How Volcanoes Work

 Image Described in Caption
 RETICULITE -- Frothy basaltic rock sometimes called basalt pumice or thread-lace scoria. This delicate rock forms during vigorous fire-fountaining. Reticulite has the lowest density of any rock, with a porosity (vesicularity) up to 98 percent. The open honeycomb network of bubbles (vesicles) results from the bursting of cell walls against one another during vesiculation. Courtesy of J.D. Griggs , USGS.