File:West Thumb Caldera.jpg

English: West Thumb Caldera (looking ~NE) - Yellowstone Lake has a large, irregularly circular embayment on its western side called West Thumb. Reported ages for this feature include 140 ka, 161-162 ka, 174 ka, and 198 ka. Recent studies on the geology of the West Thumb area have concluded that it formed as a result of collapse and flooding by Yellowstone Lake’s water following a series of three, relatively large, explosive ash eruptions. These three ash deposits together form an estimated 50 km3 unit called the Bluff Point Tuff, which ranges in age from 170 ka to 200 ka. The collapse event formed the West Thumb Caldera, the original margin of which is now covered by post-caldera rhyolite lava flows.