Lava Flows

pahoehoe lava
Pahoehoe lava and Hawaii Volcanoes National Park, Hawaii.

USGS photo.

When magma is erupted from a volcano, it moves down the slope of the volcano and surrounding topography as it cools. Lava can flow for distances of over 100km, and at speeds near 30 miles per hour. Depending on the chemistry, viscosity, and eruption style, lava flows can have very different and distinct appearances.

Pahoehoe is a smooth, ropy lava, common on the Hawaiian islands. 'A'a is a more chunky and thick flow, that is most commonly associated with composite volcano eruptions. 'A'a lava flows are also called clinkers, because of their rough and jagged appearance.

The eruptions of submarine volcanoes result in pillow lavas, which form as the lava cools in seawater. Plateau basalts occur as a result of fissure eruptions of basaltic lava. Fissure eruptions occur along elongated cracks in Earth's surface rather than from a single vent or pipe. Magma wells up through the crack, and pours out on either side in effusive eruptions. These eruptions form extensive sheets of basaltic lava. Columnar basalts are another feature that can form on basaltic lava flows when they cool uniformly. The lava cracks into vertical columns, typically hexagonal in shape.

‘A’a

`A`a (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough surface. The top layers of these flows are composed of broken lava blocks. The sharp and rugged surface of a hardened `a`a flow makes for treacherous and slow walking conditions.

The most active area of the flow is the core, which is much denser than the rough surfaces at the top of the flow, and underneath it. The dense, pasty lava in the core of the flow travels downslope, carrying the jagged chunks along the top of it. The cooler chunks can fall down the leading edge of the flow and be covered with advancing lava. This creates the rough layer below the core of an 'a'a flow.

It is possible for a pahoehoe flow to become and 'a'a flow, but 'a'a flows never become pahoehoe flows.

Pahoehoe

"Pahoehoe" is a Hawaiian word used to describe a lava flow with a smooth, ropy surface. Pahoehoe flows advance slowly, with small amounts of lava squeezing out of a cooler crust. Pahoehoe flows can exhibit all kinds of different shapes as they form and cool. These are sometimes called "lava sculpture."

Last updated: June 12, 2018