| Usually microscopic crystals such as those in basalt or the absence of crystals indicate an extrusive rock. |
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| Small crystals of extrusive rock, such as basalt, are usually produced by rapid cooling on the crust of the Earth. |
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| Obsidian as an igneous extrusive rock can be either glassy basaltic or rhyolitic, which commonly exhibits lava flow textures. |
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| Intrusive rock tends to cool more slowly than extrusive rock, thus allowing crystals to grow larger. |
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| If the magma is about 60 percent plagioclase feldspar and 35 percent mafic minerals, the intrusive rock diorite or the extrusive rock andesite will be formed. |
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| At the intersection of the Mulholland Highway with the Laurel Canyon road this pyroclastic character is associated with pronounced bedding in this extrusive rock. |
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| The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. |
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