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"Every rock has a story to tell. The story of an igneous rock
begins when rock in the lower crust or upper mantle melts to form
molten material called magma, which rises up through the crust. Some
magma fl ows or spatters out on the surface as lava, or it explodes
into the air as tiny particles of volcanic ash or larger blocks.
Igneous rock that forms from solidify ed lava or ash is called
extrusive igneous rock because it comes out (extrudes) onto the
surface. Other magma never reaches the surface and solidifi es
underground to form intrusive igneous rock, so called because it
squeezes into (intrudes) the surrounding rocks."
From: Laboratory Manual for Introductory Geology
by Ludman and Marshak