Geology And Geophysics

What are Metamorphic Rocks



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A Metamorphic rock is an igneous or sedimentary rock that has changed from its original form to another completely different rock, through a process called metamorphism. Metamorphism involves the alteration of existing rocks into new rocks either with intense heat or intense pressure or a little of both. Metamorphism may even take place through the chemical action of fluids. Studying metamorphic rocks give valuable clues to the temperatures and pressures that are occurring deep in the Earth’s crust; or an event that happened millions of years ago.

Most of the Earth’s crust is made of metamorphic and igneous rocks. Metamorphic rocks are also found at the core of most of the world’s mountain ranges. Marble which is a metamorphic rock is a popular material used in both sculpture and architecture.

There are two types of metamorphic rocks; regional and thermal. Regional metamorphic rocks, which are created through intense pressure, you usually find them in mountainous regions. The different amounts of pressure applied to the previous rocks produce a different type of metamorphic rock.

Thermal or Contact metamorphic rocks are created with intense heat. The temperature cannot be hot enough to melt the rock, otherwise you will have igneous rocks instead of metamorphic rocks. The temperature must be hot enough for the crystalline structures to be broken down and be combined with nearby elements and compounds. Another way thermal metamorphosis can take place is by magma seeping into the cracks and crannies of other rocks and between the heat and the pressure causing a metamorphic rock to be formed.

Metamorphosis through the chemical action of fluid takes place when fluids dissolve the ions and therefore create a chemical reaction and new minerals by substitution, removal or addition of elements. Oftentimes water and carbon dioxide will be found in small amounts in the perimeter between mineral crystals and pore spaces of a rock.

Some common metamorphic rocks

Slate, which is fine-grained, is a minor metamorphism between shale and mudstone. Slate is characterized by its foliation (layering within the rock) and fine grains. Phyllite, is fine grained, foliated and primarily made of quartz, silicate mica and chlorite. Schist has a medium to coarse grain, foliated and created as the result of the rearrangement of mica, chlorite, talc and hematite. Schist is a result of a very intense metamorphism. Gneiss (pronounced as “nice”) is coarse grained, metamorphosed igneous rock and recrystallized from the foliation of quartz, feldspars, micas, amphiboles into alternating light and dark bands. Marble is non-foliated, metamorphosed limestone or dolomite. Quartzite is formed by the recrystallization of the silica found in sandstone.

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