Igneous Rocks in the Missouri Ozarks

By Robert Davidson
April 23, 2010
For ES767:Global Tectonics

Introduction

When most of us think of the Missouri Ozarks we think of sedimentary geology. Whether we picture a high bluff along one of the many scenic rivers or road cut on the way to Branson, or a cave or spring deep in a secluded Ozarks hollow, we are thinking of sedimentary rocks. However the Ozarks do contain igneous rock, especially in the St. Francois Mountains of southeastern Missouri, and this igneous rock has played an important geologic and economic role in the development of the state. Igneous rocks can also be found at the surface in some areas just outside the St. Francois Mountains along deeply eroded rivers and streams where the resistant igneous rocks have produce what the locals term “Shut Ins." In addition, deep drilling at various sites in Missouri has shown that igneous rock underlies most of the state. This basement rock only comes to the surface in a few places. For a generalized view of Missouri Geology, see figure 1 below. The St. Francois Mountains are shown in red and orange in the southeast part of the state.

Generalized Geologic Map of Missouri

Figure 1. Generalized Geologic Map of Missouri. From the Missouri Department of Natural Resources. Used with permission.

The St. Francois Mountains

This rugged rocky area of the Ozarks includes Taum Sauk Mountain, the highest point in the state. The igneous rocks in this area are of Precambrian-Proterozoic origin and some are as old as 1.5 billion years. These rocks would represent the oldest rocks anywhere in the mid-continent region, and point to the great age of the Ozarks as a whole. Geologists generally consider the Ozarks to be the oldest mountain range in the United States. The Appalachian Mountains are perhaps 460 million years old and the Rockies began uplifting around 70-90 million years ago. In contrast the St. Francois Mountains are the eroded remnants of huge volcanic calderas that are 1.5 billion years old.

St. Francois Mountains

Figure 2. St. Francois Mountains located in red. From Wikipedia Commons Author KBh3rd. Used with permission.

Formation of the St. Francois Mountains began with a series of volcanic calderas over what was probably a tectonic hot spot similar to the one in Yellowstone National Park. Magma from this hot spot moved up and erupted onto the surface through many vents in a series of eruptions. Eventually pressure in the magma chamber became so great that a huge eruption formed a crater miles in diameter. Layers of igneous rock form thousands of feet thick. Generally these eruptions were of a rhyolite rock, rather than the basalt lavas found in Hawaii and Iceland. The high silica rhyolites form a thick lava that tends to hold more gas and to erupt more forcefully. Pyroclastic flows were common. As the rock settled on the ground it was hot enough that the ash welded together into massive units of solid rock. One such rhyolite flow can bee seen at Rocky Falls on Rocky Creek just east of Eminence, Missouri on the edge of the St. Francois Mountains.

Rhyolite lava flow

Figure 3. Rhyolite lava flow at Rocky Falls near Eminence, Mo. Photo by author.

Rhyolite lava flow

Figure 4. Rhyolite lava flow at Rocky Falls. Photo by author.

Weathered rhyolite at Rocky Falls

Figure 5. Weathered rhyolite at Rocky Falls. Photo by author.

This process of eruption, ash and lava flow, cooling and solidification continued for millions of years. As the magma chambers emptied and the rock cooled, the volcanic calderas would collapse. The overlying rock fractured, tilted and slipped. Over time the magma chambers filled, erupted and then cooled and collapsed many times. Eventually the hot spot cooled and the area became inactive. Roughly 520 million years ago the Ozarks area subsided due to continental plate movement. The area was flooded by the sea and the vast layers of sedimentary rock that we more commonly think of as being typical of the Ozarks began to form. Along the edges of the igneous rocks sedimentary reef structures formed. These areas trapped significant amounts of brines rich in metallic ores. As these brines precipitated there mineral content, large concentrations of iron and lead formed. These ore bodies became important in the development of the mining industry in the Ozarks. More recently, in geologic terms, the Ozark Plateau has been uplifted and foms a lagre dome of rock, with the St Francois Mountains as the peak of the dome. Millions of years of erosion have worn away the sedimentary rock in the St. Francois Mountain region, exposing the underlying igneous rocks.


The Economic Role

The igneous geology of the Missouri Ozarks has played a central role in the development of the mining and quarry industry in the state. Granite and rhyolite from the region has been mined extensively for building stone, headstones and railroad ballast. The earliest iron mines west of the Mississippi River were located at Ironton, Mo in the St. Francois Mountains starting in 1815. Early blast furnaces were constructed at Ironton and at St. James. Eventually iron ore was shipped to St. Louis for smelting on the St. Louis and Iron Mountain Railroad. This railroad later became the Missouri Pacific Railroad, now part of the Union Pacific system. Silver mining began in this area in 1879 near Fredericktown in Madison County. Tungsten was also briefly mined there. Lead mining became an important economic force in the region as early as 1720. From the late 1800’s until 1975 vast amounts of lead were pulled from the Bonne Terre area and the associated Old Lead Belt. The Viburnum trend, just to the west became known as the “New Lead Belt” and continues to be a major source of lead for the industry. Both are closely associated with the igneous rocks formed in the Proterozoic, and the fringing algal reefs that formed sedimentary areas that trapped the actual ore.

St Joseph Lead Company

Figure 6. St Joseph Lead Company smelter and Mill Photo courtesy of Missouri Department of Natural Resources. Used with permission.

Additional information

Some links are included here for those of you who might be interested in a visit to the area.

Johnson Shut Ins State Park…a great place to see the igneous rock at the surface. Good swimming hole too! http://www.mostateparks.com/jshutins.Htm

Elephant Rock State Park ……Huge granite boulders http://www.mostateparks.com/elephantrock.htm

St. Joseph Mining Company mill and smelter… buildings and museum http://www.mostateparks.com/momines.htm

Maramec Springs Park …..Early iron mine and smelter. Great museum and picnic area. Trout hatchery too. http://www.missouritrout.com/maramec.html

References

Unklesbay, A.G. and Vineyard, J.D. 1992. “Missouri Geology Three Billion Years of Volcanoes, Seas, Sediments and Erosion” University of Missouri Press 189p.

Kisvarsanyi,G. “ The Role of the Precambrian Igneous Basement in the Formation of the Stratabound Lead-Zinc-Copper Deposits in Southeast Missouri” 1977. Economic Geology 72 p.435-442

Gillman, J., Duley,B. and Seeger,C. “Igneous Rocks, The Volcanic Stone of Missouri” 2008 The Geologic Column of Missouri vol.3 issue 1 6p.

Cullers, R.L., Koch, R.J. and Bickford, M. “Chemical Evolution of Magmas in the Proterozoic Terrane of the St. Francois Mountains, Southeastern Missouri 2. Trace Element Data” 1981 Journal of Geophysical Research 86(B!!) p.10388-10401

Missouri Department of Natural Resources “ Missouri Ozarks” Division of Geology and Land Survey Fact Sheet #20 2008 Web page “Geology of Missouri- Proterozoic/Precambrian” Retrieved on 22nd April 2010 http://members.socket.net/~joschaper/proto.html

Web page “St. Francois Mountains” Statemaster encyclopedia Retrieved on 22nd April 2010 http://www.statemaster.com/encyclopedia/Saint-Francois-Mountains

Web page “Missouri Mines State Historic Site” http://www.mostateparks.com/momines.htm