When heat and pressure change the environment of a rock, the crystals may respond by rearranging their structure. They will form new minerals that are more stable in the new environment. Extreme pressure may also lead to the formation of foliation , or flat layers in rocks that form as the rocks are squeezed by pressure.
Foliation normally forms when pressure was exerted on a rock from one direction. If pressure is exerted from all directions, then the rock usually does not show foliation. This is an example of contact metamorphism. It is important to note that metamorphism does not cause complete melting of the initial rock. It only causes changes to a rock by heat or pressure. The rearrangement of the mineral crystals is the most common way that we notice these changes. Table 4. Hornfels, with its alternating bands of dark and light crystals is a good example of how minerals rearrange themselves during metamorphism.
Foliates are composed of large amounts of micas and chlorites. These minerals have very distinct cleavage. Foliated metamorphic rocks will split along cleavage lines that are parallel to the minerals that make up the rock. Slate, as an example, will split into thin sheets.
Foliate comes from the Latin word that means sheets, as in the sheets of paper in a book. Silt and clay can become deposited and compressed into the sedimentary rock shale. The layers of shale can become buried deeper and deeper by the process of deposition. Deposition is the laying down of rock forming material by any natural agent wind, water, glaciers over time. Because these layers are buried, temperatures and pressures become greater and greater until the shale is changed into slate.
Slate is a fine-grained metamorphic rock with perfect cleavage that allows it to split into thin sheets. Slate usually has a light to dark brown streak. Slate is produced by low grade metamorphism, which is caused by relatively low temperatures and pressures.
Slate has been used by man in a variety of ways over the years. One use for slate was in the making of headstones or grave markers. Slate is not very hard and can be carved easily.
The problem with slate though is its perfect cleavage. The slate headstones would crack and split along these cleavage planes as water would seep into the cracks and freeze which would lead to expansion. This freeze-thaw, freeze-thaw over time would split the headstone. Today headstones are made of a variety of rocks, with granite and marble being two of the most widely used rocks. Slate was also used for chalk boards. The black color was good as a background and the rock cleaned easily with water.
Today it is not very advantageous to use this rock because of its weight and the splitting and cracking over time. Schist is a medium grade metamorphic rock. This means that it has been subjected to more heat and pressure than slate, which is a low grade metamorphic rock. As you can see in the photo above schist is a more coarse grained rock. The individual grains of minerals can be seen by the naked eye.
Many of the original minerals have been altered into flakes. Because it has been squeezed harder than slate it is often found folded and crumpled. Schists are usually named by the main minerals that they are formed from.
Bitotite mica schist, hornblende schist, garnet mica schist, and talc schist are some examples of this. Gneiss is a high grade metamorphic rock. This means that gneiss has been subjected to more heat and pressure than schist. Gneiss is coarser than schist and has distinct banding.
This banding has alternating layers that are composed of different minerals. The minerals that compose gneiss are the same as granite. Feldspar is the most important mineral that makes up gneiss along with mica and quartz. Gneiss can be formed from a sedimentary rock such as sandstone or shale, or it can be formed from the metamorphism of the igneouse rock grantite. As a result, the rocks are heated and put under great pressure. They do not melt, but the minerals they contain are changed chemically, forming metamorphic rocks.
Marble is formed from limestone and contains tiny interlocking grains. The layers in slate, formed from shale, are arranged in layers.
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