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(green phenocrysts are Olivine ).]] 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 Phenocryst s. Phenocrysts are Crystal s 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 enbedded in a very fine grained matrix. Volcanic rocks are named according to their Chemical Composition . Basalt is a very common volcanic rock with low Silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has the same chemical composition as Granite and basalt is compositionally equal to Gabbro . Intermediate volcanic rocks include Andesite , Dacite , Trachyte and Latite . Volcanic rocks often have a Vesicular Texture , which is the result voids left by Volatile s escaping from the molten Lava . Pumice is a rock, which is an example of Explosive Volcanic Eruption . It is so vesicular that it floats in Water . Pyroclastic Rock s are the product of explosive volcanism. They are usually Felsic (high in silica). Examples of pyroclastic rocks are Tuff and Ignimbrite . Shallow Intrusion s, which possess structure similar to volcanic rather than plutonic rocks are also considered to be volcanic. SEE ALSO |
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