Why Is It Pahoehoe or Aa? (Grades 7-8)

Use slides, photographs, or field trip stops to discuss the factors that probably controlled the type of lava produced at a given location.

Pahoehoe and aa lava flows on Poliokeawe Pali. Photo by D.W. Peterson, U.S. Geological Survey, March 4, 1972.

For example, many, but not all, of the flows that drape the Hilina Pali in Hawaii Volcanoes National Park are aa because the steep slopes caused the rate of shear strain in the lava to increase.

Tube-fed pahoehoe flows near the coast on Kilauea. Photo by J.D. Griggs, U.S. Geological Survey, May, 12, 1987.

In contrast, most of the flows along the coast in the park are pahoehoe. The lava was probably transported in a tube that kept the lava hot and the viscosity low. When the lava poured out from the tube, it flowed at a low velocity and cooled slowly. These conditions favor the formation of pahoehoe.

Two more photos are below. What factors do you think were important in determining what type of lava form in each photo?

Aa lava flows on the pali at Royal Gardens. Photo courtesy of U.S. Geological Survey, July, 19, 1983.

Pahoehoe lava from a fissure eruption near Puu Oo. Photo courtesy of the U.S. Geological Survey, July 21, 1986.

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