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Extrusive Rocks 

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Spattercone USGS

Rocks which have been erupted onto the surface by volcanic activity such as lava flows. Extrusive rock are characterised by a fine groundmass with individual crytals to small to discern .Rapid cooling does not allow crystals to grow in the erupted liquid magma . Explosive eruptions can disentegrate any crystals already formed. Pyroclastic material such as ash,tephra,pumice,scoria and bombs are included . In some instances cooling is so rapid that no crsytallisation can occur and the melt resembles glass.

Erupted rocks have the same composition as the melt they came from just rate of cooling and crystal growth is different. So an extrusive basaltic lava is the equivalent of an intrusive gabbro. crystal