Lava rock from Craters of the Moon, national monument, Idaho. National Park. Inside Indian Tunnel lava tube. 'The strangest 75 miles on the North American Continent claimed an early traveler. Great basaltic lava fields from when the earth opened in a great fissure erupting approx 15,000 years ago with eruptions up until last 2,000 years. There is no one volcano, rather a series of volcanic cones and lava flows forming part of the Great Rift zone. A surreal place to walk about with dark jagged rocks of peculiar shapes and sizes. Oct 2002.