Were/are there volcanoes in Montana?
If you take a look at the geologic map of Montana depicted on the Bureau’s post card, you will see areas of rocks labeled as “igneous extrusive”. These are volcanic rocks and most of them are concentrated in southwestern or north-central Montana. The volcanoes that produced most of these rocks have not been active for 50 million years or so, and because of erosion they do not look like volcanoes any more. Some of the better exposures of these volcanic rocks are preserved southwest of Great Falls where I-15 traverses the Missouri River Canyon between Craig and Cascade. Here, peculiar structures that look like rock walls extend for miles across the country side; geologists call these structures dikes, and they represent magma that rose in fissures toward the earth’s surface but froze before it actually made it there. Yellowstone National Park itself is a volcano (caldera) that produced huge eruptions of volcanic ash only 600,000 years ago. Although the caldera does not extend into Montan
