“Bread-crust” bombs

“Bread-crust” bombs are the typical eruption material of vulcanian activity. They are formed of a compact and intensely fractured outer “crust” and a spongy inner portion visible only inside the fractures.
The resemblance to bread crust is often surprising and even the method of formation is similar. In the case of bread, the crust initially formed in the baking process is subsequently broken by the expansion of its soft insides, when the heat makes the water vapour of the dough and the carbon dioxide produced by the yeast swell.
In the case of volcanic bombs, the crust is formed by the cooling of the outer part of the large lava crumb during flight. The rigid crust is subsequently broken by the expansion of the hot and molten core that swells to release gas.