They are not made from molten rock — rocks that do melt form igneous rocks instead. Earth movements can cause rocks to be deeply buried or squeezed. 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. As the mountain ranges are eroded away over millions of years, the metamorphic rocks are eventually uncovered. The Appalachian Mountains used to be a very large mountain range. Over time the mountains have eroded away and we now can see many different types of metamorphic rocks in the region.
Find Your Rock:. Metamorphic Rocks Graphite Diamond Marble Metamorphic When rocks are heated up or put under a lot of pressure, they can change drastically. So why are these rocks interesting? Previous Item: Chemical Sedimentary Rocks.
The three types of metamorphism are Contact, Regional, and Dynamic metamorphism. Contact Metamorphism occurs when magma comes in contact with an already existing body of rock. When this happens the existing rocks temperature rises and also becomes infiltrated with fluid from the magma. The area affected by the contact of magma is usually small, from 1 to 10 kilometers.
Contact metamorphism produces non-foliated rocks without any cleavage rocks such as marble, quartzite, and hornfels. In the diagram above magma has pushed its way into layers of limestone, quartz sandstone and shale. The heat generated by the magma chamber has changed these sedimentary rocks into the metamorphic rocks marble, quartzite, an hornfels. Regional Metamorphism occurs over a much larger area. This metamorphism produces rocks such as gneiss and schist.
Regional metamorphism is caused by large geologic processes such as mountain-building. These rocks when exposed to the surface show the unbelievable pressure that cause the rocks to be bent and broken by the mountain building process. Regional metamorphism usually produces foliated rocks such as gneiss and schist.
Dynamic Metamorphism also occurs because of mountain-building. These huge forces of heat and pressure cause the rocks to be bent, folded, crushed, flattened, and sheared. Metamorphic rocks are almost always harder than sedimentary rocks. They are generally as hard and sometimes harder than igneous rocks.
They form the roots of many mountain chains and are exposed to the surface after the softer outer layers of rocks are eroded away. Many metamorphic rocks are found in mountainous regions today and are a good indicator that ancient mountains were present in areas that are now low hill or even flat plains.
Metamorphic rocks are divided into two categories- Foliates and Non-foliates. 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.
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