DEFINITION OF PAHOEHOE LAVA

A term of Hawaiian origin for a basaltic

lava flow with a twisted billowy surface that resembles a

coiled rope. The term was introduced by Clarence E. Dutton

in 1882. It is contrasted with other main types of basaltic

flows in Hawaii known as aa, characterized by a blocky, rubbly

surface, resembling bulbous pillows. There seem to be

several variables that cause one type of flow to form instead

of the other. Pahoehoe flows are generally richer in gas, are

hotter, and are slightly less viscous than their aa counterparts.

Pillow lavas form underwater.

There are several varieties of pahoehoe flows. Massive

pahoehoe are 6.5–50 feet (2–15 m) thick and are smooth for

large parts of the flow, whereas scaly pahoehoe flows resemble

a fish’s scales or a shingled roof, where numerous typically

2–12-inch (5–30-cm) thick lobes overlap preceding lobes.

Scaly pahoehoe grade into entrail pahoehoe, in which the

lobes expand into piled up masses or may grade into pillowlike

lobes. Shelly pahoehoe is characterized by numerous

burst bubbles on the surface producing a frothy texture, and

when the bubbles (vesicles) are very numerous they produce a

flow with large holes and thin vesicle walls known as reticulite

or thread-lace scoria. Slabby pahoehoe have surfaces

consisting of a number of broken, piled-up slabs, probably

produced by cracking of the surface when underlying lava

tubes drained.

Pahoehoe lava flows may have many other irregular features

on their surface, including spatter cones, linear ridges,

lava blisters, pressure plateaus, and squeeze-ups, where lava

from beneath the surface has pushed its way through the

overlying crust. Explosion tubes, or thin pipes known as pipe

amygdules, form where lava has overrun water, and the water

is converted to steam which explodes and bores a tube to the

surface. Where the lava flow has overrun something, such as

a house or tree, the object may leave a mold in the lava flow,

especially if it burns after the lava has surrounded it. Lava

tubes and tunnels are well developed in many pahoehoe

flows, forming where lava on the surface has cooled and lava

from the tube has drained by flowing to lower elevation

areas, leaving a tube-like cave behind. These tubes typically

form a branching network following the branching paths of

the lava flows across the surface. These tubes may be small,

but may be up to several tens of meters in diameter and many

are even tens of kilometers long. Lava tubes have made excellent

hiding places for bandits and chased armies at various

times and places in history.

See also AA; HAWAII; VOLCANO.

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