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Where molten basalt cools rapidly, as at the earth's surface, fine-grained rocks are formed. Basalt may be compact or vesicular, i.e., porous because of gas bubbles contained in the lava while it is solidifying. If the vesicles become subsequently filled with secondary minerals, e.g., quartz or calcite, the rock is called amygdaloidal basalt. Basalt may form as columns of rock, such as the Devil's Tower in Wyoming; or it may form as twisted coils of rope, or cinders of jagged rock, called "pahoehoe" and "aa," respectively.
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