At Taum Sauk Mountain, Missouri’s highpoint (1,772 feet), a light-colored rock can be found along the ground. Upon closer inspection, it is not a sedimentary rock, like one might expect in the middle of the contiguous United States. Instead, it is an igneous volcanic rock composed mainly of the minerals of quartz and feldspar. It’s known as rhyolite and it forms when lava and ash erupts from volcanoes and cools very rapidly. But…rhyolite? And volcanoes? In Missouri?

The geologic history of Taum Sauk Mountain and the St. Francois Mountains in Missouri began almost 1.5 billion years ago with a series of eruptions from a volcanic system. The upper part consists of rhyolitic rocks, including lava flows and remains of a caldera-like structure, indicating explosive eruptions. Underneath this are granites representing the plumbing system that supplied the molten rock. The chemistry of these granitic rocks indicates they mostly came from re-melting of sedimentary layers at the edge of Laurentia (the ancient growing North American continent), due to heat supplied by melting the mantle beneath. As the crust was pulled apart, molten rock from the mantle intruded, causing the rocks of the crust to re-melt, and that molten rock supplied the volcanic system now exposed in the region.

The St. Francois ridge, which Taum Sauk Mountain is a part of, is much, much older than the surrounding Ozark Mountains. While much of the United States started out underneath the waters of ancient seas, Taum Sauk may be one of the very few peaks which would have jutted from the Paleozoic waters as an island. Vertical relief in the rest of the Ozarks region is the result of erosion of sedimentary rocks while the St. Francois are an ancient Precambrian igneous uplift several times older than the Appalachian Mountains.

The 3-mile Mina Sauk loop trail that we took passed by Mina Sauk Falls, the highest waterfall in Missouri, which drops 132 feet down a series of rocky rhyolite ledges into a clear, rock-bottom pool that sits at the base.

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photo: by Ulyana Peña