Rocks are formed on Earth as igneous, sedimentary, or metamorphic rocks. Igneous rocks form when rocks are heated to the melting point which forms magma. Sedimentary rocks are formed from the cementing together of sediments, or from the compaction (squeezing together) of sediments, or from the recrystallization of new mineral grains which are larger than the original crystals. Metamorphic rocks form from heat and pressure changing the original or parent rock into a completely new rock. The parent rock can be either sedimentary, igneous, or even another metamorphic rock. The word "metamorphic" comes from Greek and means "To Change Form".
The diagram above shows you how the rocks on Earth have been changed continually over time from one rock type to another. This changing of rock types is called the "Rock Cycle".
Solid rock can be changed into a new rock by stresses that cause an increase in heat and pressure. There are 3 main agents that cause metamorphism. Factors that cause an increase in Temperature, Pressure, and Chemical changes are the three agents that we are going to study.
Temperature increases can be caused by layers of sediments being buried deeper and deeper under the surface of the Earth. As we descend into the earth the temperature increases about 25 degrees Celsius for every kilometer that we descend. The deeper the layers are buried the hotter the temperatures become. The great weight of these layers also causes an increase in pressure, which in turn, causes an increase in temperature.
The descending of rock layers at subduction zones causes metamorphism in two ways; the shearing effect of the plates sliding past each other causes the rocks coming in contact with the descending rocks to change. Some of the descending rock will melt because of this friction. When rock melts it is then considered igneous not metamorphic, but the rock next to the melted rock can be changed by the heat and become a metamorphic rock. The diagram above shows you where metamorphic rock (YELLOW ZONE) can be produced at a subduction zone.