Geology of Hell's Half Acre
The Hell's Half Acre Lava Flows are within the Snake River Plain. The Snake River Plain is an expansive crescent-shaped depression 50 to 70 miles wide and 350 miles long that stretches across southern Idaho. This plain is composed of lava flows formed over the last 15 million years that originated from volcanic vents and fissures.
The Snake River used to flow across the northern portion of the Snake River Plain, but now flows along the southern margin of the Plain. The river was pushed there by successive lava flows. Hell's Half Acre flows over about 222 square miles or 162,000 acres. Sixty-six thousand acres are within a Wilderness Study Area (WSA).
The Hell's Half Acre flows came from a vent that is located along a rift. This rift parallels the 62 mile long Great Rift that goes through Craters of the Moon National Monument. A rift zone is a line of weakness in the earth's crust associated with volcanism. The main vent for the Hell's Half Acre flow is 95 to 200 feet wide and 730 feet long. The highest point is 5350 feet; whereas, the lowest point is 4600 feet. The vent has 13 pit craters where lava flowed out and then receded. The last flow from the main vent occurred 2,000 years ago. The older flows near Highway 20 are estimated to be over 4,100 years old and came from an eruptive crater about 3.5 miles in diameter. These older flows were covered by more recent flows from the main vent.