Hornfels facies rocks typically occur under the medium to high temperature and low pressure conditions associated with contact aureoles. Each metamorphic facies is associated with a group of index minerals from which it can be identified. The exact set of indicator minerals from each group that is present in a particular instance depends on the chemical composition of the protolith. Information about the mineralogy of each metamorphic facies may be found in Grotzinger and Jordan. Metamorphic facies.
Greenschist, amphibolite and granulite conditions are typically associated with regional metamorphism. Blueschist and eclogite are associated with the type of regional metamorphism that occurs at subduction zones. Hornfels and sanidinite are typically associated with contact metamorphism. Image from Wikipedia Commons. Regional metamorphism usually results in the formation of rocks that are strongly foliated, such as slates, schists, and gneisses see below.
The differential stress needed to produce such foliated rocks usually results from tectonic forces that produce compressional stresses in the rocks such as occur when two continental masses collide. Compressive stresses result in folding of rock and thickening of the crust, which results in some of the rocks being pushed to deeper levels where they are subjected to higher temperatures and pressures.
The pressure and heat which drive metamorphism come from three sources: the internal heat of the Earth; the weight of the overlying rock; and the horizontal pressures developed as rocks become deformed.
Grades of metamorphic rock. Image from Tulane. Metamorphic rocks formed in the shallow crust are referred to as low-grade metamorphic rocks ; those at higher temperatures and pressures as high-grade metamorphic rocks. Contact metamorphism typically occurs around intrusive igneous rocks, and is the result of temperature increase associated with the introduction of hot magma into the colder country rocks.
The area surrounding an intrusion, where metamorphism is present, is called a metamorphic aureole. Metamorphism is greatest closest to the intrusion and dissipates as distance from the intrusion increases.
The size of the aureole depends on the temperature of the igneous intrusion, the size of the intrusion and the temperature of the country rock. 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. 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. Metamorphic rocks Metamorphic rocks are formed from other rocks that are changed because of heat or pressure. Metamorphic rocks may form from rocks heated by nearby magma What are metamorphic rocks like? Slate is a metamorphic rock that can be split into thin sheets, which makes it ideal for roof tiles.
Marble is used for statues and decorative items like vases Figure 4. Ground up marble is also a component of toothpaste, plastics, and paper. Quartzite is very hard and is often crushed and used in building railroad tracks Figure 4. Schist and slate are sometimes used as building and landscape materials. Skip to main content. Search for:. Metamorphic Rocks In this lesson you will learn about metamorphic rocks, how they form, and some of their common uses.
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