Slabs of 'a'a lava make up the surface of Bonito Lava Flow
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'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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