INTRUSIVE (Plutonic): Igneous rocks which form deep within the earth's crust or upper mantle. These have never reached the surface and so cool slowly in the earth, perhaps for thousands or tens of thousands of years. As long as the magma remains in a liquid or semi-liquid state the mineral crystals in the rock have time to grow and become very large. Intrusive igneous rocks have large crystal size, also called 'coarse texture'.
Intrusive = Slow cooling deep underground = Large crystals (coarse texture)/
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LEFT-Samples of polished granite: Note the many colors due to different mineral composition. |
 Rough, unpolished granite |
 Pegmatite |
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Diorite |
 Gabbro |
 Dunite |
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Peridotite |
 Diabase |
Note that diabase (left) crosses the line between extrusive and intrusive igneous rocks. | |
