Lava Pillows
Images (c) 2001 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Lava pillows—or pillow lavas? Geologists use both terms, but I think the first is a little better. Strictly speaking, this is a lava flow that exhibits pillow structure. It consists of rounded lumps of lava, probably of Miocene age (around 20 million years old), that range in size from a bread loaf to a beanbag chair.
Pillow lava is typical of eruptions underwater, where the cold seawater quickly freezes the hot rock so that long streams, like those on land, cannot form. Geologists concluded this because 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. See more examples in the Pillow Lava Picture Gallery. Pillow lava is also common in subduction-zone rocks.
This particular set of lava pillows is actually tilted nearly vertically, as you can see from the photo below. The tilting happened during the millions of years since the pillows were laid down, as crustal movements assembled present-day California. The location is at the farthest north end of Avila Beach. (This picture is also available as a free wallpaper image.)
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