A common igneous rock rule is that if lava cools at a faster rate, the rock formed will have finer grains and have a glassy appearance; if rock cools at a slower rate, the grains will be larger and more coarse. Porphyritic rock is a type that has a combination of large and small grains; this occurs when a rock has a mixed cooling history.
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A common igneous rock rule is that if lava cools at a faster rate, the rock formed will have finer grains and have a glassy appearance; if rock cools at a slower rate, the grains will be larger and more coarse. Porphyritic rock is a type that has a combination of large and small grains; this occurs when a rock has a mixed cooling history.