The Behavior of Magma

Why do some volcanoes explode violently while others erupt gently? Why do some kinds of magma (molten rock) solidify within Earth to form hard rocks, and others rise all the way to the surface to erupt from volcanoes as lava? To answer these questions, we must consider the properties and behavior of magma. Once magma forms, it rises toward Earth’s surface because it is less dense than surrounding rock. As it rises, two changes 0ccur. First, it cools as enters shallower and cooler levels of Earth. Second, pressure drops because the weight of overlying rock

decreases. Cooling and decreasing pressure have opposite effects on magma: Cooling tends to solidify it, but decreasing pressure tends to keep it liquid.

Whether magma solidifies or remains liquid as it rises toward Earth’s surface depends on the type of magma.Basaltic magma commonly rises to the surface to erupt from a volcano. In contrast, granitic magma usually solidifies within Earth’s crust (outer layer). 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, whereas basaltic magma contains only 1 to 2 percent water.

As in silicate minerals, silicate tetrahedral (four-faced solids, similar to triangular pyramids) in magma link together to form chains, sheets, and framework structures. They form long chains 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. In granitic magma, the long chains become tangled, making the magma 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 easily. Because of its fluidity. It rises rapidly to erupt at Earth’s surface.

A second difference is that granitic magma contains more water than basaltic magma. Water lowers the temperature at which magma solidifies. Thus, if dry granitic magma solidifies at 700C, the same .Magma with 10 percent water may remain liquid until its temperature drops below 600C. Water tends to escape as steam from hot magma, but deep in Earth’s crust where granitic magma forms, high pressure prevents the water from escaping. As magma rises, pressure decreases and Water escapes. Because the magma loses water, its solidification temperature rises causing it to become hard rock. Thus, water loss causes rising granitic magma to Solidify within the crust. For this reason, most granitic magmas solidify at depths of 5 to 20 kilometers beneath Earth’s surface. Because basaltic magmas have only 1 to 2 percent water to begin with, water loss is relatively unimportant. As a result of its comparatively low viscosity, rapidly rising basaltic magma remains liqudl1 the way to Earth’s surface, and basal volcanoes are common.

In most cases, granitic magma solidifies within Earth’s crust to form a rocky mass called a pluton. Many granite plutons are large, measuring tens of kilometers in diameter. To form a large pluton, a huge volume of granitic magma must rise through continental crust. But how can such a large mass of magma rise through solid rock? If you place oil and water in a jar, screw the lid on, and shake the jar, oil droplets disperse throughout the water. When you set the jar down, the droplets coalesce to form larger bubbles, which rise toward the surface, easily displacing the water as they ascend. Granitic magma rises in similar way. It forms near the base of continental crust, where surrounding rock is soft and flexible and behaves plastically , it flows and deforms without fracturing, because it is hot.As the magma rises, it shoulders aside the hot, plastic rock, which then slowly flows back to fill in behind the rising bubble. After a pluton forms, forces in the crust may push it upward, and erosion may expose parts of it at Earth’s surface.

1.Why do some volcanoes explode violently while others erupt gently? Why do some kinds of magma (molten rock) solidify within Earth to form hard rocks, and others rise all the way to the surface to erupt from volcanoes as lava? To answer these questions, we must consider the properties and behavior of magma. Once magma forms, it rises toward Earth’s surface because it is less dense than surrounding rock. As it rises, two changes 0ccur. First, it cools as enters shallower and cooler levels of Earth. Second, pressure drops because the weight of overlying rock decreases. Cooling and decreasing pressure have opposite effects on magma: Cooling tends to solidify it, but decreasing pressure tends to keep it liquid.