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Pillow
lavas are bulbous, spherical, or tubular lobes of lava. They form during
eruptions with relatively low effusion rates. Slow extrusion gives enough
time for a thick crust to form on all sides of a pillow lobe, and prevents
individual pillows from coalescing into a sheet. Internally the pillows
are fed via a distributary system of interconnected channels. Pillows
are not typically hollow and tend to solidify all the way through. Pillows
often have lineations or scrape marks on their sides that form during
extrusion. Pillow flows are produced by the piling up of individual pillow
lava lobes. As a pillow flow forms, the newest pillows are erupted from
the top of the stack and flow outward a limited distance before freezing,
a process which tends to produce steep-sided mounds or ridges which can
grow to be 10's of meters thick. Within
the 1998 eruption area at Axial Volcano, pillow lavas only occur along
the edges of the flow, where lobate lava fronts were thin and stalled
out. Castle Vent is located on an older pillow ridge that formed prior
to the 1998 eruption.
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