9.Pahoehoe, aa, and blocky lavasDan Barker March 2009

Mauna Loa

Basaltic lava flows show a variety of styles that depends on viscosity, cooling history, and topography. The most distinctive is pahoehoe or "ropy lava". While it is moving, this has a glassy skin only a few millimeters thick that is still hot and pliable enough to be folded by drag from the liquid within. Singlepahoehoe flows are usually less than 1 meterthick, with shiny and ropy or billowy surfaces.Low extrusion rates (less than 5 to 10 m3 persec, high temperature, and low crystal contentfavor the formation of pahoehoe.Pahoehoe lavas can travel surprisingly long distances through tubes that are thermallyinsulated by solidified porous lava.

Pahoehoe, Mauna Ulu, Hawaii

Pahoehoe, Mauna Ulu, Hawaii. Note the "sticky" behavior.

Corrugations on pahoehoe are usually (but not always) convex in the direction of flow.

Pahoehoe draped on Holei Pali fault scarp, Hawaii

Young pahoehoe covers a real estate development at Kalapana, Hawaii.

Active pahoehoe (arrow), Kalapana Gardens, Hawaii

"Breakout" of fresh lava from under a pahoehoe slab, Kalapana Gardens, HI

Pahoehoe swirls, Mauna Ulu

Swirl, Royal Gardens, HI

Still another swirl, Royal Gardens, HI

Pahoehoe, Isla Fernandina, Galapagos

Lava lizard on pahoehoe, Isla Fernandina, Galapagos. Note the adaptive coloring.

1855 pahoehoe, Mauna Loa, Hi

Superimposed pahoehoe flows at Krafla, Iceland, resemble breaking waves of water.

A filled pahoehoe tube is outlined by layers with varying concentrationsof vesicles. Makapuu Point, Oahu.

Broken pahoehoe tubes, Mauna Ulu

Lava tube, Devil's Orchard, Craters of the Moon, Idaho

Tube in pahoehoe, Mauna Loa

Tube in 10th century flow, Blafell, Iceland

Filled lava tube, Makapuu Point, Oahu. G.P.L. Walker for scale.

Inflated pahoehoe, 1969 flow, Mauna Ulu

Shelly pahoehoe, the ankle destroyer,1783 eruption, Laki, Iceland.

Oxidized pahoehoe interior, Royal Gardens, Hawaii

Another lava type, transitional from pahoehoe, has been proposed. Toothpaste lava has a smooth but not as shiny surface, decorated with grooves and drawn-out spines.Toothpaste lava is more viscous, and has a higher crystal content, than pahoehoe. Few volcanologists think there is a need for defining this type.

Here are the only two examples I have seen.

Toothpaste lava, 1951 eruption, Fogo, Cape Verdes

Toothpaste lava, Fleener Chimneys, Lava Beds National Monument, CA

Upturned slab of pahoehoe, Royal Gardens, HI

Upturned slabs of pahoehoe crust, Bonito flow, Sunset Crater AZ

The two previous images illustrate thebeginning of a transition from pahoehoe to aa lava. As pahoehoe flows, its crustbecomes thicker and more brittle, sosharp-edged plates rotate and produce arough spiny surface. Pahoehoe lavascommonly change to aa flows downstream,but the reverse has never been observed.Also, aa can emerge directly from a vent, without going through the pahoehoe stage.There is a handy way to remember the distinction between pahoehoe and aa;ah-ah is the sound you make when walkingbarefoot on aa.

Aa flow of 1910, Etna

Section through 1983 aa flow, Etna. Notethe rubbly base and top, and the massive interior.Aa flows are thicker than pahoehoe, andsome are fed by interior tubes similarto those in pahoehoe.

1986 aa flow in caldera, Izu Oshima, Japan

Aa flow of 1986 in caldera, Izu Oshima, Japan

Road diverted by 1983 aa, Etna

House overrun by 1983 aa, Etna

Aa and pahoehoe, Mauna Ulu

Blocky lava flows are the otherextreme from pahoehoe. Thesurface is covered with blocks,but generally the interior is massiveand crystalline. Blocky flows advance in a "tank tread"manner, in which the brittle top is carried forward, falls off the front, andis overridden by the flow. This resultsin thick breccia at the top and bottomof the flow. Some aa flows also actthis way.

This is a blocky dacite flow, Medicine Lake volcano, CA. J. Keller for scale.

The front of a blocky flow, Montana Rojada, Tenerife

Blocky flow, McKenzie Pass, OR

Surface of blocky flow, McKenzie Pass

Uncommonly, blocky flows may produce "toes" as evidence of a liquid interior.Snaefellsness, Iceland.

The base of this blocky flow at Volvic, France, is only remarkable because in 1751 Guettard followed this rock layer upslope into a scoria cone, proving that the lava was not sedimentary.

Finally, two bits of miscellany. This lava flow invaded a stream channel and engulfed the alluvium. Rio Blanco, Rincon de la Vieja, Costa Rica.

Here basalt lava, informally called the Thelma and Louise flows, spilled into the western Grand Canyon.