巖漿成因外文翻譯-全文預(yù)覽
【正文】 formation. 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, silica poor rock produces magma having a basaltic position. 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 cool and crystallize before reaching the suface. However, as dry basaltic magma moves upward, the confining pressure steadily diminishes and further reduces the melting point. 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 mon on the earth’s surface. Conversely, granitic magmas are thought to be generated by partial melting of waterrich 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 occasion
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