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Facts about Metamorphic Rocks

Metamorphic rocks are formed when existing igneous or sedimentary rocks are fused together by high heat and pressure. Finding the oldest rocks in the world is a harder task than it might seem. ... Technically, the oldest known rocks existing on Earth are not from Earth at all, but are instead of extraterrestrial origin .Aug 13, 2010

They do not melt, but the minerals they contain are changed chemically, forming metamorphic rocks. Sometimes, metamorphic rocks are formed when rocks are close to some molten magma, and so get heated up. Remember that metamorphic rocks are not made from melting rock. (Rocks that do melt form igneous rocks instead.)

Metamorphic Rocks. Metamorphic rocks were once igneous or sedimentary rocks, but have been changed (metamorphosed) as a result of intense heat and/or pressure within the Earth's crust. They are crystalline and often have a “squashed” (foliated or banded) texture.

Marble is used for statues and decorative items like vases (Figure). Quartzite is very hard and is often crushed and used in building railroad tracks. Schist and slate are sometimes used as building and landscape materials.

There are two basic types of metamorphic rocks. Foliated metamorphic rocks such as gneiss, phyllite, schist, and slate have a layered or banded appearance that is produced by exposure to heat and directed pressure.

As with igneous and sedimentary rocks, metamorphic rocks are classified on the basis of texture (grain size, shape, orientation) and mineral composition. However, the classifiction of metamorphic rocks is more difficult to get into than with igneous and sedimentary rocks, for a couple of reasons.Oct 25, 2000

The main difference is that the crystals in an igneous rock is shaped as it crystallized from a melt. The structures in a metamorphic rock are earlier strucures (crystals, sedimentary fragments, etc.) that have been deformed or recrystallized due to heat or pressure, or some chemical change.Apr 24, 2017