Explosive volcanic activity in Yellowstone did not stop with the major eruption that formed the Yellowstone caldera 630,000 years ago. Another later eruption blew out a crater beneath West Thumb. The crater later filled with water becoming part of Yellowstone Lake.
Calderas differ from craters in size and way they form. Craters are smaller and are formed by the explosive removal of material. A caldera is formed following the removal of magma from a relatively shallow source either through eruption or withdrawal of the magma to a new reservoir. With out the support of the magma, the overlying rock collapses into the void left by the magma forming a typically circular depression.
The hydrothermal features along the shore are evidence of the continued presence of volcanic activity in the area.
Surrounding these features is siliceous sinter. Super heated ground water deep below this area is flowing through rhyolite, a volcanic rock rich is silica. The ground water dissolves small amounts of silica and transports it up to the surface. At the surface, the water temperature and pressure drop and the silica redeposited at the surface as siliceous sinter. This sometimes forms thin sheets that appear to be solid ground. It is believed that siliceous sinter only forms above ground.
Logging requirements:
Send me a note through geocaching.com with :
- The text "GC15A0V West Thumb Geyser Basin and Crater" on the first line
- The number of people in your group.
- Based on the information panel (at the coordinates), when did the west thumb crater form and how does that relate to the formation of the Yellowstone Caldera
- Relate the size and shape (check the informational panel) of the Yellowstone Caldera and the West Thumb crater and explain why one is a crater and the other is a caldera
- look at the sides of the feature(s) at (N44 24.987 W110 34.300) and explain why the Park Service build the boardwalk in this area.
- Look at where siliceous sinter is present near Lakeshore Geyser, Fishing Cone and Big Cone. What does that say about the water level of Yellowstone Lake over time.
The above information was compiled from the following sources:
- NPS informational Pannel
- Fritz, William J., Roadside Geology of the Yellowstone Country, Mountain Press Publishing Company, May 1989.
- West Thumb Geyser Basin Trail Guide including Fishing Cone and Thumb Paint Pots, Yellowstone National Park, Yellowstone Association, April 2007
- The Geologic Story of Yellowstone National Park, William R. Keefer Illustrated by John R. Stacy, GEOLOGICAL SURVEY BULLETIN 1347 1975.
Placement approved by the
Yellowstone National Park