page_title: Adel Mountains Volcanic Field
section_title: Description
hierarchical_section_title: Adel Mountains Volcanic Field / Description
caption_attribution_description: English: Northwest view of Square Butte, a laccolith in the Adel Mountains Volcanic Field in west-central Montana. From: Stephen S. Harlan, Lawrence W. Snee, Mitchell W. Reynolds, Harald H. Mehnert, R.G. Schmidt, Steve D. Sheriff, and Anthony J. Irving. "40Ar/39Ar and K-Ar Geochronology and Tectonic Significance of the Upper Cretaceous Adel Mountain Volcanics and Spatially Associated Tertiary Igneous Rocks, Northwestern Montana." Professional Paper 1696. U.S. Department of the Interior, U.S. Geological Survey. Reston, Virginia: 2005
context_page_description: The Adel Mountains Volcanic Field is an ancient volcanic field of heavily eroded 75-million-year-old igneous rocks about 40 miles long and 20 miles wide 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.
context_section_description: 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. Two concentric rings of laccoliths lie in the northwest section of the field 11 miles (18 km) and 14 miles (23 km) from the center of the area. Square Butte, Shaw Butte (near the town of Ft. Shaw), and Cascade Butte (next to the town of Cascade) are three laccoliths belonging to the Adel field which are famous local landmarks. More than 2,000 feet (610 m) of rock which used to exist on top of Square Butte has subsequently eroded to reveal the igneous rock which forms the laccolith. Each laccolith was fed by a radial dike at its distal end where it rolled over from vertical to form the horizontal laccolith. Laccoliths spread from the end of their feeder dike towards the more-elongate axis of the parent volcano, in response to load-imposed stresses of the volcano. 
  Adel Mountain is the highest peak in the volcanic field, at 7,093 feet (2,162 m) above sea level, and an unnamed peak in the field is the second-highest at 6,845 feet (2,086 m) above sea level.
caption: Northwest view of Square Butte, a laccolith in the Adel Mountains Volcanic Field.
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