Adel Mountains Volcanic Field, Montana

June 15, 2018

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June 2018 Viewer's ChoicePhotographer: Rod Benson
Summary Author: Rod Benson 

The rocky slopes shown in this photo are in the northern part of the Adel Mountains Volcanic Field, about 30 miles (48 km) southwest of Great Falls, Montana. Volcanoes were active here during the late Cretaceous Period, but have undergone extensive erosion so none of the classic cone-shaped volcanic mounts remain. However, the layers in the photo are made of materials (lava, pyroclastics, etc.) that flowed down the side of one of the volcanoes some 70-80 million years ago. What’s left of the volcanic field forms a mountainous area about 40 miles (64 km) long and 20 miles (32 km) wide between Great Falls and Montana's capital city of Helena. Both the Missouri River and Interstate 15 cut right through the middle of it, providing drivers (or floaters) an opportunity to travel through the guts of an ancient volcano.

Volcanic activity in Adel Field was also responsible for numerous dikes and laccoliths that can be seen from the interstate highway and other roads in the area. Magma beneath the ancient volcanoes worked its way laterally through vertical fractures in surrounding sedimentary rock. In places, its upward movement was halted by a durable layer of sandstone that bulged as the magma pooled below. Eventually, the magma hardened, creating a network of dikes and laccoliths beneath the surface. Over millions of years, erosion has worn away hundreds of feet of overlying sedimentary rock, leaving remnants of these more resistant igneous formations. Today, those remnants form the impressive ridges (dikes) and iconic buttes (laccoliths) that makes this area geologically interesting and visually stimulating.

Photo Details: Camera: Panasonic DMC-ZS60; Software: iPhoto 9.6.1; Exposure Time: 0.0010s (1/1000); Aperture: ƒ/3.3; ISO equivalent: 80; Focal Length (35mm): 26.