Metamorphic rocks occur when rocks go through a change. This change happens under high temperatures, pressure and, at times, chemical fluids. When a rock goes through metamorphosis, it changes its texture and even the mineral composition. Metamorphic rocks are changed through a solid state unlike igneous rocks which changes if it were to melt. Gneiss, slate, marble, schist and quartzite are all types of metamorphic rocks. Gneiss is recognized as being a coarse-grained metamorphic rock, while slate is a very fine-grained and foliated one.
Metamorphic rocks have foliation in their make-up. However, there are different types of foliation: rock cleavage, schistosity, gneiss, and non-foliated. Over all these rocks are characterized by foliated and non-foliated. Foliation is the layering that can be seen in these rocks. This is due to the shortening of the rock along one axis while it is being recrystallized. When this occurs the elongated crystals become perpendicular to the orientation of th eshortening.
Any rock can become a metamorphic rock and the parent rock is what determines the process the rock needs to go through to change. When exposed to high temperatures and pressures metamorphic minerals can also be produced. Calcite crystals in limestone, a sedimentary rock, and chalk change into larger crystals in metamorphosed sandstone. The original quartz sand grains become a tight quartzite or metaquartzite, where the larger crystals are interlocked.
When most of a rocks chemical composition goes through a major change during metamorphism, it is called metasomatism. This change occurs from the chemicals that are in rocks that surround the parent rock.
When magma or hot lava is introduced to the rocks, it fills up any empty spaces. This process is called contact metamorphism and the solid rock that is the parent rock is called country rock. At one time these rocks were called hornstones, infact today the term hornfels is used to characterize fine-grained, compact, non-foliated rocks which were formed by contact metamorphism. An example of a rock going through this process could be shale which changes to a dark hornfel that is filled with plates of brown biotite.
Gneisses are inclined to have segregated bands. When there are bands of quarts and mica, it is called a mica schist. These bands may be thin but they are, generally, made up of one mineral. Gneisses can also have folia which alternate between widths and the bands may be irregular. These also are less micaceous and are quickly dying.