Photograph by Gordon Tribble and courtesy of U.S. Geological Survey. Eruptions under water or ice make pillow lava. Pillow lavas have elongate, interconnected flow lobes that are elliptical or circular in cross-section. Pahoehoe near the coast of Kilauea. Photo by Steve Mattox, 1989.
Pahoehoe lava is characterized by a smooth, billowy, or ropy surface.
Pahoehoe flows tend to be relatively thin, from a few inches to a few feet thick. In map-view the flows tend to be narrow and elongate. Photograph by R.W. Decker, U.S. Geological Survey, July 2, 1983. Aa is characterized by a rough, jagged, spinose, and generally clinkery surface. Aa lava flows tend to be relatively thick compared to pahoehoe flows.