page_title: Medicine Lake Volcano
section_title: Glass Mountain
hierarchical_section_title: Medicine Lake Volcano / Eruptive history / Glass Mountain
caption_attribution_description: Glass Mountain from Medicine Lake caldera rim, northern California
context_page_description: Medicine Lake Volcano is a large shield volcano in northeastern California about 30 miles northeast of Mount Shasta. The volcano is located in a zone of east-west crustal extension east of the main axis of the Cascade Volcanic Arc and the Cascade Range. The 0.6-mile thick shield is 22 miles from east to west and 28 to 31 miles from north to south, and covers more than 770 square miles. The underlying rock has downwarped by 0.3 miles under the center of the volcano. The volcano is primarily composed of basalt and basaltic andesite lava flows, and has a 4.3-by-7.5-mile caldera at the center.
  The Medicine Lake shield rises about 3,900 feet above the Modoc Plateau to an elevation of 7,795 feet. Lavas from Medicine Lake Volcano are estimated to be at least 140 cubic miles in volume, making Medicine Lake the largest volcano by volume in the Cascade Range. Lava Beds National Monument lies on the northeast flank of the volcano.
  Medicine Lake Volcano has been active for 500,000 years. The eruptions were gentle rather than explosive like Mount St. Helens, coating the volcano's sides with flow after flow of basaltic lava.
context_section_description: 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.  Fitch cites reports that a light ash fall that occurred in 1910 may have come from a small eruption at Glass Mountain.  No field evidence has been found to substantiate the 1910 eruption.
  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 1,050 years before present (1990) by the Little Glass Mountain and Lassen Peak data.
caption: Glass Mountain from Medicine Lake caldera rim. USGS photo by Julie Donnelly-Nolan.
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