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Metamorphic Rocks

How are they formed?gneiss
Metamorphic rocks are formed when existing igneous or sedimentary rocks are changed by pressure and heat, but without melting. This can happen by contact with cooling igneous rocks, or at the edges of tectonic plates, where very high temperatures and pressures occur as mountains are being formed.

What are they like?
Metamorphic rocks usually contain regularly-aligned crystals, and they may be shiny and hard. They sometimes flake into layers.

Marbleslateschist and gneiss are examples of metamorphic rock.

Chemical composition
Metamorphic rocks may have the same chemical composition as other rocks, and this provides evidence for how they were formed. For example, marble is formed from limestone, and both are forms of calcium carbonate.

VOCAB

Metamorphic:  Meaning to change shape. Changes with temperature and pressure, but remains solid.  Usually takes place deep in the Earth.

Contact Metamorphism – heated by nearby magma.  Increased temperature changes the composition of the rock, minerals are changed into new minerals.  (Example:  hornfels)

Regional Metamorphism – pressure builds up in rocks that is deep within the Earth.  Large pieces of the Earth’s crust collide and the rock is deformed and chemically changed by heat and pressure

Foliated:  contain aligned grains of flat minerals (example:  gneiss).

Non-Foliated:  mineral grains are not arranged in bands (example:  marble).

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