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Based on available scientific evidence, the earth’s crust and mantle are composed primarily of solid rock. Further, although the outer core is a fluid, this iron- rich material is very dense and remains deep within the earth. If this is true, what is the source of magma that produces the earth’s volcanic activity?

Since the molten outer core is not a source of magma, geologists conclude that magma must originate from essentially solid rock located in the crust and mantle. The most obvious way to generate magma from solid rock is to raise its temperature. In a near surface environment, silica-rich rocks of granitic composition begin to melt at temperatures around 750^oC, whereas basaltic rocks must reach temperatures above 1oo5r0^oC, before melting commences.

What is the source of heat that melts rock? One source is the heat liberated during the decay of radioactive elements that are found in the mantle and crust.

Workers in underground mines have long recognized that temperatures get higher as they descend to greater depths. Although the rate of temperature increase varies from place to place, it averages between 20^oC and 30^oC per kilometer in the upper curst. This gradual increase in temperature with depth is known as the geothermal gradient.

The geothermal gradient is thought to contribute to magma production in tow important ways. First, at deep-ocean trenches, slabs of cool oceanic lithosphere descend into the hot mantle. Here hear supplied by the surrounding rocs is thought to be sufficient to melt the subducting oceanic curst and produce basaltic magma. Second, a hot, mantle derived magma body as just described could migrate to the base of the crust and intrude silica-rich. Because granitic rocks have melting temperatures well below those required to melt basalt, heat derived from the otter basaltic magma could melt the already warm crustal rocks. The volcanic activity that produced that vast ash flows in Yellowstone National Park is believed to have resulted from such activity. Here basaltic magma from the mantle transported heat to the crust, where the melting of silica-rich rocks generated explosive lavas.

If temperature were the only factor that determined whether or not rock melts, the earth would be a molten ball covered with a thin, solid outer shell.

This, of course, is not the case. The reason is that pressure also increases with depth. Since rock must expand when heated extra heat is required to melt buried rocks in order to overcome the effect of confining pressure. In general, an increase in the confining pressure causes an increase in the rock’s melting temperature, and a reduction in confining pressure causes the rock’s melting temperature to decline.

Consequently, a drop in confining pressure can lower the melting temperature, and a reduction in confining pressure causes the rock’s melting temperature to decline.

rock sufficiently to trigger melting. This occurs whenever rock ascends, thereby moving into zones of lower pressures.

Another important factor affecting the melting temperature of rock is its water content, in general the more water present, the lower the melting temperature. The effect of water on lowering the melting point is magnified by increased pressure. Consequently , “wet” rock under pressure has a much lower melting temperature that “dry” rock of the same composition. For example at a depth of 10 kilometers, wet granite has a melting temperature of about 657^oC,whereas dry granite begins to melt at temperatures approaching 1000^oC.

Therefore, in addition to rock’s composition, its temperature, confining pressure, and water content determine whether the rock exists as a solid or liquid.

One important difference exists between the malting of a substance that consists of a single compound, such as ice, and the melting of igneous rocks.

Which are mixtures of several different minerals. Whereas ice melts at a definite temperature most igneous rocks melt over a temperature range of a few hundred degrees. As a rock is heated, the first liquid to form will contain a higher percentage of the low-melting temperature minerals than the original rock. Should melting continue, the composition of the melt will steadily approach the overall composition of the rock from which it is derived. Most often, however, melting is not complete. This process, known as partial melting produces most, if not all magma.

An important consequence of partial melting is that it generates a melt with a chemical composition that is different from the original rock. In particular, partial melting generates a melt that: (1) is enriched in the elements found in the low-melting-temperature silicate minerals; and (2) is higher in silica than the original material. Recall that ultramafic rocks contain mostly high-melting –temperature minerals that are comparatively low in silica, whereas granitic rocs are composed primarily of low-melting-point silicates that are enriched in silica. Consequently,

magmas generated by partial melting are nearer the granitic end of the compositional spectrum then the material from which they formed.

Most basaltic magmas are believed to originate from the partial melting of the rock peridotite, the major constituent of the upper mantle. Laboratory studies confirm that partial melting of this dry; silicapoor rock produces magma having a basaltic composition. Since mantle rocks exist in environments that are characterized by high temperatures and pressures, melting often results from a reduction in confining pressure. This can occur, for example, where mantle rock ascends as part of a slowmoving convection cell.

Due to the fact that basaltic magmas form many kilometers below the surface, we might expect that most of this material would cooled and crystallize before reaching the surface. However, as dry, basaltic magma flows upward, the confining pressure steadily diminishes and further reduces the melting temperature.

Basaltic Magmas appear to ascend rapidly enough so that as they enter cooler environments the heat loss is offset by a drop in the melting point. Consequently, large outpourings of basaltic magmas are common on the earth’s surface.

Conversely, granitic magmas are thought to be generated by partial melting of water-rich rocks that were subjected to increased temperature. As a wet granitic melt rises, the confining pressure decreases, which in turn reduces the effect of water on lowering the melting temperature. Further, granitic melts are high in silica and thus more viscous than basaltic melts. Thus in contrast to basaltic magmas that produce vast outpourings of lava, most granitic magmas lose their mobility before reaching the surface and therefore tend to produce large intrusive features such as batholiths. On those occasions when silicarich magmas reach the surface, explosive pyroclastic flows, such as those that produced the Yellowstone Plateau, are the rule.

Even though most magma is thought to be generated by partial melting, once formed the composition of a magma body can change dramatically with time. For

surrounding country rock. As the country rock is assimilated, the composition of the magma is altered. Further, magma often undergoes magmatic differentiation.

This produces a magma quite unlike the parent material. These processes account, at least in part, for the fact that a single volcano can extrude lavas with a wide range of chemical compositions.