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Types of Rock

Deep in the earth's crust, temperatures are high enough to melt rock into magma. Magma sometimes erupts to the surface as lava, or it may force its way into other solid rock underground. In either case, when magma cools, it solidifies, forming igneous rocks, such as granite and basalt.

Sedimentary rocks are rocks formed in horizontal layers, or strata, from sediments. A sediment can consist of eroded particles of older rock (igne­ous, sedimentary, or metamorphic) that wash downhill to lakes or to the oceans. Sediment may also consist of minerals that precipitate out of water. In any case, water is a crucial ingredient in forming sedimentary rocks. Over tens of thousands of years, the layers become thick, and the weight of the overlying sediments compacts the earlier deposits (fig. 1.10). Minerals in the water cement these deposits together into sedimentary rocks. Limestone, sandstone, and clay are typical sedimentary rocks.


Figure 1.10 The weight of overlying sediments, combined with minerals in water, compacts sediments into rock.

Metamorphic rocks are rocks—either igneous, sedimentary, or other metamorphic rocks—that have been buried deep in the earth where they were subjected to high temperatures and pressures. The term comes from the Greek meta, to change, and morphe, form or shape. During the metamor­phic process, the original rock undergoes physical and chemical changes that may greatly alter its composition and appearance. So, for example, lime­stone can be metamorphosed into marble, and sandstone into quartzite. A belt of one type of rock is known as a formation. Formations are stacked on top of one another and then deformed by folding, warping, and faulting.

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