You are standing at the site of one of the largest volcanic explosions in geologic history. The Henrys Fork Caldera is located in eastern Idaho and is part of the Yellowstone hotspot track, which has a significant volcanic history. It is a key feature of the larger volcanic province associated with the movement of the North American Plate over a stationary mantle plume (the Yellowstone hotspot).
Formation of Henrys Fork Caldera
Henrys Fork Caldera was formed around 1.2 million years ago during a massive volcanic eruption, part of the Island Park volcanic complex. This eruption produced an enormous volume of rhyolitic lava and volcanic ash, leading to the collapse of the land above the emptied magma chamber, forming the large, circular depression known as a caldera.
- Volcanic Eruption: A major explosive volcanic event occurred, ejecting vast amounts of pyroclastic material into the atmosphere.
- Caldera Collapse: Once the magma chamber was emptied, the overlying crust collapsed, creating the Henrys Fork Caldera. The collapse was catastrophic, resulting in the large, 23-mile-wide (37 km) caldera.
- Aftermath: Following the collapse, volcanic activity persisted, producing lava flows, smaller eruptions, and further reshaping the landscape.
Geological History
- Yellowstone Hotspot Movement: The Henrys Fork Caldera is part of the volcanic history of the Yellowstone hotspot, which has been active for at least 17 million years. As the North American Plate moved southwestward over the stationary hotspot, it triggered a series of volcanic eruptions. The hotspot created a track of volcanic features, including the Snake River Plain and several other calderas, such as the one at Yellowstone National Park.
- Island Park Caldera and Relationship to Henrys Fork: Henrys Fork Caldera is a part of the Island Park Caldera, a larger caldera system formed during different volcanic phases. The Island Park Caldera is older, with Henrys Fork being the product of one of its later eruptions.
- Tuff and Ash Deposits: The eruption that formed Henrys Fork Caldera created vast ash deposits, known as the Mesa Falls Tuff. These tuff layers spread over a wide area, adding to the complex stratigraphy of the region.
- Post-Caldera Volcanism: After the main eruption, smaller volcanic events continued for thousands of years, producing rhyolitic lava flows and domes. These events helped shape the modern landscape and contributed to the area's geothermal activity.
- Yellowstone Caldera: About 640,000 years ago, the Yellowstone Caldera to the east formed in another massive eruption, making the entire region one of the most volcanically active in the world. Both calderas are part of the larger Yellowstone volcanic system.
Modern Landscape
Today, Henrys Fork Caldera is a topographically subdued caldera compared to the more famous Yellowstone Caldera, but it remains geologically significant. The area around it features high plateaus, rhyolite lava flows, and evidence of glaciation that occurred after the volcanic activity. It is part of the greater volcanic and geothermal landscape of the Yellowstone region.
In summary, Henrys Fork Caldera was formed by a massive volcanic eruption 1.3 million years ago, associated with the movement of the North American Plate over the Yellowstone hotspot. It is part of the complex volcanic history of the region, which includes several caldera systems formed over millions of years.
References
Information above was referenced from the following sources:
Activities
From GZ read the information signs and also look around to make some observations:
- The curving ridgeline you see to the southwest forms a portion of the Henrys Fork Caldera -- the remains of a volcano that exploded and collapsed into itself. Estimate the angle of the slope of the caldera and provide your thoughts as to what this says about how it was formed
- Is the caldera rim rounded or rigid and what does this say about the geologic history?
- Look around GZ for examples of tuff, a type of igneous rock that forms when volcanic ash or dust is compacted and cemented together, and describe it (color, texture, etc).