Pillow lavas—or lava pillows? Geologists tend to use "pillow lavas," but both are OK. Strictly speaking, they are lava flows that exhibit pillow structure. Pillow lavas are rounded lumps of lava that range in size from a bread loaf to a beanbag chair. They form when lava is erupted into water: the outer layer hardens so rapidly that the molten rock is forced to start new pillows instead of becoming a steady flow. The pillow lavas in this gallery range in age from the present to about 450 million years ago, a vivid instance of the old saying that in geology, the present is the key to the past.
Lava pillows are commonly interbedded with sediments from a deep-sea setting, like fine muds. In the 20th century, submarine observers filmed lava pillows forming, and they're a staple of programs about the mid-ocean ridges. They crackle furiously, as it happens. Pillow lava is common in ophiolites and subduction-zone rocks, but it can be found wherever lava is erupted in water.
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