The Boulder Batholith originated as part of the Elkhorn Mountains Volcanics, as molten magma rose through the earth's crust from 81 to about 74 million years ago and, when it reached the surface, created violent explosions that hurled chunks of rock, cinders, and volcanic ash into the air. This volcanic field was about 100 miles in diameter and up to 3 miles thick. After the pile of volcanic rocks became too thick, magma stopped reaching the surface and accumulated near the bottom of the pile, where it cooled into a granitic body called a batholith. Granite like that exposed along Interstate Highway 15 and east of Butte became the host rock for the ores mined at Butte, having formed by slow cooling deep below the earth's surface about 76 million years ago. Faults and fractures in the Butte area later cut the granite and created pathways for hot water carrying dissolved metals; as these solutions reacted with the enclosing granite, they cooled and deposited quartz and metallic minerals in veins, some as large as 50 feet wide and 4,500 feet long. The discovery of copper-rich veins, together with the need for copper wire for electrical use from 1880 on, stimulated the development of many underground mines and the growth of Butte, which rose from a few dozen gold prospectors in 1864 to a reported population of 91,000 in 1917. Headframes from some former underground shaft mines still stand above the tan rock on the sides of the Berkeley Pit. Open-pit mining began there in 1955 and continued until mid-1982, and copper and molybdenum ore are currently mined in the Continental Pit west of the Interstate Highway.