What is the difference between extrusive and intrusive igneous rocks?
Answers
Answer:
Igneous rocks form whenever molten rock cools and minerals crystallize to produce a solid rock.
Hence the prerequisite for the formation of igneous rocks is the melting of rocks. Molten rock is called magma if it occurs within the lithosphere and becomes lava when the molten rock is extruded on Earth’s surface through volcanic activity.
Explanation:
Extrusive igneous rocks:
These rocks cool rapidly on or very near the Earth’s surface.
The rapid cooling is due to the great difference in temperature between the hot molten rock and the relatively cool surrounding environment.
These rocks are fine-grained since minerals crystallize too rapidly to form large crystals.
A common example of an extrusive igneous rock is basalt.
Basalt is a major component of the oceanic crust and produced in great abundance at mid-ocean ridges.
The extremely fast cooling of a lava extruded on Earth’s surface results in the production of
volcanic glass in which atoms do not have time to form a regular crystal lattice.
Intrusive igneous rocks:
These rocks cool slowly within the Earth’s crust. The slow cooling enables minerals to grow large.
The texture of these rocks is coarse-grained. An example of an intrusive igneous rock is granite.
Granite is a major component of the continental crust.
Igneous rocks may sometimes consist of a combination of fine-grained and coarse-grained
textures which tells us something interesting about their history.
Porphyry is the name given to an igneous rock composed of large crystals (or phenocrysts) “floating” in a fine-grained matrix.
This particular texture indicates that the rock started to cool slowly at some depth beneath the surface
producing large crystals, and then began to cool faster as it approached the surface forming tiny
crystals out of the remaining melt.
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