Glass Mountain from Medicine Lake caldera rim. USGS photo by Julie Donnelly-Nolan.
The most recent eruption occurred around 1,000 years ago when rhyolite and dacite erupted at Glass Mountain and associated vents near the caldera's eastern rim. No field evidence has been found to substantiate a report of an eruption in 1910.
Glass Mountain consists of a spectacular, nearly treeless, steep-sided rhyolite and dacite
obsidian flow that erupted just outside the eastern caldera rim and flowed down the steep
eastern flank of Medicine Lake Volcano. Ten additional small domes of Glass Mountain rhyolite and rhyodacite lava lie on a N25degreesW trend to the north and one to the south.
The age of Glass Mountain and its preceding pumice deposits has been a matter of discussion for some time. A radiocarbon dating age of 885+/-40 years before present (1990) was obtained on a dead incense-cedar tree without limbs or bark that is preserved in the edge of one of the distal tongues of the flow. The dated material consisted of a piece of exterior wood containing about 30 annual growth rings.
This age may be too old, because some of the outside of the tree is missing.
The tephra deposits that precede the flow and domes may be somewhat older but are constrained to be less than about 1050 years before present (1990) by the Little Glass Mountain and Lassen Peak data.

Source: Wikipedia