Amboy Crater is an ash-and-cinder cone 250 feet high and 15,000 feet in diameter in one of the youngest volcanic fields in the United States. It lies in the Barstow-Bristol trough, a conspicuous west-northwest trending physiographic feature that approximately straddles the boundary between the Mojave and Sonoran tectonic blocks. This field was created by at least four distinct periods of eruptions, producing a coaxially nested group of volcanic cinder cones. The most recent eruption of Amboy Crater occurred about 10,000 years ago. Its lava flows are basalt rich in magnesium, iron, and calcium, and close examination may reveal minute green olivine crystals. The crater’s red color indicates ferric iron and results from steam on heated rocks. Its recent origin and near-perfect shape led to its designation as a National Natural Landmark in 1973.