2 IGNEOUS ROCKS: THE

䊳 4.2 IGNEOUS ROCKS: THE

Figure 4–3 Sedimentary layers of sandstone and coal form

ORIGINS OF MAGMA

steep cliffs near Bryce, Utah. If you drilled a well deep into the crust, you would find

that Earth temperature rises about 30ºC for every kilo- cycle (Fig. 4–5). The transformations from one rock type

meter of depth. Below the crust, temperature continues to another can follow many different paths. For example,

to rise, but not as rapidly. In the asthenosphere (between weathering may reduce a metamorphic rock to sediment,

depths of about 100 to 350 kilometers), the temperature

Weathering

Igneous rock

Figure 4–5 The rock cycle shows that rocks change continuously over geologic time. The arrows show paths that rocks can follow as they change.

60 CHAPTER 4 I G N E O U S RO C K S

To crystallize magma: cool the liquid or increase pressure or remove water to raise the melting point

To melt rock: increase temperature or decrease pressure or add water to decrease the melting point

Figure 4–6 The lower box shows that increasing temperature, addition of water, and de- creasing pressure all melt rock to form magma. The upper box shows that cooling, increasing pressure, and water loss all solidify magma to form an igneous rock.

is so high that rocks melt in certain environments to form because spaces open up between the dancers as they magma.

move.

If a rock is heated to its melting point on the Earth’s surface, it melts readily because there is little pressure to PROCESSES THAT FORM MAGMA

keep it from expanding. The temperature in the as- thenosphere is more than hot enough to melt rock, but

Three different processes melt the asthenosphere: rising there, the high pressure prevents the rock from expand- temperature, decreasing pressure, and addition of water

ing, and it cannot melt (Fig. 4–7). However, if the pres- (Fig. 4–6). We will consider these processes and then

sure were to decrease, large volumes of the astheno- look at the tectonic environments in which they generate

sphere would melt. Melting caused by decreasing pressure magma.

is called pressure-release melting. In the section enti- tled “Environments of Magma Formation” we will see

Rising Temperature how certain tectonic processes decrease pressure on parts Everyone knows that a solid melts when it becomes hot

enough. Butter melts in a frying pan, and snow melts un- der the spring sun. For similar reasons, an increase in

High

Low

temperature will melt a hot rock. Oddly, however, in-

pressure

pressure

creasing temperature is the least important cause of magma formation in the asthenosphere.

Decreasing Pressure

A mineral is composed of an ordered array of atoms bonded together. When a mineral melts, the atoms be- come disordered and move freely, taking up more space than when they were in the solid mineral. Consequently, magma occupies about 10 percent more volume than the

Figure 4–7 When most minerals melt, the volume increases. rock that melted to form it. As an analogy, think of a

If a deeply buried mineral is near its melting point, high pres- crowd of people sitting in an auditorium listening to a

sure prevents expansion and it doesn’t melt. If the pressure concert. At first, they sit in closely packed, orderly rows.

decreases, the mineral can expand more easily and it melts, But if everyone gets up to dance, they need more room

even though the temperature remains constant.

Igneous Rocks: The Origins of Magma 61

of the asthenosphere and cause large-scale melting and pressure-release melting forms basaltic magma (the terms magma formation.

basaltic and granitic refer to magmas with the chemical compositions of basalt and granite, respectively). Because

Addition of Water the magma is of lower density than the surrounding rock,

A wet rock generally melts at a lower temperature than

it rises toward the surface.

an otherwise identical dry rock. Thus, addition of water Most of the world’s spreading centers are in the to rock that is near its melting temperature can melt the

ocean basins, where they form the mid-oceanic ridge. rock. Certain tectonic processes add water to the hot rock

The magma created by pressure-release melting forms of the asthenosphere to form magma.

new oceanic crust at the ridge. The oceanic crust then spreads outward, riding atop the separating tectonic plates. Nearly all of the Earth’s oceanic crust is created

ENVIRONMENTS OF MAGMA FORMATION in this way at the mid-oceanic ridge. Some spreading Magma forms in three tectonic environments: spreading

centers, like the East African rift, occur in continents, centers, mantle plumes, and subduction zones. Let us con-

and here, too, basaltic magma erupts onto the Earth’s sider each environment to see how the three aforemen-

surface.

tioned processes melt the asthenosphere to create magma. Magma Production at a Hot Spot Magma Production in a Spreading Center

Recall from Chapter 2 that a mantle plume is a rising As lithospheric plates separate at a spreading center, hot,

column of hot, plastic mantle rock that originates deep plastic asthenosphere oozes upward to fill the gap (Fig.

within the mantle. The plume rises because it is hotter 4–8). As the asthenosphere rises, pressure drops and

than the surrounding mantle and, consequently, is buoy-

Spreading center (mid-ocean ridge)

Oceanic Continental crust

Lithosphere 125 km

Asthenosphere

Figure 4–8 Pressure-release melting occurs where hot asthenosphere rises beneath a spreading center.

62 CHAPTER 4 I G N E O U S RO C K S

TERMS COMMONLY USED BY GEOLOGISTS

T Bedrock is the solid rock that lies beneath soil or

he terms basement rock, bedrock, parent rock,

and country rock are commonly used by geolo- unconsolidated sediments. It can be igneous, meta- gists.

morphic, or sedimentary.

Basement rock is the igneous and metamorphic Parent rock is any original rock before it is changed rock that lies beneath the thin layer of sediment and

by metamorphism or any other geologic process. sedimentary rocks covering much of the Earth’s sur-

The rock enclosing or cut by an igneous intrusion face, and thus it forms the basement of the crust.

or by a mineral deposit is called country rock.

ant. As a mantle plume rises, pressure-release melting Earth’s surface directly above a mantle plume. Because forms magma that erupts onto the Earth’s surface (Fig.

mantle plumes form below the asthenosphere, hot spots 4–9). A hot spot is a volcanically active place at the

can occur within a tectonic plate. For example, the Yellowstone hot spot, responsible for the volcanoes and hot springs in Yellowstone National Park, lies far from the nearest plate boundary. If a mantle plume rises be- neath the sea, volcanic eruptions build submarine volca- noes and volcanic islands.

Active volcano over hot spot

Magma Production in a Subduction Zone

At a subduction zone, a lithospheric plate sinks hundreds of kilometers into the mantle (Fig. 4–10). As you learned in Chapter 2, a subducting plate is covered by oceanic crust, which, in turn, is saturated with seawater. As the wet rock dives into the mantle, rising temperature drives off the water, which ascends into the hot asthenosphere directly above the sinking plate.

Direction of plate movement

Lithosphere

As the subducting plate descends, it drags plastic asthenosphere rock down with it, as shown by the ellip- tical arrows in Figure 4–10. Rock from deeper in the asthenosphere then flows upward to replace the sinking rock. Pressure decreases as this hot rock rises.

Finally, friction generates heat in a subduction zone

Partial melting forms magma

as one plate scrapes past the opposite plate. Figure 4–10 shows that addition of water, pressure release, and fric- tional heating combine to melt portions of the astheno-

sphere, at a depth of about 100 kilometers, where the

Mantle plume

spreads out

subducting plate passes into the asthenosphere. Addition

at base of

of water is probably the most important factor in magma

lithosphere

Rising mantle

production in a subduction zone, and frictional heating is

plume

probably the least important.

As a result of these processes, igneous rocks are common features of a subduction zone. The volcanoes of

Asthenosphere

the Pacific Northwest, the granite cliffs of Yosemite, and the Andes Mountains are all examples of igneous rocks formed at subduction zones. The “ring of fire” is a zone

Figure 4–9 Pressure-release melting occurs in a rising man- of concentrated volcanic activity that traces the subduc- tle plume, and magma rises to form a volcanic hot spot.

tion zones encircling the Pacific Ocean basin. About 75

Igneous Rocks: The Origins of Magma 63

Continental crust (lithosphere)

Sea level

Granitic Andesitic plutons

Oceanic crust

Mantle Asthenosphere

(asthenosphere)

Circulation of asthenosphere

Water rising from heated oceanic crust

Friction drags the plastic rock of the asthenosphere downward

Figure 4–10 Three factors contribute to melting of the asthenosphere and production of magma at a subduction zone: (1) Friction heats rocks in the subduction zone; (2) water rises from oceanic crust on top of the subducting plate; and (3) circulation in the asthenosphere decreases pressure on hot rock.

percent of the Earth’s active volcanoes (exclusive of the comparison, an iron bar turns red hot at about 600ºC and submarine volcanoes at the mid-oceanic ridge) lie in the

melts at slightly over 1500ºC.

ring of fire (Fig. 4–11).

Chemical Composition

CHARACTERISTICS OF MAGMA Because oxygen and silicon are the two most abundant elements in the crust and mantle, nearly all magmas are

We have just described how, why, and where magma silicate magmas. In addition to oxygen and silicon, they forms. Now we consider its properties and behavior. also contain lesser amounts of the six other common el-

Temperature ements of the Earth’s crust: aluminum, iron, magnesium, calcium, potassium, and sodium. The main variations The temperature of magma varies from about 600º to

among different types of magmas are differences in the 1400ºC, depending on its chemical composition and the

relative proportions of these eight elements. For exam- depth at which it forms. Generally, basaltic magma forms

ple, basaltic magma contains more iron and magnesium at great depth and has a temperature near the high end of

than granitic magma, but granitic magma is richer in sil- this scale. Granitic magmas, which form at shallower

icon, potassium, and sodium. A few rare magmas are of depths, tend to lie near the cooler end of the scale. As a

carbonate composition. The rocks that form from these

64 CHAPTER 4 I G N E O U S RO C K S

Bezymianny Katmai

Surtsey Hekla

Vesuvius

Fujiyama 10,000 Smokes")

Mt. St. Helens ("Valley of

Canary Is.

Paricutin Mauna Loa

Tonga Is.

Tambora

South Sandwich Is. Deception Is.

Transform boundary Figure 4–11 Seventy-five percent of the Earth’s active volcanoes (yellow dots) lie in the

Divergent boundary

Convergent boundary

“ring of fire,” a chain of subduction zones (heavy red lines with teeth) that encircles the Pacific Ocean. (Tom Van Sant, Geosphere Project)

are called carbonatites and contain carbonate minerals called plutonic rocks after Pluto, the Greek god of the such as calcite and dolomite.

underworld.

Behavior TEXTURES OF IGNEOUS ROCKS

When a silicate rock melts, the resulting magma expands The texture of a rock refers to the size, shape, and by about 10 percent. It is then of lower density than the arrangement of its mineral grains, or crystals (Table 4–1). rock around it, so magma rises as it forms—much as a Some igneous rocks consist of mineral grains that are too hot air balloon ascends in the atmosphere. When magma small to be seen with the naked eye; others are made up rises, it enters the cooler, lower-pressure environment of thumb-size or even larger crystals. Volcanic rocks are near the Earth’s surface. When temperature and pressure usually fine grained, whereas plutonic rocks are medium drop sufficiently, it solidifies to form solid igneous rock.

or coarse grained.