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USGS Geology in the Parks

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Lava flow trail stop 9

'A'a and Pahoehoe

Basically basalt

Virtually all of Sunset Crater Volcano National Monument's surface is covered with basalt flows and cinders. Despite the fact that its all pretty much the same stuff, the black and red basalt surface exhibits a remarkable variety of form and texture.

HVO citation
This picture of pahoehoe was taken from the USGS volcanoes website.
Slab pahoehoe on Bonito Lava Flow
Slabs of 'a'a lava make up the surface of Bonito Lava Flow

'A'a

Most of Sunset Crater's lava flows at are 'A'a flows. The surfaces of 'A'a flows are covered by an extremely rough jumble of broken lava blocks. Beneath this forbidding cover is a dense core of basalt. 'A'a flows form from relatively cool, sticky, pasty (high viscosity) lava. The surface and base of an 'A'a flow solidifies to form a crusty sheet that breaks into rough blocks as it flows away from its volcanic vent.

Pahoehoe

Pahoehoe lava is much less common at Sunset Crater. In contrast to A'a' lava, pahoehoe lava is far more fluid (low viscosity) and has a distinctive ropy to smooth surface. Some lava flows begin as hot pahoehoe, then change to a'a' flows as they cool or slow down. Click here to learn more.

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