Rhyolite is an igneous, extrusive volcanic rock high in silica forming typically in continental arc settings. The flow-banding in this sample is caused by the friction of the viscous magma in contact with a solid rock surface or by compaction, causing flattening and welding of shards and pummice.
[Be prepared for pictures of a bunch of cool volcanic rocks in the coming days! We’ve got a whole bunch of cool specimens out for our lab!]
Image from author
Emily Zawacki, Ph.D. 