Magma – AEAS精读 (Y10-12)

Any rock that has cooled and solidified from a molten state is an igneous rock. Therefore, if the Earth began as a superheated sphere in space, all the rocks making up its crust may well have been igneous and thus the ancestors of all other rocks. Even today, approximately 95 percent of the entire crust is igneous. Periodically, molten material wells out of the Earth’s interior to invade the surface layers or to flow onto the surface itself. This material cools into a wide variety of igneous rocks. In the molten state, it is called magma as it pushes into the crust and lava when it runs out onto the surface.

All magma consists basically of a variety of silicate minerals (high in silicon-oxygen compounds), but the chemical composition of any given flow may differ radically from that of any other. The resulting igneous rocks will reflect these differences. Igneous rocks also vary in texture as well as chemistry. Granite, for instance, is a coarse-grained igneous rock whose individual mineral crystals have formed to a size easily seen by the naked eye. A slow rate of cooling has allowed the crystals to reach this size. Normally, slow cooling occurs when the crust is invaded by magma that remains buried well below the surface. Granite may be found on the surface of the contemporary landscape, but from its coarse texture we know that it must have formed through slow cooling at a great depth and later been laid bare by erosion. Igneous rocks with this coarse-grained texture that formed at depth are called plutonic.

On the other hand, if the same magma flows onto the surface and is quickly cooled by the atmosphere, the resulting rock will be fine-grained and appear quite different from granite, although the chemical composition will be identical. This kind of rock is called rhyolite. The most finely grained igneous rock is volcanic glass or obsidian, which has no crystals. Some researchers believe this is because of rapid cooling; others believe it is because of a lack of water vapor and other gases in the lava. The black obsidian cliffs of Yellowstone National Park are the result of a lava flow of basalt running head on into a glacier. Some of the glacier melted on contact, but suddenly there also appeared a huge black mass of glassy stone.

 

任何从熔融状态冷却和凝固的岩石都是火成岩。因此,如果地球开始是太空中的过热球体,构成地壳的所有岩石都可能是火成岩的,因此也是所有其他岩石的祖先。即使在今天,整个地壳的大约95%都是火成岩。熔融物质定期从地球内部流出,以侵入表面层或流到地表本身。这种材料可以冷却成各种各样的火成岩。在熔融状态下,它被称为岩浆,当它流到地表时会推入地壳和熔岩。

所有岩浆基本上由各种硅酸盐矿物(硅氧化合物含量高)组成,但任何给定流动的化学成分可能与其他任何一种都不同。由此产生的火成岩将反映这些差异。火成岩在质地和化学方面也各不相同。例如,花岗岩是一种粗粒火成岩,其单个矿物晶体形成的尺寸很容易被肉眼看到。缓慢的冷却速度使晶体达到这个尺寸。通常情况下,当地壳被岩浆侵入时,会发生缓慢冷却,岩浆仍然埋在地表下方。花岗岩可以在当代景观的表面上找到,但是从它粗糙的纹理中我们知道它必须通过在很大的深度缓慢冷却而形成,然后由于侵蚀而暴露出来。具有在深处形成的粗粒纹理的火成岩被称为深成岩。

另一方面,如果相同的岩浆流到表面并被大气快速冷却,所得到的岩石将是细粒的并且看起来与花岗岩完全不同,尽管化学成分是相同的。这种岩石被称为流纹岩。最细粒度的火成岩是火山玻璃或黑曜石,没有晶体。一些研究人员认为这是因为快速冷却;其他人认为这是因为熔岩中缺乏水蒸气和其他气体。黄石国家公园的黑色黑曜石悬崖是玄武岩熔岩流入冰川的结果。一些冰川在接触时融化,但突然间也出现了巨大的黑色玻璃状石块。