巖漿成因外文翻譯-wenkub
2023-05-19 03:40:39 本頁面
【正文】 nd 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 lowmeltingpoint minerals than does the original rock. Should melting continue, the position of the melt will steadily approach the overall position of the rock from which it is derived. Most often, however, melting is not plete. This process, known as partial melting, produces most, if not all, magma. A significant result of partial melting is the production of a melt with a higher silica content than the parent rock. Recall that basaltic rocks have a relatively low silica content and that granitic rocks have a much 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 m
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