Read the text. IGNEOUS ROCKS: THE ORIGINS OF MAGMA

 

IGNEOUS ROCKS: THE ORIGINS OF MAGMA

If you drilled a well deep into the crust, you would find that Earth temperature rises about 30 °C for every kilometer of depth. Below the crust, temperature continues to rise, but not as rapidly. In the asthenosphere (between depths of about 100 to 350 kilometers), the temperature is so high that rocks melt in certain environments to form magma.

 

PROCESSES THAT FORM MAGMA

 

Three different processes melt the asthenosphere: rising temperature, decreasing pressure, and addition of water.

 

Rising Temperature

A solid melts when it becomes hot enough. An increase in temperature will melt a hot rock. Oddly, however, increasing 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 become disordered and move freely, taking up more space than when they were in the solid mineral. Consequently, magma occupies about 10 percent more volume that the rock that melted to form it.

If a rock is heated to its melting point on the Earth’s surface, it melts readily because there is little pressure to keep it from expanding. The temperature in the asthenosphere is more than hot enough to melt rock, but there, the high pressure prevents the rock from expanding, and it cannot melt. However, if the pressure were to decrease, large volumes of the asthenosphere would melt. Melting caused by decreasing pressure is called pressure-release melting.

 

Addition of Water

A wet rock generally melts at a lower temperature than an otherwise identical dry rock. This, addition of water to rock that is near its melting temperature can melt the rock. Certain tectonic processes add water to the hot rock of the asthenosphere to form magma.

 

ENVIRONMENTS OF MAGMA FORMATION

 

Magma forms in three tectonic environments: spreading centers, mantle plumes, and subduction zones.

 

Magma Production in a Spreading Center

As lithospheric plates separate at a spreading center, hot, plastic asthenosphere oozed upward to fill the gap. As the athenosphere rises, pressure drops and pressure-release melting forms basaltic magma (the terms basaltic and granitic refer to magmas with the chemical compositions of basalt and granite, respectively). Because the magma is of lower density than the surrounding rock, it rises toward the surface.

Most of the world’s spreading centers are in the ocean basins, where they form the mid-oceanic ridge. The magma created by pressure-release melting forms 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 in this way at the mid-oceanic ridge. Some spreading centers, like the East African rift, occur in continents, and here, too, basaltic magma erupts onto the Earth’s surface.

 

Magma Production at a Hot Spot

A mantle plume is a rising column of hot, plastic mantle rock that originates deep within the mantle. The plume rises because it is hotter than the surrounding mantle and, consequently, is buoyant. As a mantle plume rises, pressure-release melting forms magma that erupts onto the Earth’s surface. A hot spot is a volcanically active place at the Earth’s surface directly above a mantle plume. Because mantle plumes form below the asthenosphere, hot spots 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 beneath the sea, volcanic eruptions build submarine volcanoes and volcanic islands.

 

Magma Production in a Subduction Zone

At a subduction zone, a lithospheric plate sinks hundreds of kilometers into the mantle. 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.

As the subducting plate descends, it drags plastic asthenosphere rock down with it. 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 as one plate scrapes past the opposite plate. Addition of water, pressure release, and frictional heating combine to melt portions of the asthenosphere, at a depth of about 100 km, where the subducting plate passes into the asthenosphere. Addition of water is probably the most important factor in magma production in a subduction zone, and frictional heating is probably the least important.

As a result of these processes, igneous rocks are common features of a subduction zone. The volcanoes of 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 of concentrated volcanic activite that traces the subduction zones encircling the Pacific Ocean basin. About 75 percent of the Earth’s active volcanoes (exclusive of the submarine volcanoes at the mid-oceanic ridge) lie in the ring of fire.

 

CHARACTERISTICS OF MAGMA

 

Temperature

The temperature of magma varies from about 600°C to 1400° C, depending on its chemical composition and the depth at which it forms. Generally, basaltic magma forms at great depth and has a temperature near the high end of this scale. Granitic magmas, which form at shallower depths, tend to lie near the cooler end of the scale. As a comparison, an iron far turns red hot at about 600°C and melts at slightly over 1500° C.

 

Chemical Composition

Because oxygen and silicon are the two most abundant elements in the crust and mantle, nearly all magmas are silicate magmas. In addition to oxygen and silicon, they also contain lesser amounts of the six other common elements of the Earth’s crust: aluminum, iron, magnesium, calcium, potassium, and sodium. The main variations among different types of magmas are differences in the relative proportions of these eight elements. For example, basaltic magma contains more iron and magnesium than granitic magma, but granitic magma is richer in silicon, potassium, and sodium. A few rare magmas are of carbonate composition. The rocks that form from these are called carbonatites and contain carbonate minerals such as calcite and dolomite.

 

Behavior

When a silicate rock melts, the resulting magma expands by about 10 percent. It is then of lower density than the rock around it, so magma rises as it forms – much as a hot air balloon ascends in the atmosphere. When magma rises, it enters the cooler, lower-pressure environment near the Earth’s surface. When temperature and pressure drop sufficiently, it solidifies to form solid igneous rock.