Extrusive

Extrusive

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 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. Often, a residual portion of the matrix fails to crystallize at all, instead becoming an interstitial 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.

External links

* [http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures.html Igneous and volcanic textures - images]
* [http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures/7VesicularAmygdaloidal.html Vesicular and Amygdaloidal Textures]


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  • Extrusive — Ex*tru sive, a. [See {Extrude}.] (Geol.) Forced out at the surface; as, extrusive rocks; contrasted with {intrusive}. [Webster 1913 Suppl.] …   The Collaborative International Dictionary of English

  • extrusive — 1816, from L. extrusus, pp. of EXTRUDERE (Cf. extrudere) (see EXTRUSION (Cf. extrusion)) + IVE (Cf. ive). Related: Extrusively …   Etymology dictionary

  • extrusive — [ek stro͞o′siv] adj. 1. extruding or tending to extrude 2. Geol. designating or of igneous rock formed from lava that hardened on the surface of the earth, moon, etc.: cf. INTRUSIVE (sense 2) …   English World dictionary

  • extrusive — ● extrusif, extrusive adjectif Relatif à une extrusion …   Encyclopédie Universelle

  • extrusive rock — Any igneous rock derived from magma that is poured out or ejected at the Earth s surface. Extrusive rocks are usually distinguished from intrusive rocks on the basis of their texture and mineral composition. Lava flows and pyroclastic debris… …   Universalium

  • extrusive — adjective Date: 1816 relating to or formed by geological extrusion from the earth in a molten state or as volcanic ash …   New Collegiate Dictionary

  • extrusive — /ik strooh siv, ziv/, adj. 1. tending to extrude. 2. pertaining to extrusion. 3. Also, effusive. Geol. noting or pertaining to a class of igneous rocks that have been forced out in a molten or plastic condition upon the surface of the earth.… …   Universalium

  • extrusive — 1. adjective Jutting out; extruding. See Also: extrude, extrusion, detrusive, intrusive, protrusive, retrusive 2. noun Rock which has been formed through volcanic extrusion …   Wiktionary

  • extrusive —   lit. forced out. In geography, applied to volcanic material that reaches the surface. May be lava, pyrolclasts or gases …   Geography glossary

  • extrusive —    (adjective)    Said of igneous rocks and sediments derived from deep seated, molten matter (magma), deposited and cooled on the earth s surface (e.g., including lava flows and tephra deposits).    Compare: intrusive, volcanic.    HP …   Glossary of landform and geologic terms

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