Extrusive rock

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
IUGS classification of aphanitic extrusive igneous rocks according 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.
An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope

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]This is as opposed to intrusive rock formation, in which magma does not reach the surface.[2]

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. 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. Examples of extrusive rocks include basalt, rhyolite, andesite, obsidian and pumice, scomia, fledspar and gabbro.

See also


  1. ^ "Extrusive rock - geology". Retrieved 21 October 2018.
  2. ^ "Intrusive rock - geology". Retrieved 21 October 2018.

External links

  • Igneous and volcanic textures - images
  • Vesicular and Amygdaloidal Textures
Retrieved from "https://en.wikipedia.org/w/index.php?title=Extrusive_rock&oldid=899657480"
This content was retrieved from Wikipedia : http://en.wikipedia.org/wiki/Extrusive_rock
This page is based on the copyrighted Wikipedia article "Extrusive rock"; it is used under the Creative Commons Attribution-ShareAlike 3.0 Unported License (CC-BY-SA). You may redistribute it, verbatim or modified, providing that you comply with the terms of the CC-BY-SA