pillow lava

^{ALASKA VOLCANO OBSERVATORY GLOSSARY}

Interconnected, sack-like bodies of lava formed underwater.

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^{GLOSSARY OF VOLCANIC TERMS}

A type of submarine lava flow consisting of interconnected, elongated lava tubes. Cross-sections of individual lava tubes resemble pillows with convex upper surfaces and flat or concave lower surfaces (Schmidt and Schminke, 2000, p. 383). Both radial and concentric cooling fractures may be present along the margins of individual pillows, and these fractures are brought on by thermal contraction during cooling. Growth of the pillow tubes takes place as the outer, commonly striated outer glassy surface of the pillow tube fractures, and a new tube "buds" from the fracture in a manner similar to the way that toothpaste is squeezed out of a tube.

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^{USGS PHOTO GLOSSARY OF VOLCANIC TERMS}

When basalts erupt underwater, they commonly form pillow lavas, which are mounds of elongate lava "pillows" formed by repeated oozing and quenching of the hot basalt. First, a flexible glassy crust forms around the newly extruded lava, forming an expanded pillow. Next, pressure builds until the crust breaks and new basalt extrudes like toothpaste, forming another pillow. This sequence continues until a thick sequence may be deposited. When geologists find pillow basalts in ancient rock sequences, they may conclude that the area was once under water.

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Photograph by Richard D. Grigg

Diver examines elongate pillowed flow lobe off the coast of Kilauea volcano, Hawai`i.