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. There are 3 factors that cause an increase in pressure which also causes the formation of metamorphic rocks. These factors are;.
The huge weight of overlying layers of sediments. Stresses caused by plates colliding in the process of mountain building. Stresses caused by plates sliding past each other, such as the shearing stresses at the San Andreas fault zone in California. Factors that cause chemical changes in rocks also contribute to the formation of metamorphic rocks. Very hot fluids and vapors can, because of extreme pressures, fill the pores of existing rocks. These fluids and vapors can cause chemical reactions to take place, that over time, can change the chemical makeup of the parent rock.
Metamorphism can be instantaneous as in the shearing of rocks at plate boundaries or can take millions of years as in the slow cooling of magma buried deep under the surface of the Earth. There are three ways that metamorphic rocks can form. 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.
One ways rocks may change during metamorphism is by rearrangement of their mineral crystals. 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. In highly deformed rocks that have undergone both folding and shearing, it may be more difficult to determine that the compositional layering represents original bedding. As shearing stretches the bedding, individual folded beds may be stretched out and broken to that the original folds are not easily seen. Similarly, if the rock had been injected by dikes or sills prior to metamorphism, these contrasting compositional bands, not necessarily parallel to the original bedding, could be preserved in the metamorphic rock.
Thus felsic minerals could be dissolved from one part of the rock and preferentially nucleate and grow in another part of the rock to produce discontinuous layers of alternating mafic and felsic compositions. Examples of questions on this material that could be asked on an exam. Metamorphic Rock Textures. Metamorphic rocks exhibit a variety of textures. These can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith.
Textural features of metamorphic rocks have been discussed in the previous lecture. Here, we concentrate on the development of foliation, one of the most common purely metamorphic textures, and on the processes involved in forming compositional layering commonly observed in metamorphic rocks. If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock.
Rounded grains can become flattened in the direction of maximum compressional stress. Minerals that crystallize or grow in the differential stress field may develop a preferred orientation. Sheet silicates and minerals that have an elongated habit will grow with their sheets or direction of elongation orientated perpendicular to the direction of maximum stress.
Slate Slates form at low metamorphic grade by the growth of fine grained chlorite and clay minerals. The preferred orientation of these sheet silicates causes the rock to easily break planes parallel to the sheet silicates, causing a slatey cleavage. Schist - The size of the mineral grains tends to enlarge with increasing grade of metamorphism.
Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates mainly biotite and muscovite. Quartz and feldspar grains, however show no preferred orientation. The irregular planar foliation at this stage is called schistosity. Search Search. Apply Filter. It is smoothly rounded on three sides and a sheer vertical face on the fourth.
Half Dome, which stands nearly 8, feet 2, meters above sea level, is composed of granodiorite, and is the remains of a magma chamber that cooled slowly and crystallized Is glacier ice a type of rock? Glacier ice, like limestone for example , is a type of rock. Glacier ice is actually a mono-mineralic rock a rock made of only one mineral, like limestone which is composed of the mineral calcite. The mineral ice is the crystalline form of water H 2 O. Most glacier ice forms through the metamorphism of tens of thousands of individual What are sedimentary rocks?
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Common Sedimentary Rocks Are there geologic maps or publications for where I live?
Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations. Geologic maps at many scales and from many sources are listed in the National Geologic Map Database. Download digital geologic maps for entire states Where can I find information about the geology and natural history of National Parks?
Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history.
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