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Volcanic rock

Volcanic rock is an igneous rock of volcanic origin.

Volcanic rocks are usually fine-grained or aphanitic to glassy in texture.

They often contain clasts of other rocks and phenocrysts.

Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye.

They were created during fractional crystallization of magma before extrusion.

Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a very fine grained matrix.

Note:   The above text is excerpted from the Wikipedia article "Volcanic rock", which has been released under the GNU Free Documentation License.
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August 3, 2015

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