruption altered weather patterns in Iceland and Europe. In Iceland, violent thunderstorms and hailstorms killed cattle and destroyed crops. The crop failure resulting from the reduced solar energy and extreme weather events are estimated to have killed about 24 percent of the human population there. In Europe, the summer of 1783 was more like a winter, with the Sun remaining a pale ghost in the sky or a strange blood-red color. The cold summer temperatures were followed by an extremely harsh winter in 1783 to 1784, and for several years afterward the destruction of crops and livestock brought about famine and poverty that probably contributed directly to the French Revolution, which started in 1789. A plot of global temperatures before and after eight recent major volcanic eruptions shows a correlation between global cooling and volcanic eruptions.
ash-flow tuff
A volcanic rock formed when a pyroclastic flow solidifies. If the deposit is thick enough, the ash at the bottom of the pile begins to compact and may partially melt from the residual heat, causing the ash to fuse together and form a welded tuff. Typically, only the lower portion of a pyroclastic flow becomes welded; the upper portion usually remains a relatively porous accumulation of ash particles. Welded tuffs form several prized sport climbing areas in the western United States because of their strength.
Volcanic Explosions: Ash-Flow Tuffs and Calderas
Although granitic magma usually solidifies within the crust, under certain conditions it rises to Earth's surface, where it erupts violently. Granitic magmas that rise to the surface contain only a few percent water, like basaltic magma. But decreasing pressure allows the small amount of dissolved water in granitic magmas to form a frothy, pressurized mixture of gas and liquid magma that may be as hot as . As the mixture rises to within a few kilometers of Earth's surface, it fractures overlying rocks and explodes skyward through the fractures. A large and violent eruption can blast a column of pyroclastic material 10 or 12 kilometers into the sky, and the column might be several kilometers in diameter. A cloud of fine ash may rise even higher—into the upper atmosphere. The force of material streaming out of the magma chamber can hold the column up for hours or even days.
welded tuff
An ash-flow tuff that compacts from the weight of overlying tuff deposits and fuses together because of the residual heat from the pyroclastic flow.
pyroclastic flow
An extremely destructive incandescent mixture of volcanic ash, larger pyroclastic particles, minor lava, and hot gas that forms from collapse of an eruptive column and flows rapidly along Earth's surface. When a pyroclastic flow stops, most of the gas escapes into the atmosphere, leaving behind a chaotic mixture of volcanic ash and rock fragments called ash-flow tuff.
vent
An opening in a volcano, typically in the crater, through which lava and rock fragments erupt.
Magma Production in a Spreading Center
As lithospheric plates separate at a spreading center, hot, plastic asthenosphere wells upward to fill the gap. As the asthenosphere rises, the surrounding pressure drops and pressure-release melting forms magma with a basaltic composition. Because the magma is of lower density than the surrounding rock, it rises buoyantly toward the surface. Most of the world's spreading centers lie in the ocean basins, where they form the Mid-Oceanic Ridge system. The rising basaltic magma is injected into the spreading center where it solidifies to form new oceanic crust. Some of the magma erupts onto the seafloor. Once formed, the new oceanic crust then drifts away from the spreading center on both sides, riding atop the separating tectonic plates. Nearly all of Earth's oceanic crust is created in this way at the Mid-Oceanic Ridge system. In most places, the ridge lies beneath the sea. In a few places, such as Iceland, the ridge rises above sea level and basaltic magma erupts onto Earth's surface. Some spreading centers, such as the East African Rift or the North American Basin and Range Province, occur in continents, and here too basaltic magma erupts onto the surface in addition to magma with other compositions.
Basalt
Basalt and granite are the most common igneous rocks. Basalt makes up most of the oceanic crust. Recall that basaltic magma forms by the melting of the asthenosphere. But the asthenosphere is peridotite, an ultramafic rock. Basalt and peridotite are quite different in composition: Peridotite contains about 40 percent silica , but basalt contains about 50 percent.
flood basalt
Basaltic lava that erupts gently and in great volume from vents or fissures at Earth's surface, to cover large areas of land and form lava plateaus.
fissures
Breaks, cracks, or fractures in rocks. Fissures and fissure eruptions vary greatly in scale. In some cases, lava pours from small cracks on the flank of a volcano. Fissure flows of this type are common on Hawaiian and Icelandic volcanoes. In other cases, however, fissures extend for tens or hundreds of kilometers and pour thousands of cubic kilometers of basaltic lava onto Earth's surface.
Cinders
Glassy, pyroclastic volcanic fragments 4 to 32 millimeters in size.
Granite
Granite is the most abundant rock in continental crust. Granite contains more silica than basalt and therefore melts at a lower temperature—typically between 700 degrees Celsius and 900 degrees Celsius. Thus, basaltic magma is hot enough to melt continental crust made of granite. Basaltic magma that forms beneath a continent and then rises into the continental crust will cause the crust to partially melt. Because the lower continental crust is hot, a small volume of basaltic magma can melt a large volume of lower continental crust to form granitic magma. Typically, the granitic magma rises a short distance and then solidifies within the crust to form granitic plutons. Most granitic plutons solidify at depths between about 5 and 20 kilometers. In continental rift zones where tectonic stretching has thinned the crust, some granitic magma reaches the surface where it erupts as rhyolite. Granite forms by this process in a subduction zone at a continental margin, a continental rift zone, and a mantle plume rising beneath a continent.
super-heating
Heating of a substance above a phase-transition (gas to liquid or liquid to solid) without the transition occurring. For example, high pressure can keep solid rock from melting even though it is above its melting temperature.
Partial Melting and the Origin of Granitic Continents
If the earliest crust had a composition similar to that of the mantle, we must describe how oceanic and continental crust evolved from mantle rocks. In section on "Basalt and Granite," we explained that partial melting produces magma that contains a higher proportion of silica than the rock from which the melt formed. Geologists infer that the earliest crust was peridotitic lava, formed from the melted mantle as Earth cooled from the surface downward after its early pervasive melting event. Later, partial melting of this primordial crust formed a basaltic crust that was richer in silica. Then, partial melting of the basalt probably formed intermediate rocks such as andesite, which underwent another partial melting to form the silica-rich granitic continents. The process of partial melting may explain how the silica-rich continents evolved in steps from the silica-poor mantle. As stated earlier, in the modern Earth, magma forms in three geologic environments: spreading centers, subduction zones, and mantle plumes. Similar magma-forming environments may have existed in early Precambrian time, but geologists are uncertain which were most important. Some observations imply that Archean tectonics was similar to modern horizontal plate movements and that most magma formed at spreading centers and subduction zones. Other evidence indicates that horizontal plate motion was minor and that vertical mantle plumes dominated early Precambrian tectonics.
Andesite and Intermediate Magma
Igneous rocks of intermediate composition, such as andesite and diorite, form by processes similar to those that generate granitic magma. Their magmas contain less silica than granite, either because they form by the partial melting of continental lithosphere or asthenosphere with low silica content or because basaltic magma has mixed with granitic magma.
Magma Production in a Subduction Zone
In a subduction zone, the addition of water, decreasing pressure, and heat from friction all combine to form huge quantities of magma. A subducting plate is covered by water-saturated oceanic sediments, and the upper portions of the underlying basalt also are saturated with water. As the wet rock and sediments dive into the hot mantle, the heated water ascends into the hot asthenosphere directly above the sinking plate. As the subducting plate descends, it drags plastic asthenosphere rock down with it, as shown by the elliptical arrows. Rock from deeper in the asthenosphere then flows upward to replace the sinking rock. Pressure decreases as this hot rock rises. Friction generates heat in a subduction zone as the downgoing plate scrapes past the overriding plate. the addition of water, pressure release, and frictional heating combine to melt asthenosphere rocks in the zone where the subducting plate passes into the asthenosphere. The addition of water is probably the most important factor producing melting in a subduction zone, and frictional heating is the least important. The subduction process leads directly to the formation of large plutons and volcanoes. The volcanoes of the Pacific Northwest, the granite cliffs of Yosemite, and the Andes Mountains are all examples of volcanic and plutonic rocks formed through subduction. The Ring of Fire is a chain of active volcanoes that runs parallel to the subduction zones encircling the Pacific Ocean basin. About 75 percent of Earth's active volcanoes (exclusive of the submarine volcanoes at the Mid-Oceanic Ridge) lie in the Ring of Fire.
Short-Term Prediction
In contrast to regional predictions, short-term predictions attempt to forecast the specific time and place of an impending eruption. They are based on instruments that monitor an active volcano to detect signals that the volcano is about to erupt. The signals include changes in the shape of the mountain and surrounding land, earthquake swarms indicating movement of magma beneath the mountain, increased emissions of ash or gas, increasing temperatures or changing compositions of nearby hot springs, and any other signs that magma is approaching the surface. In 1978, two U.S. Geological Survey (USGS) geologists, Dwight Crandall and Don Mullineaux, noted that Mount St. Helens had erupted more frequently and violently during the past 4,500 years than any other volcano in the contiguous 48 states. They predicted that the volcano would erupt again before the end of the 20th century. In March 1980, about two months before the great May eruption, puffs of steam and volcanic ash rose from the crater of Mount St. Helens, and swarms of earthquakes occurred beneath the mountain. This activity convinced other USGS geologists that Crandall and Mullineaux's prediction was correct. In response, they installed networks of seismographs, tiltmeters, and surveying instruments on and around the mountain.
Effects of Silica on Magma Behavior
In the silicate minerals, silicate tetrahedra link together to form the chains, sheets, and framework structures. Silicate tetrahedra link together in a similar manner in magma. They form long chains and similar structures if silica is abundant in the magma, but shorter chains if less silica is present. Because of its higher silica content, granitic magma contains longer chains than does basaltic magma. The long chains become tangled, causing the magma to become stiff, or viscous. It rises slowly because of its viscosity and has ample time to solidify within the crust before reaching the surface. In contrast, basaltic magma, with its shorter silicate chains, is less viscous and flows more easily. Because of its fluidity, it rises rapidly to erupt at Earth's surface.
Partial Melting and the Origin of Continents
It is hypothesized that Earth melted shortly after its formation about 4.6 billion years ago. Magma at the surface then cooled to form the earliest crust. From the evidence of a few traces of very old crust combined with computer models of Earth's early formation, geologists surmise that the first crust was lava with the composition of peridotite. Our explanation of the formation of granitic magma by the melting of granitic continental crust leaves us with two interesting questions: (1) When did granitic continents form? (2) If the early crust and mantle were composed of peridotite, how did granitic continental crust evolve at Earth's surface?
Lava and Pyroclastic Rocks
Lava is magma that flows onto Earth's surface; the word also describes the rock that forms when the magma solidifies. Lava with low viscosity may continue to flow as it cools and stiffens, forming smooth, glassy-surfaced, wrinkled, or "ropy" ridges As hot lava cools and solidifies, it shrinks. The shrinkage pulls the rock apart, forming cracks that grow as the rock continues to cool. As the lava continued to cool and solidify, the cracks grew downward through the flow.
aa
Lava that has a jagged, rubbly, broken surface.
pahoehoe
Lava with a smooth, billowy, or ropy surface.
Environments of Magma Formation
Magma forms abundantly in three tectonic environments: spreading centers, mantle plumes, and subduction zones. Let us consider each environment to see how rising temperature, decreasing pressure, and the addition of water can melt rock to create magma.
pressure-release melting
Melting caused by a decrease in pressure, expansion of rock volume, and melting. Usually occurs in the asthenosphere.
Magma Behavior
Once magma forms, it rises toward Earth's surface because it is less dense than surrounding rock. As it rises, two changes occur: (1) magma cools as it enters shallower and cooler levels of Earth, and (2) pressure drops because the weight of overlying rock decreases. Cooling tends to solidify magma but decreasing pressure tends to keep it liquid. Basaltic magma commonly remains liquid and rises to the surface to erupt from a volcano or flow onto the seafloor at the Mid-Oceanic Ridge. In contrast, granitic magma usually solidifies within the crust. The contrasting behavior of granitic and basaltic magmas is a consequence of their different compositions. Granitic magma contains about 70 percent silica, whereas the silica content of basaltic magma is only about 50 percent. In addition, granitic magma generally contains up to 10 percent water, but basaltic magma contains only 1 to 2 percent water.
Processes That Form Magma
Recall that the asthenosphere is the layer in the upper mantle that extends from a depth of about 100 kilometers to 350 kilometers. In that layer, the combined effects of temperature and pressure are such that 1 or 2 percent of the mantle rock is molten, as explained in Chapter 6. Although the majority of the asthenosphere is solid rock, it is so hot and so close to its melting point that large volumes of rock can melt with relatively small changes in temperature, pressure, or the volume of water present
columnar joints
Regularly spaced cracks that commonly develop in lava flows, grow downward starting from the surface, and typically form five- or six-sided columns.
Regional Prediction
Risk assessment for regional predictions is based both on the frequency of past eruptions and on potential violence. However, regional predictions based on the concentration of volcanoes in an area can only estimate probabilities and cannot be used to determine exactly when a particular volcano will erupt or the intensity of a particular eruption.
pyroclastic rock
Rock made up of liquid magma and solid rock fragments that were ejected explosively from a volcanic vent.
Vertical Mantle Plume Tectonics
Several researchers have proposed, instead, that mantle plumes dominated early Precambrian tectonics. In this view, upwellings of mantle rock led to partial melting within the upper mantle. This magma then solidified to form basaltic crust. Continued partial melting eventually formed granitic continental crust. As more information about the nature of early Earth accumulates through scientific study, some geologists have suggested that both vertical and horizontal mechanisms were important. According to one hypothesis, mantle plumes formed thick basaltic oceanic plateaus, which then oozed outward to initiate horizontal motion. This motion caused subduction and another melting episode that generated continental crust by partial melting of the basalt plateaus.
tectonic accretion
Tectonic accretion is the process of material such as island arcs and sediment being added to tectonic plates at subduction zones.
When Did Continents Form?
The 3.96-billion-year-old Acasta Gneiss in Canada's Northwest Territories is among Earth's oldest known rock. It is metamorphosed granitic rock, similar to modern continental crust, and this implies that at least some granitic crust had formed by early Precambrian time. The presence of these zircon grains suggests that granitic rocks existed 4.4 billion years ago. Geologists have also found granitic rocks in Greenland and Labrador that are nearly as old as the Acasta Gneiss and the Australian zircon grains. These dates tell us that some granitic continental crust probably existed by 4.4 billion years ago.
partial melting
The process in which a silicate rock only partly melts as it is heated, forming magma that is more silica rich than the original rock.
volcanic ash
The smallest pyroclastic particles, less than 2 millimeters in diameter.
Volcano Types
Volcanoes differ widely in shape, structure, and size. Lava and rock fragments commonly erupt from an opening called a vent, located in a crater, a bowl-like depression at the summit of the volcano that was itself created by volcanic activity.
Magma Production in a Mantle Plume
that a mantle plume is a rising column of hot, plastic rock that originates within the mantle. The plume rises because it is hotter than the surrounding mantle and, consequently, is less dense and more buoyant. As a plume rises, pressure-release melting forms magma, which continues to rise toward Earth's surface. Because mantle plumes form below the lithosphere, they commonly occur within tectonic plates rather than at a boundary. For example, the Yellowstone Volcano—responsible for the volcanic activity, geysers, and hot springs in Yellowstone National Park—results from a shallow mantle plume that lies far from the nearest plate boundary. If a mantle plume rises beneath oceanic crust, volcanic eruptions build submarine volcanoes and volcanic islands. For example, the Hawaiian Islands are a chain of hot-spot volcanoes that formed over a long-lived mantle plume beneath the Pacific Ocean.
