Question

Extrusive Igneous rocks generally have small crystals. Give reason.
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Answer 1

Answer:

Extrusive igneous rocks come from lava. Lava, at the surface, is exposed to air and water which causes the molten rock to cool rapidly. Solidifying rocks at the surface cool too quickly for large crystals to form. Molecules in the lava do not have time to arrange themselves to form large crystals. Extrusive rocks have crystals that are too small to see without magnification. A common example of an extrusive igneous rock is basalt. Some extrusive rocks, such as obsidian and pumice, cool so rapidly that they completely lack crystal structure and are considered a volcanic glass. Pumice is just like obsidian except it is tiny shards of glass .Extrusive rocks generally have small crystals because in the formation of these rocks, the solidification of magma takes place at a slower rate.