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The St. Francois Mountains

Decent Essays
When thinking about volcanic outcrops, one generally thinks of islands and plate boundaries such as the northwestern coast of the United States, Hawaii, or New Zealand. However, 1.4 billion years ago, super volcanoes characterized the Midwestern United States. Today, the Midwest is known for its’ rather simple “layer cake geology.” The Midwest is a part of the North American Craton, meaning that it has remained relatively stable for the past 600 million years. Thus, a volcanic field is definitely not the first thought to pop into someone’s mind when thinking about this region. However, the Midwest is home to the St. Francois Mountains, which is a topic that is under represented in the geologic curriculum. The St. Francois Mountains are located…show more content…
We went on a weekend trip to Fredericktown, Missouri and stopped at different locations in the area trying to understand the different igneous features we were seeing. The area is packed with igneous exposures from columnar basalts, Precambrian basement rock, granitic ring complexes, volcanic ejecta, to mantle xenoliths. Together, these features provide insight into the overall geology of the region, indicating that this terrain was once an active nested caldera complex (multitude of calderas within calderas). The highly silicic nature of these rocks indicates that calderas formed from the collapsing of rhyolitic dome structures, which sometimes in geologic literature is referred to as “super volcanoes.” The “super volcanoes” here are thought to have formed, the way many calderas do, by a subsurface pluton pushing its way up into the crust and erupting. Once the plutonic material erupts, the land subsides and the central part of the once outstretched region becomes a crater, which is termed caldera. In the case of the St. Francois Mountains and the highly silicic volcanics that are found in the region the plutonic source is potentially granitic in nature. However, there are mafic exposures, which are result of occasional basalt and mafic melts making their way to the surface through mantle source. Geologically, the St.Francois Mountains are complex, and as a regional feature, I do not know the entire evolutionary history for their
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