岩浆成因外文翻译(编辑修改稿)

岩浆成因外文翻译(编辑修改稿)内容摘要：

uch higher silica content. Consequently,magmas generated by partial melting are nearer the granitic end of the positional spectrum than the parent material from which they formed. As we shall see, this idea will help us to understand the global distribution of the various types of volcanic activity. What is the heat source to melt rock? One source is the heat liberated during the decay of radioactive elements that are thought to be concentrated in the upper mantle and crust. Workers in underground mines have long recognized that temperatures increase with depth. Although the rate of increase varies from place to place, it is thought to average between 20℃ and 30℃ per kilometer in the upper crust. This gradual increase in temperature with depth is known as the geothermal gradient. If temperature were the only factor to determine whether or not a rock melts, the earth would be a molten ball covered with only a thin, solid outer shell. However, pressure also increases with depth. Since rock expands when heated, extra heat is needed to melt buried rocks, in order to overe the effect of confining pressure. In general, an increase in the confining pressure causes an increase in the rock’s melting point. Another important factor affecting the melting point of rock is its water to a point, the more water present, the lower the melting point. The effect of water on lowering the melting point is magnified by increased pressure. Consequently, “wet”rock 4 under pressure has a much lower melting temperature than “dry” rock of the same position. Therefore, in addition to a rock’s position, its temperature, confining pressure, and water content determine。