Basaltic lavas are thin and fluid and come in two varieties: pahoehoe and aa. Pahoehoe has a characteristic smooth, ropy surface; aa is jagged, cindery, and very difficult to walk upon. Aa forms from slightly more viscous magma than pahoehoe. Columnar jointing is common in basalt flows. More silicic lavas are called block lavas and form as a viscous lava flow fragments upon flowing.
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Pahoehoe on Mauna Loa |
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Smooth-sided basaltic vent on Mauna Loa |
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Hornito on aa flow, Lava Beds National Monument |
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Columnar jointing in the Devils Postpile, California |
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Margin of Obsidian Dome, California |
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Obsidian and interfingered pumice at Obsidian Dome |
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Tuff from eruption of Mt. Mazama (Crater Lake) |
