"Puffed obsidian" from Panum Crater

This close-up view of some of the rhyolite that makes up Panum Dome shows some of the neat features of glassy volcanic lava. You can clearly see the dark, dense volcanic glass (obsidian) but there are also layers that look lighter and distinctly "puffed" out -- these are also made of volcanic glass, but they likely had higher concentrations of gas bubbles, something akin to pumice. As the bubbles expanded when the magma reached the surface and cooler, the layers puffed and cracked like breadcrust.

Image: Erik Klemetti