Hell's Half Acre: A Case Study

The purpose of this paper is to compare the angular basalt cobbles lining the paved trails at Hell’s Half Acre with samples taken from the basalt flow at the same location in order to determine if the cobbles are from the surrounding lava flow. A trip was made to Hell’s Half Acre where observations were recorded and three samples of both the basalt from the trail and the flow were collected to visually compare physical and mineral properties. The study area is a unit of relatively young Quaternary lava flows found on the Snake River Plain known as Hell’s Half Acre. Trails through the lava flow are located at the Blackfoot Rest Area on I-15 in southeastern Idaho between Blackfoot, Idaho and Idaho Falls, Idaho. All observations were made and samples collected from the trails at the northbound rest area.

The rocks are pebble to cobble sized basalt fragments. The samples collected ranged from 50mm to 120 mm. The rocks are angular. They have a gray, very fine ground mass. Fine plagioclase feldspars are visible throughout the ground mass and sparkle in the sunlight. The rocks do not show signs of heavy weathering. There is some white staining visible, but there is very little red or brown staining. The rocks have abundant vesicles. There is a mix of rocks with varying sizes and numbers of vesicles. Samples collected were representative of this mix. Light green olivine phenocrysts are visible with the naked eye and more are visible when viewed through a hand lens. The olivine is visible on both weathered and fresh surfaces in all three samples. Also found mixed with the basalt rocks lining the trails are intermittent sub-rounded quartzite pebbles and