National monument, western New Mexico, U.S. Located at an elevation of 6,4008,400 ft (1,9502,560 m), it covers 114,716 acres (46,424 hectares), including a lava flow area of 85,000 acres (34,400 hectares).
Features include a 17-mi (27-km) lava tube system, a number of ice caves, volcanic cinder cones, one of New Mexico's largest natural arches, and more than 20 gas and lava spatter cones. Designated a national natural landmark with the name Grants Lava Flow in 1969, it became a national monument in 1987.