How Volcanoes Work

 Image Described in Caption
Breadcrust Bomb -- The cracks on this breadcrust bomb developed when the brittle outer surface cooled and cracked due to expansion associated with the continued growth of gas bubbles in the interior. This example is a dacite breadcrust bomb (~15 cm across) that erupted from the Mt. St. Helens lava dome. Courtesy of D.W. Wieprecht.