Hell's Half Acre Lava Field
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The Hells Half Acre lava field, the easternmost of the young basaltic lava fields of the Snake River Plain, covers an area of about 400 km2 (155 mi2) southwest of Idaho Falls and is the second largest on the Snake River Plain.
Location: Idaho, Bonneville/Bingham Counties
Latitude: 43.5° N
Longitude: 112.45° W
Elevation: 1,631 (m) 5,351 (f)
Volcano type: Shield volcano
Most recent eruption: 5,200 years ago
Threat Potential: Low/Very Low *
The broad, low shield volcano was dominantly formed from basaltic pahoehoe lava flows that were erupted from a 3-km-long (1.9-mi-long) northwest to southeast trending vent system at the northwest part of the field during a brief eruptive episode about 5,200 years ago. The summit vent area contains an irregular, elongate 0.8-by-0.3-km (0.5-by-0.2-mi) wide central depression that was the site of a former lava lake that fed late-stage flows. About 10 circular pit craters truncate the surface of the lava lake, and two prominent lava tube systems are located near the summit vent complex. Two major lava flow lobes, each about 5 km (3 mi) wide and 10 km (6 mi) long, extend to the south and southwest to the flood plain of the Snake River and surround Morgans Pasture, a large kipuka. Interstate 15 crosses the southeast margin of the lava field southwest of the city of Idaho Falls.
There is no threatening hazard from Hell's Half Acre Lava Field, as it was a one-time (monogenetic) basaltic shield eruption, which will not recur. It is in the far southeastern region of the Lava Ridge-Hells Half Acre volcanic rift zone. Other eruptions in the past few hundred thousands of years have occurred from along all parts of this volcanic rift zone. A low degree of hazard exists along this rift zone, but it is much less than the hazard farther west along the Great Rift volcanic rift zone, particularly at its northern edge in the Craters of the Moon lava field.