Global Volcanism Program | Image GVP-03346
The thick cliffs are composed of massive ash-flow deposits of the Lava Creek Tuff, erupted about 640,000 years ago. Eruption of the 1000 cu km Lava Creek Tuff resulted in the formation of the present 45 x 85 km wide caldera, the latest of three large calderas formed at Yellowstone during the past 2 million years.
Photo by Lee Siebert, 1994 (Smithsonian Institution).
This image is made available under the Public Domain Dedication CC0 license, but proper attribution is appreciated.