Historic Resource Study, Crater Lake National Park, Oregon, 1984
V. Geological and Biological Information on Crater Lake Area
E. Points of Geological Interest in Crater Lake National Park
16. The Pinnacles
These towering needle-like formations of rock, or stalagmites, called fossil fumaroles, projecting from the Sand Creek Canyon floor, were formed under sheets of volcanic pumice that preceded Mazama’s collapse. As the surface of the hot pumice cooled over the years, steam and gases were released by the hot rocks underneath through vents and tubes that were welded into cement hardness by their passage. These ancient vents now stand alone due to the erosion of the surrounding softer materials.
Illustration 3. The Pinnacles. Courtesy Oregon Historical Society.