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老托福阅读文本passage13

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.

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