Rhyolite is a rock formed from silica-rich magma, a much rarer rock than basalt. it often forms small bodies of rock that piles up over its vent to form a lava dome.  Domes are found along the keweenaw, in places like mt houghton. Rhyolite is full of quartz, a very hard mineral, and the rock itself is very hard.

Earth’s youngest rhyolite, Chaiten, Chile; March 2010

Dave Schneider photo

Bill Rose
Professor of Petrology
15 November 2010
Cornwall, 1954, USGS
Bedrock Geology, Lake Medora Quad GQ 52