Geology of Craters of the Moon National Monument, Idaho

Murtaugh, J

The Craters of the Moon National Monument, Idaho is an area of Quaternary basaltic flows representing the latest phase of the volcanic eruptions of the Snake River Plain. These rocks overlie folded Oligocene quartz latite vitrophyres and basaltic vitrophyres, and Miocene quartz monzonites. Most of the basaltic flows issued from vents 1n the Great Rift Zone, a northwest-trending, elongate area of weakness in the crust that is parallel to fold axes 1n the older rocks. There may have been four periods of eruption, but a significant time interval existed only between the first· period and the later periods. The primary vent structures include cinder cones, spatter cones, spatter ramparts, and lava domes. Spindle, ribbon and breadcrust bombs are common. Cow-dung bombs are rare. The rootless vents which are those not directly connected to the magma conduit, formed over cracks in the surfaces of t some flows. Secondary lava domes are formed by pahoehoe flow units that have broken through the roofs of lava tubes. Hornitos are cones of spatter formed over rootless vents. More than fifty flows were recognized. in the Monument. They are mostly pahoehoe and aa, with a few block lava flows. The pahoehoe is basalt with a generally smooth crust that may be corded and festooned. Lava tubes as much as sixty feet in diameter are exposed by collapse. Pressure ridges are iv common. Squeeze-ups have broken through the crust, in places flowing out and forming now units. Squeeze-outs ot pahoehoe have broken through the margins of aa flows, in places becoming flow units. The as is basalt with a Jagged, spinose surface, and a central massive layer. Open channels braid the surfaces or some aa flows. The block lava flows are andesitic basalt. They are composed mostly of smooth, angular, discrete blocks, that may be mixed with aa. Monoliths found in the Monument are fragments of cinder cones that have been carried by some or the thick flows. Grooved lava occurs where plastic lava has been rubbed by hard surfaces. Xenoliths are fragments of pyroxene granulite that have been carried up from depth by the basaltic magma.