Interpretation of Heat Flow and Seismic Data from

Yellowstone Lake, Yellowstone National Park

Petru T. Negraru and David D. Blackwell (AGU2004 abstract)

Abstract

The heat flow map of Yellowstone Lake shows a complex thermal regime with variations of more than two orders of magnitude. The major geothermal systems mapped are within the caldera boundary, in the northern part of the lake and in West Thumb. In the northern part of the lake two areas of extremely high heat flow are identified: an area east of Stevenson Island (heat flow more than 10000 mW/m²), not previously known to have high heat flow and Mary Bay-Sedge Bay geothermal system (up to 40000 mW/m²). These are separated by an area with background heat flow of 1500 to 2000 mW/m² which correlates with large sediment thickness. Therefore the whole area from east of Stevenson Island to the Mary Bay is interpreted as a single geothermal system variably attenuated by the conductive effects of the sediments. Seismic data show that the western boundary of this high heat area is the complex graben which extends from the Lake Hotel in an approximately north-south direction east of Stevenson Island. We present evidence (heat flow, seismic and geologic features observed from underwater video) that the Hotel Fault, one of the graben faults, extends on the east side of the Stevenson’s Island, not on the west. West of the graben the heat flow could be as low as 150 mW/m². Although several topographic notches similar to the hydrothermal vents east of Stevenson Island were identified west of the graben structure, the nearby heat flow values are low, suggesting the presence of cold, not hot springs. The decrease of the gradients to the south is more gradual and is not associated with major faulting or with the caldera boundary. The other important geothermal system is West Thumb, but no extremely high heat flow values were found.

The areas east of Stevenson Island and Mary Bay – Sedge Bay are identified as potential geothermal hazards. However, in Mary Bay, at the hottest point located in the lake, the boiling point temperatures are reached at more than 5 m below the bottom of the lake, in a water depth of 40 m. If a geothermal explosion were to occur it would require a sudden drop of water level with at least 6 m. By connecting several discontinuous features it has been argued that the maximum possible length of the Hotel Fault, the largest fault in the lake, is 25 km. However, the fault was mapped only north of Stevenson Island (for about 3 km). In the south it could be masked by deformed area in the east of Stevenson Island (up to 8 km). No link to the Eagle Bay fault to the south was observed in sparker records and thus the maximum fault length is only 12 km. We argue that a fault 12 km long could not offset the bottom of the lake by 6 m. The overall low stress condition in the caldera (weak crust) support the presence of a high number of small events rather than large single fault events.

Heat flow map of Yellowstone Lake (in mW/m²) and location of heat flow points. The heat flow values for different areas of the lake are: West Thumb ~2300 mW/m², East of Stevenson Island ~10000 mW/m², Mary Bay – Sedge Bay ~40000 mW/m², West of Stevenson Island ~150 mW/m², southern Yellowstone Lake <150 mW/m².

Comments could be sent at:

Petru Negraru negraru@mail.smu.edu

David D. Blackwell blackwel@mail.smu.edu

Additional information about Yellowstone Hot Spot can be found at:

The Yellowstone Volcanic Observatory

Dr. Robert Smith’s (University of Utah) research web page