The broad McDermitt Caldera is a prominent geologic feature in the Trout Creek Mountains . The oval @-@ shaped caldera is a collapsed lava dome that straddles the Oregon – Nevada border on the eastern side of the range and south of the Oregon Canyon Mountains . It is about 28 miles ( 45 km ) long and 22 miles ( 35 km ) wide . The lava dome was created by volcanic eruptions in the early Miocene . A total of five large ash flows were produced along with a large rhyolite dome structure . The caldera formed when the dome collapsed about 16 million years ago . The caldera contains significant ore deposits , and mercury and uranium have been mined at eight or more sites in and around the caldera . Other areas in the caldera were mined for ores of antimony , cesium , and lithium .