20150705 - pillow lava

Pillow lava 

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.

 http://www.pmel.noaa.gov/eoi/nemo/explorer/concepts/pillows.html
 https://en.wikipedia.org/wiki/Pillow_lava