Photograph by L. Simmons on
November 29, 1988
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| Tube-fed pahoehoe flow creeps across flat-lying ground
near the coast. A crust forms on these flows as soon as
lava is exposed to air. As the flow front slowly advances,
the crust at the margins of the flow lifts and molten material
oozes out, further extending the flow.
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Photograph by T. Mattox on
August 16, 1990
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| This photo shows examples of both forms of basaltic
lava: a rubbly `a`a flow advances over smooth silvery pahoehoe.
There is no difference in the composition of these two forms, and a
single flow can start as pahoehoe and change to `a`a as it loses
heat and gas and starts to crystallize. Flowing down steep slopes,
or very rapidly, also accelerates the transition from pahoehoe to `a`a.
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