“A volcanic bomb of basaltic andesite, broken in cross section and clearly illustrating the cause of its "breadcrust" nature. A mass of porous, viscous, gas-charged lava was ejected during an explosive eruption. Gases immediately diffused from the exterior of the mass as it flew through the air, creating a dense, non-porous outer layer that cooled and hardened into a shell. However, being no longer confined in the volcanic conduit, the gases trapped in the interior of the mass continued to expand, causing the shell to crack like the crust of baking bread. The still-plastic interior held the pieces together at least briefly, but the cracking process causes many (probably most) volcanic bombs to shatter in flight or upon landing. However, some survive relatively intact--see https://www.mindat.org/photo-1168743.html . Collected on a Smithsonian Research Expedition to Arenál in May 1989. ”

“Exterior of the bomb. The cross-section surface is at the bottom. ”