There are three main types of rocks: sedimentary, igneous, and metamorphic. Each type of rock has its own characteristics and properties, are part of the rock cycle, and can be found all over the world. However, metamorphic rocks are those that have been changed by extreme heat and pressure. The original rocks are usually sedimentary, igneous, or other metamorphic rocks. The name metamorphic is derived from the word morph, which means change.
The original rock changes due to extreme heat, greater than 300°F or 400°F, and pressure. Under the surface of the Earth, there are physical and chemical changes that transform the original rock into a metamorphic rock. The changes take place in the root of a mountain chain or volcano. The different forces are enough to change the shape of the strata (layer) and the minerals of the changing rocks.
For example, a sedimentary rock that has changed may seem to have been twisted and heated over a hot fire. Various types of other rocks change into new rocks. For example, marble is a metamorphic rock that was once limestone, a sedimentary rock. Another sedimentary rock, sandstone, is changed into a metamorphic rock called quartzite. If any of the sedimentary rocks contained fossils, the recrystallization of the rocks destroys the fossils.
There are two main types of metamorphism or changes that take place with rocks: regional and contact. The first type, regional, occurs in great masses of rock and is also called dynamic metamorphism. It is the most common. The rocks can be found at very great depths below the Earth's surface underneath the weight of the rock layers above and exposed to the high temperatures which cause the changes to occur.
Much of the lower continental crust layer is metamorphic, except for some recent igneous intrusions, rocks that are created inside the earth's surface from magma. The collision of the tectonic plates of continents create belts or metamorphic rocks, and they are uplifted and exposed by erosion and appear as long belts or other large areas on the earth's surface.
The second type of formation, contact, also called thermal, are formed by both extreme pressure and by intense heat. Molten rock pushes its way up to the Earth's crust, and the pressure fills empty spaces with magma. The heat chemically changes the rocks around it and change them into a new rock. The closer the rocks are to the magma, the greatest the change in the rock.
There are two basic types of metamorphic rocks: foliated and non-foliated. The foliated have a layered or banded appearance, and the non-foliated do not have a layered or banded appearance.
Foliated metamorphic rocks include gneiss, schist, slate and many others. They can be used as crushed stone for construction, landscaping projects, countertops, flooring, roofing, chalkboards, and other uses. Non-foliated metamorphic rocks include hornfels, marble, quartzite and many more. Hornfels is mostly used for the construction industry and roads, with marble powder used as filler in paints and putty, as well as window sills and other decorative stone structures. Quartzite is hard, tough, durable rock used by early people to make cutting tools such as ax heads and scrapers.
In summary, as with all three major types of rocks, metamorphic rocks go through many changes during the rock cycle and many igneous and sedimentary rocks become metamorphic rocks. Metamorphic rocks have been modified by heat, pressure, and chemical processes while buried deep below the surface of the Earth.
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