Bread-crust bubble
Bred krəst ˈbəb(ə)l n.
Tiny, gas-filled beads of volcanic ash with a scaly surface.
Scientists have identified a new type of volcanic ash that erupted from a volcano in central Oregon roughly 7 million years ago. The particles are similar to larger bread-crust bombs, which form as gases trapped inside globs of lava expand, cracking the bombs’ tough exterior. Bread-crust bubbles, each no more than a millimeter wide, have a distinctly crackled surface that can reveal secrets about how volcanoes erupt, researchers reported November 4 at the Geological Society of America annual meeting in Indianapolis.