Adel Mountains Volcanic Field

The Adel Mountains Volcanic Field (also known as the Adel Mountains, Adel Volcanics, and Adel Mountain Volcanics) is an ancient volcanic field of heavily eroded 75-million-year-oldigneous rocks about 40 miles long and 20 miles wide (800 square miles, or 2,071 square kilometers) in west-central Montana about 30 miles southwest of the city of Great Falls. The area was named by geologist John Bartholomew Lyons, who first described the general geology of the region in 1944. The Adel Mountains Volcanic Field is a significant and abundant source of shonkinite, a very uncommon type of intrusive igneous rock found primarily in Montana, Ontario, and Timor. Because of its geologic structure, the Adel Mountains Volcanic Field has drawn the attention of geologists for more than 100 years.

The Adel Mountains Volcanic Field is a 3,280-foot (1,000 m) thick unit which lies unconformably on top of Cretaceous sedimentary rock of the Two Medicine Formation on the edge of the Great Falls Tectonic Zone. The extrusive rock is composed of breccia and conglomerate. The intrusive rock has formed numerous dikes, sills, and plugs. An "enormous number of dikes...emanate radially from its center", many of which are visually striking and vertical, and can be seen from the interstate highway and other roads in the area. Many of the dikes clearly fed laccoliths in the area.

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