Extrusive rock

Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff.[1] In contrast, intrusive rock refers to rocks formed by magma which cools below the surface.[2]

IUGS classification of aphanitic extrusive igneous rocks to their relative alkali (Na2O + K2O) and silica (SiO2) weight contents. Blue area is roughly where alkaline rocks plot; yellow area where subalkaline rocks plot. Original source: *Le Maitre, R.W. (ed.); 1989: A classification of igneous rocks and glossary of terms, Blackwell Science, Oxford.
A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope

The main effect of extrusion is that the magma can cool much more quickly in the open air or under seawater, and there is little time for the growth of crystals.[3] Sometimes, a residual portion of the matrix fails to crystallize at all, instead becoming a natural glass or obsidian.

If the magma contains abundant volatile components which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in pumice, scoria, or vesicular basalt. Other examples of extrusive rocks are rhyolite and andesite.

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