Below: SCUBA-diving scientist's view of incandescent lava breaking through the solidified shell of a pillow-lava lobe to form another tongue as underwater flow advances during the 1969-71 Mauna Ulu eruption. (Photograph by Richard Grigg, University of Hawaii.) Right: Pillow lava on the submerged western slope of Mauna Loa at a water depth of about 2,500 feet. The research submarine's mechanical arm (right) can be manipulated by scientists on board to collect samples. (Photograph by Daniel Fornari, Lamont-Doherty Geological Observatory of Columbia University.)

Lava breaking through pillow-lava lobe
The contrast between the surfaces of pahoehoe and aa flows is immediately obvious to anyone hiking Hawaiian lava fields. Walking on dense pahoehoe can almost be as easy as strolling on a paved sidewalk. But walking across aa is like scrambling over a building-demolition site or battle zone, strewn with loose, unstable debris of all shapes and sizes. The jagged rubble of aa flows quickly destroys field boots and, should the hiker stumble or fall (not at all uncommon), it can tear clothing and flesh.

Many Hawaiian lava flows solidify as pahoehoe throughout their extent, and a few flows solidify completely as aa. Most flows, however, consist of both pahoehoe and aa in widely varying proportions. In a given flow, pahoehoe upstream commonly changes to aa downstream, but aa lava flows do not change into pahoehoe flows. The explanation for this oneway change lies in the delicate balance between the initial gas content of the lava, the changes in lava viscosity, and the rate of deformation ("shear strain") of the lava during flow and cooling. Once this critical balance is upset, pahoehoe can change to aa.

Pillow lava, 2,500 feet below

Hawaiian lava is fluid enough to travel great distances, especially if it is transported through lava tubes. Some historic flows are longer than 30 miles; in general, pahoehoe flows tend to be longer than aa. Lava tubes may be preserved when the eruption ends and the lava drains away to leave open tunnels. They may be as much as several tens of feet in diameter, and some have been followed by spelunkers (cave explorers) for nearly 10 miles. Ancient Hawaiians used lava tubes as places of shelter and as burial caves. Visitors to Hawaii Volcanoes National Park can walk through Thurston Lava Tube, which formed in a pahoehoe flow a few hundred years ago.

Fluid lava erupted or flowing under water may form a special structure called pillow lava. Such structures form when molten lava breaks through the thin walls of underwater tubes, squeezes out like toothpaste, and quickly solidifies as irregular, tongue-like protrusions. This process is repeated countless times, and the resulting protrusions stack one upon another as the lava flow advances underwater. The term pillow comes from the observation that these stacked protrusions are sack- or pillow-shaped in cross section. Typically ranging from less than a foot to several feel in diameter, each pillow has a glassy outer skin formed by the rapid cooling of the lava by water. Much pillow lava is erupted under relatively high pressure created by the weight of the overlying water; there is little or no explosive interaction between hot lava and cold water. The bulk of the submarine part of a Hawaiian volcano is composed of pillow lavas.

Back | Home | Forward

Maintained by John Watson
Updated 05.01.97