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Generalized Geologic Map of the Conterminous United States
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  The North American Tapestry of Time and Terrain

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  Distribution of Metamorphic Rocks
 
  Map of Metamorphic Rock Distribution of North America

METAMORPHIC ROCKS

Metamorphic rocks result when an existing rock is altered by some combination of changes in pressure or temperature. That change occurs while the rock in in a solid state--it doesn't actually melt, though the molecules are re-organized.

The changes in pressure or temperature can come from a number of possible circumstances. As the slow deposition of overlying sediment buries a rock, those sediments may over time put enough pressure on the rock to cause metamorphism. The stresses arising from the interaction of plates, such as mountain-building plate collisions or subduction, can put stress on a rock. Heat emanating from a nearby magma body can alter rock, as does the intense heat and pressure of a meteor impact.

Metamorphism will change the rock's mineralogic or textural features. The shift in pressure or temperature may knock the rock's mineral composition out of chemical equilibrium. As conditions change, the molecules rearrange themselves, some forming different minerals, until the rock reaches a new balance. The textural changes, meanwhile, may include changes in grain size or the alignment of minerals as a result of directional forces that can be manifested as a sort of layering within the rock called foliation.

In some cases, significant amounts of hot water can infiltrate the rock through cracks. This water, and the dissolved ions it contains, can change the surrounding rock's mineralogy. In addition, the dissolved material may precipitate from the water and remain as mineral deposits in the cracks; sometimes, these veins are filled with deposits of valuable ores of copper, silver, or gold.