How Volcanoes Work

 Image Described in Caption
 Accretionary Lava ball -- Note spiral appearance of this accretionary lava ball which rolled down the flow front of an a'a flow derived from the Pu'u O'o volcano, Hawaii. Rolling of a small fragment of solidified lava above sticky lava results in the accretion of lava in a spiral layer fashion. Courtesy of J.D. Griggs, USGS.