They have a coarse texture with large mineral grains, indicating that they spent thousands or millions of years cooling down inside the earth, a time course that allowed large mineral crystals to grow. Granite and diorite are examples of common intrusive rocks. If they are formed outside or on top of Earth’s crust, they are called extrusive, or volcanic, igneous rocks. When they are formed inside of the earth, they are called intrusive, or plutonic, igneous rocks. Igneous rocks can also be made a couple of different ways. Igneous Rocks Igneous rocks (derived from the Latin word for fire) are formed when molten hot material cools and solidifies. Nonfoliated rocks can also form by metamorphism, which happens when magma comes in contact with the surrounding rock. Sedimentary rocks like bituminous coal, limestone, and sandstone, given enough heat and pressure, can turn into nonfoliated metamorphic rocks like anthracite coal, marble, and quartzite. Nonfoliated rocks are formed the same way, but they do not contain the minerals that tend to line up under pressure and thus do not have the layered appearance of foliated rocks. When granite undergoes this process, like at a tectonic plate boundary, it turns into gneiss (pronounced “nice”). Granite contains long and platy minerals that are not initially aligned, but when enough pressure is added, those minerals shift to all point in the same direction while getting squeezed into flat sheets. An example of this transformation can be seen with granite, an igneous rock. Foliation is the aligning of elongated or platy minerals, like hornblende or mica, perpendicular to the direction of pressure that is applied. When a rock with flat or elongated minerals is put under immense pressure, the minerals line up in layers, creating foliation. Metamorphic rocks have two classes: foliated and nonfoliated. Metamorphic Rocks Metamorphic rocks are rocks that have been changed from their original form by immense heat or pressure. These dissolved minerals are precipitated when the water evaporates. This occurs as water travels through Earth’s crust, weathering the rock and dissolving some of its minerals, transporting it elsewhere. A chemical precipitate is a chemical compound-for instance, calcium carbonate, salt, and silica-that forms when the solution it is dissolved in, usually water, evaporates and leaves the compound behind. Chemical sedimentary rocks, like limestone, halite, and flint, form from chemical precipitation. Once the sediment settles somewhere, and enough of it collects, the lowest layers become compacted so tightly that they form solid rock. Through the process of erosion, these fragments are removed from their source and transported by wind, water, ice, or biological activity to a new location. The formation of clastic and organic rocks begins with the weathering, or breaking down, of the exposed rock into small fragments. Organic sedimentary rocks, like coal, form from hard, biological materials like plants, shells, and bones that are compressed into rock. Clastic sedimentary rocks, like sandstone, form from clasts, or pieces of other rock. There are three different types of sedimentary rocks: clastic, organic (biological), and chemical. Sedimentary Rocks Sedimentary rocks are formed from pieces of other existing rock or organic material. Each of these rocks are formed by physical changes-such as melting, cooling, eroding, compacting, or deforming-that are part of the rock cycle. There are three main types of rocks: sedimentary, igneous, and metamorphic.
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