Extrusive rock

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

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.

External links

• Igneous and volcanic textures - images
• Vesicular and Amygdaloidal Textures

Geologic principles and processes
Stratigraphic principles	Principle of original horizontality Law of superposition Principle of lateral continuity Principle of cross-cutting relationships Principle of faunal succession Principle of inclusions and components Walther's law
Petrologic principles	Intrusive Extrusive Volcanic Exfoliation Weathering Pedogenesis Diagenesis Compaction Metamorphism
Geomorphologic processes	Plate tectonics Salt tectonics Tectonic uplift Subsidence Marine transgression Marine regression
Sediment transport	Fluvial processes Aeolian processes Glacial processes Mass wasting processes