Figure 20 Heat, pressure, and chemical reactions change the form of rocks. In this sequence, shale changes to slate. Additional heat and pressure may change slate into schist or gneiss. Comparing and Contrasting How does gneiss appear to be similar to schist? How is it different?
dMetamorphic Rock
The word metamorphic comes from the Greek for “changing form.” Metamorphic rock is rock that has been changed by temperature, pressure, or reactions with hot water. Geologists call this process of change metamorphism. 
Metamorphic rock forms when a rock is transformed by heat, pressure, or chemical reactions. Most metamorphic rocks form under high temperatures and pressures deep underground. The original rock could have been any kind of rock: igneous, sedimentary, or even another metamorphic rock. The result is a new kind of rock.
Metamorphism can result in a rock with a mineral content that is different from that of the original rock. The process begins when heat deep inside Earth allows the minerals to recrystallize, and small crystals to enlarge. As chemical changes occur, new minerals may replace the original minerals. For example, shale is a fine-grained clastic rock that contains clay. During metamorphism, some of the clay changes to the mineral mica, forming the metamorphic rock slate, as you can see in Figure 20.
Metamorphism also changes the texture of rocks. The particles that make up sedimentary and igneous rocks tend to be distributed randomly through the rock. But when pressure is applied from one direction, the particles may line up and lock together in layers or bands. Metamorphic rocks with crystals arranged in parallel layers or bands are foliated rocks. Some foliated rocks have a striped appearance. For example, when schist is subjected to heat and pressure, the result is gneiss (nys). Other metamorphic rocks have a nonfoliated texture. Nonfoliated rocks don't have bands. Their crystals are arranged randomly.
The Rock Cycle
Earth is a dynamic planet. Constructive forces form new igneous rock. Destructive forces break down rock, forming sediment. Other forces push rock deep beneath the surface, where heat and pressure form metamorphic rock.
These changes form a cycle—a set of events that repeats again and again. The rock cycle is a series of processes in which rocks continuously change from one type to another. In the rock cycle, forces within Earth and at the surface cause rocks to change form.
To follow the rock cycle in Figure 21, imagine a newly formed igneous rock such as granite. In one path through the rock cycle, the granite is pushed to the surface where it is broken into sand. A river then carries the sand to the ocean floor. There, the sand becomes part of the sedimentary rock sandstone.
The rock cycle doesn't necessarily end with the formation of sandstone. Over millions of years, the sandstone could be pushed deep underground and form the metamorphic rock quartzite. The quartzite could be heated past its melting point and then cool to form a new igneous rock.
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