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Geology in Missouri

The general landscape of Southern Missouri was shaped by large volcanic calderas (volcanoes that erupt from many different vents). These volcanoes erupted molten ash that cooled quickly forming rhyolite, an igneous rock that has high silica content. The more silica in lava, the more viscous it is and the more effective it is at holding gas. This led to more explosive eruptions (Seeger). When the magma chambers were emptied of their magma, the now emptied space collapsed into calderas, influencing the general topography of the region.

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Iron Mountain, in the St. Francois Mountains, is the oldest continually mined property in the US. It was originally mined for iron but now produces igneous rocks. Mining activity ultimately led to human settlement of the surrounding areas (Seeger).

The mountains were formed over several hundred million years of eruption and cooling of lava. This lava layered on top of itself and solidified to form the mountains’ base. The igneous rocks became knob islands which protruded out and then were covered by more sedimentary rocks. The rocks then eroded over time and formed high and low points that eventually developed into the mountains that are present today. Due to tectonic plate activity, the Ozark region has been uplifted and continues to rise, exposing more of the buried mountain range (Seeger).

When new layers of rock are deposited on old eroded layers of rock, a clear unconformity can be seen between the layers due to the different appearances of eroded and new rocks. These unconformities oftentimes represent large time gaps between the layers of rocks. Due to the fact that igneous rocks do not have fossils, radioactive isotopes with known decay rates must be used to date the rock’s age. The igneous rocks in the St. Francois Mountains are about 1.5 to 1.2 billion years old (Seeger).

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The granite boulders at Elephant Rocks State Park were formed by the process of erosion and uplifting of granite, which also formed pools of water called “birdbaths”. The reaction between the water and granite formed even deeper fractures over time and led to a widening of the water basins (Seeger). First an exposed piece of granite cracked up or fractured. Then the rainwater is able to concentrate on the intersections of multiple sets of fractures. Those intersections would be the first to be weathered. The edges of the stone became curved and softened. When the gaps were large enough, the wind would also join the process and weather the granites even more. The rectangular pieces of the fractured granite would become individualized and circular. Then, entire free standing pieces of granite boulders would be carved out of the ground level completely. On many granite boulders, deep parallel sets of fracture lines are visible as deep grooves. From the rocks, we can see three sets of parallel fracture lines, indicating three major tectonic activities in the past from different directions left their marks on the rocks. Two sets are vertically going down, yet are perpendicular to each other. One set is parallel to the ground.

The wealth of iron mining in the mountains led to the development and settlement by humans. Starting in 1815 iron has been mined from the mountains. Due to the inefficiency of wagons, railroads were built to transport the iron, which became the Missouri Pacific railroad. Missouri granite has also been mined for a long time, and granite from the mountains has been used across the United States (Seeger).

In 2005, a hydropower reservoir facility failed, causing a massive flooding. The powerful forces from the flooding scared the land and revealed large swathes of underlying bedrock now known as the Scours.  The exposed rock provides geologists with the unique opportunity to see a large geological timeline unobscured by plant life.  At the bottom of the mountain is exposed sedimentary rock known as Cambrian Dolomite.  Further up the mountain is older sedimentary rock called conglomerate, then even older exposed granite, and finally rhyolite at the top. (Seeger) In the sedimentary rocks one can even see the fossils of an ancient sea bed.

Drill core records provide information to explore ore deposits, geologic mapping, hydrologic and environmental studies, and academic purposes. The Division of Geology and Land Survey use drill cores to archive information about Missouri’s Geology (Seeger).

While the region south of the Missouri River has some mountainous topography, the area north of the river is much more flat. This topography change is a result of massive ice sheets that covered Missouri during the Pleistocene era. The Missouri River itself is the remaining mark of the southern boundary of the continental ice sheets that were present 11,000 years ago. Because of the presence of the glaciers, the land was scoured and flattened, leading to very flat topography in the northern regions of the state.


Seeger, Cheryl. “Igneous Rocks the Volcano Stones of Missouri.” The Geologic Column of Missouri 3.1 (2008) : 1-6. Web.

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