Andrew Hall: Granite Cauldron and the Boiling Boulders | Thunderbolts

Between the Borrego Desert and the Cleveland National Forest in eastern San Diego county, California, dwell the In-Ko-Pah Mountains. It’s a barren, broken land, forsaken to brooding hoodoos, rodents, snakes and decomposing rock. Here granite boulders appear as bubbles of boiled sand. Not hollow, but solid drops of granite, formed as boils at the surface of a hot fluid matrix. ‘Cauldron of Granite’ is an apt description. The energy came from below, and by electrolysis and thermal convection, boiled through layers of sand to the surface, liquifying the sand in the process, like one would melt metals in a cauldron. Many boulders look like the kind of candy made with drops of molten sugar. A viscous, thick material that quickly cools to form a skin that holds its drop shape, while the inside remains hot and liquid. The shapes are like frozen beads of molten glass—they look like drops and bubbles because they are. Author and engineer Andrew Hall examines the reason boulders boiled from the Ear