Mya Mounds

Mya Mounds

Posted on October 17, 2012
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We’ve all chronicled Mya’s ever-growing thicky thickness over the last couple of years, but it may have gotten out of hand. Look at this picture that’s been surfacing around the net recently. Mya’s thicker than she’s ever been and if she played baseball we’d be testing her for steroids or something.
What do you think? Is Mya’s thickness still banging or are you getting a little worried? Regardless, we can just look at the pictures and enjoy the view.
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By hgregory • January 6, 2020 January 7, 2020

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Missouri is situated on an ancient geologic structure called the North American Craton, which is the core basement rock forming the North American continent. Basement rock is the rock that forms the core of all continents, and in Missouri, this basement rock was first pushed into the North American Craton by tectonic forces to form what would become the base of Missouri. Additionally, bedrock, which is the layer above the basement rock, was added to the craton over the course of geologic history. Bedrock is not sediment; rather, it is dense, hard rock, most often igneous or metamorphic. It is sometimes exposed on the surface, but it is typically hidden by layers of dirt and sediment hundreds or thousands of feet thick. Missouri contains each of the three classes of rocks that forms the basement rock and bedrock: igneous, metamorphic, and sedimentary rocks. The most common igneous rocks in Missouri are rhyolite, granite, diabase, and volcanic tuff, each of which can be seen exposed in the St. Francois Mountains. Metamorphic rocks are formed when rock changes because it is subjected to different extreme conditions that have the potential to morph rocks, including high pressures and temperatures. Metamorphic rocks are rare in Missouri, and they are usually only found in basement rock. The only partially exposed examples in Missouri are the Hawn Park Gneiss found in Hawn State Park (and there is debate as to whether or not this rock is actually metamorphic), and some samples housed in glacial sediment in Northern Missouri. Sedimentary rocks are very common in Missouri, as many cycles of ocean rise and fall deposited sediment across the basement rocks, most often leading to limestone formation. Sedimentary rocks are formed by a very long process of erosion and compression. As weather conditions erode existing rocks, grains of sediment collect and are carried to a sediment deposit, often by means of water or glacial transport. In large basins of sediment, which often are found in areas such as river bottoms, depressions in the Earth, and lake bottoms, layers of sediment gather over a large period of time. Gradually the initial layers of sediment are subjected to pressure and chemical reactions due to the sediment above. As water between the sediment is pressed out, the grains of sediment bind together in a process called lithification, which forms the sedimentary rocks (Spencer 2011).
There are several common geologic structures in Missouri. One such structure is a fold, which forms because sedimentary rocks bend under pressure rather than breaking immediately. Tectonic pressures from continental collisions during the Pennsylvanian Period (which simultaneously created the Appalachian Mountains) caused the layers of rocks to bend into the folds in Missouri. Folds are seen in road-cuts in the layers of rock in two orientations: either domed upward (an anticline) or downward (a syncline). In the event when rock cannot respond by bending into folds because forces are too sudden or powerful, the rock breaks, forming structures called joints. There are two major orientations of joints in Missouri: Northwest-Southeast and North-South. Joints and fractures are sometimes hard to distinguish, but due to erosion, the joints are widened and filled in with dirt and sediment, making the joints more visible as time passes (Spencer 2011).
Elephant Rocks State Park in southeastern Missouri exhibits the effects of rock fractures well. While initial cracks formed in the granite as it cooled over 1 billion years ago, during periods of continental uplift over the course of Missouri’s history, the larger cracks formed in the rocks, fracturing all of the underlying granite into cubes. As the rocks were slowly uplifted and exposed at the surface, wind and rain slowly eroded, rounded, and enlarged the fractures to produce the large oval shaped boulders that are known as the Elephant Rocks (Seeger 2008).
Below is a picture of one of the “elephants”:
The next picture displays an intersection of two rock fractures (a faint fracture oriented vertically and a more eroded fracture oriented horizontally). The rocks below will someday look like the rock above after enough uplift and erosion.
Faults are formed from fractures of joints when large tectonic forces pressure the fractures in the rock for long periods of time, which can result in the rocks slipping past each other in what is called a fault. There are three types of faults: transverse (sliding horizontally), normal faults (top rocks slide down), and reverse (upper rocks slide up) (Spencer 2011). The most notable fault feature of Missouri is the New Madrid Seismic Zone, found in the southeast corner of the state. This zone was responsible for the famous 1895 and 1811-1812 earthquakes (USGS 2015).
Reflecting on the timeline, a couple of interesting facts emerge that illustrates the enormous length of history that Missouri possesses since its “birth” 1.8 billion years ago with the first igneous rocks. For over 70%, of Missouri’s history, from 1.8 billion years ago to 542 million years ago, the state was composed of solely igneous rocks. For about 20% of Missouri’s history, the state was a part of the supercontinent Rodinia. However, all of Missouri’s history has taken place within the last 40% of Earth’s history as the Earth formed 4.6 billion years ago, and all of Missouri’s geologic history except for the igneous events occurred in the last 12% of Earth’s history, from the Cambrian Period onward. (Statistics based of information from Spencer 2011).
Below is a diagram that places other worldwide geological events on a timeline that helps put into perspective the localized events in Missouri described above.
Note: Image edited to include information about Missouri. Image Source: Wikimedia Commons, Woudloper, September 2007, https://commons.wikimedia.org/wiki/File:Geologic_clock.jpg
As mentioned above, the basement rock of Missouri was formed about 1.8 billion years ago as outlying volcanic islands began to crash into the southwestern edge of the ancient continent of Laurentia, also known as the North American Craton, due to tectonic forces pushing the islands into the continent. The resulting subduction and volcanic activity formed the igneous baserock that now lies across much of central and northern Missouri. About 1.5 billion years ago, the basement rock in southern Missouri was formed due to a series of large volcanic eruptions on the south coast of Laurentia. Rather than volcanic eruptions induced by tectonic subduction, these igneous rocks were formed by a rising magma plume from the mantle that erupted out of magma deposits in a series of huge volcanic eruptions. As the magma moved beneath the crust, it flowed along two older fault zones, the Grand River and Central Missouri Tectonic Zones, traversing central Missouri in a Northwest-Southeast orientation. The area between these two faults is the called the Missouri Gravity Low, an intriguing geological structure in that this stretch of granite is less dense than the surrounding rocks on either side of this area. The Missouri Gravity Low is 370 miles long and 60 miles wide. The last set of rocks to be formed in Missouri formed about 1.3 billion years ago, as more magma flows in Southeastern Missouri surfaced and hardened into diabase (Spencer 2011).
There are two major types of igneous rocks present in Missouri: granite and rhyolite. Granite is formed from magma that cooled under the surface of the Earth and did not erupt. Because it did not erupt and cooled more slowly, granite contains large amounts of mineral crystals. On the other hand, rhyolite is formed from silica rich magma that erupted explosively and then quickly cooled. Crystals do not have as much time to form when the rock cools and hardens quickly, so rhyolite does not have as many large crystals as granite. Because of the high silica content, the magma contained a lot of explosive power, thus leading to the conclusion that the eruptions that formed the rhyolite of the St. Francois Mountains were among the most intense volcanic eruptions on Earth (Seeger 2008).
Granite and rhyolite erode and weather at different rates. Granite’s crystal and larger mineral structures are easier to break apart than the harder, more solid structure of rhyolite formed from its fast cooling, so the granite erodes faster. In the St. Francois Mountains, rhyolite can often be found in the higher areas because it erodes less, while granite is found in the lower areas of the landscape because it has eroded and weathered away (Seeger 2008).
Richard Hathaway, October 2015. Exposed granite on the Johnson’s Shut-Ins Scour, formed during the Proterozoic Era volcanic eruptions.
Richard Hathaway, October 2015. A dislodged piece of rhyolite on the Johnson’s Shut-Ins Scour.
A third type of igneous rock found in Missouri is called volcanic tuff. Tuff is formed in very intense explosions by small pieces hardened magma called volcanic ash that bind together upon impact with the ground (Seeger). Below is an outcrop of volcanic tuff found at Johnson Shut-Ins State Park. Note the distinctive bands of brown coloring among the mostly black colored rock.
Rock formation during the end of the Proterozoic era was very limited; however, geological processes were still present in Missouri. Between 1.1 billion and 750 million years ago, other continents joined with Missouri and the North American Craton, forming the gigantic landmass Rodinia. After Rodinia broke apart 750 million years ago, another tectonic rift almost managed to break apart the entire craton. This fault zone still exists today, buried below Southeastern Missouri, and this fault still causes substantial earthquakes in Missouri today. Scientists also believe that between 850 and 635 million years ago, there was a massive ice age covering all of Missouri, and some scholars even think that it is possible that the entire ocean froze as a massive ice sheet covered the entire planet. Due to the ice, the entire North American continent was eroded down to a land mass of relatively equal elevation, with only some of the igneous hills remaining. When the ice melted at the end of the ice age, the waters inundated the entire continent and brought with it the first of Missouri’s sedimentary rocks (Spencer 2011).
Many more of Missouri’s sedimentary rocks were deposited in the Paleozoic Era from 542-251 million years ago during periods of rapid cycles of high sea level rise and fall. Six cycles of sea level rise and fall occurred during the Paleozoic Era, and each rise and fall brought a new round of sedimentary rocks. As the seas receded with each cycle, the newly formed rocks were exposed and eroded from weather and the air, creating clear distinctions between layers of rock from each cycle. While there are hundreds of smaller factors of sea level change, the main cause is tectonic activities at the ocean floor. During periods of high tectonic activity at the ocean floor, large amounts of igneous rock are formed from magma being pushed up through deep sea vents. This process creates ocean mountain ranges over time, which displaces water upwards and causing the sea level to rise (Spencer 2011).
From the beginning of the Cambrian Period to the present day, there have been six major cycles of sea level rise. The first cycle, called the Sauk cycle, began in the early Cambrian Period. At this time, Missouri and the rest of the North American continent were located at the mid-southern latitudes. The flooding waters deposited large amounts of sand, which formed into sandstone under pressure. Because these are the first rocks to form in Missouri since the igneous rocks in the Proterozoic Era, the distinction between the sandstone and the underlying igneous rocks represents a geological gap of 900 million years. This separation between these layers in Missouri is known as the Great Unconformity, an important feature of Missouri’s geological history. Additionally, Missouri’s oldest known sedimentary rock layer, the Lamotte Sandstone, about 520 million years old, was deposited during this period. As waters inundated Missouri in the Sauk Cycle, the weathered rhyolite hills that were formed in the Proterozoic era became isolated islands, but also were soon flooded and covered in sandstone (Spencer 2011).
The Johnson Shut-Ins scour reveals the effects of the rising sea on the rhyolite and granite. A mixture of rock called conglomerate outcrops on the scour, consisting of bits of old rhyolite and granite joined together by layers of sandstone. This is an interesting formation considering that the rocks mixed in the conglomerate were formed 900 million years apart from each other (Seeger 2010).
Richard Hathaway, October 2015. An outcrop of conglomerate at the Johnson Shut-Ins scour. This area once used to be a ancient beach during Cambrian times (Seeger 2010).
The ocean also began to deposit carbonate sediment in the form of calcite (a mineral containing calcium and carbon) and dolomite (very similar to calcite, but magnesium takes the place of roughly half of the calcium atoms), which had begun to collect on the sea floor, having been ejected by small organisms and plants. Carbonate rocks, specifically limestone and dolomite, formed from calcite and dolomite minerals and represents a large portion of the Cambrian rocks from Missouri. An intriguing observation is that a substantial portion Missouri’s oldest carbonate is actually magnesium based dolomite instead of the calcium based limestone. This is odd because limestone is much more easily formed than dolomite because calcium is more easily used by organisms in the ocean, and there isn’t enough magnesium in the water. One hypothesis to explain this observation is that the dolomite was created as limestone and then converted to dolomite over history. Thus, some of the initial limestone was transformed over time into dolomites as magnesium from the surrounding environment, possibly from flowing water or from magma deep in Proterozoic igneous rocks, created fissures in the carbonates that allowed magnesium to infiltrate the rocks (Spencer 2011).
Below is an outcrop of dolomite of Cambrian age in the Johnson Shut-Ins scour (Seeger 2010).
There are five main layers of Cambrian sedimentary rock. After the Lamotte Sandstone was deposited, the next overlying layer of rock is called the Bonneterre formation, which is comprised of dolomite and limestone. Because the entire state was not yet submerged when the Lamotte Sandstone was deposited, as some of the rhyolite hills were still exposed, Bonneterre rocks sometimes lie directly on top of igneous rocks, and these rocks lie across most of the state. Additionally, Bonneterre rocks are important because a large amount of the state’s lead has been mined from these rocks. The third layer of sediment to be deposited is the Davis formation, which due to frequent minor sea level changes, contains alternating layers of limestone, shale, and sandstone. Additionally, the Davis formation contains evidence of numerous different ancient marine fossils. The fourth layer is the Potosi formation, which is made up of carbonate layers. Finally, the top layer of Cambrian rock is called the Eminence formation, which is formed from dolomite and contains a large amount of chert. Additionally, the Eminence formation is important in Missouri because many of the caves and springs in the state are carved out from Eminence dolomite. At the end of the Cambrian Period, the sea level fell, leaving a clear unconformity spanning the border between the Cambrian and Ordovician Periods (Unklesbay and Vineyard 1992).
Two other rock structures formed during Cambrian and Ordovician time due to sea coverage: bioherms and chert. Bioherms are rock structures made up of fossils of ancient marine organisms and are often surrounded by fossilized algae that existed at the sea floor. Chert is a rock containing high amounts of silica that formed from sediment composed of shells of microorganisms called radiolaria that fell to the seafloor (Spencer 2011).
Around 488 million years ago, the second sea level rise, the Tippecanoe Cycle, began, depositing mostly carbonate and some sand. (Spencer 2011). The deposition of the two materials fluctuated so that the Ordovician column exhibits small layers of sandstone in between the larger layers of dolomite. Two of these sandstone layers are the common Gunter and Roubidoux sandstones, and prominent dolomites from this period are the Jefferson City and Cotter dolomites (Unklesbay and Vineyard 1992). There are very few examples of dolomite younger than Ordovician time. However, there are a few samples of limestone dating to Mississippian time that have some magnesium characteristics in fault areas, which indicates that magnesium flows through faults were one of the main ways through which limestone changed to dolomite (Spencer 2011).
Later in Middle Ordovician time, the North American Craton began to collide with other tectonic plates to the East, which began to force the middle of the continent upwards and resulted in the initial rise of the Ozark Dome. This displaced much of the water covering Missouri at the time. Additionally, ice sheets began to form in the Southern hemisphere which further drained the North American continent, and thus produced a time of erosion in the Middle Ordovician period. This unconformity is very visible, as there was a lack of deposition of carbonate rocks during this time. Another cycle of sea level rise deposited another layer of sandstone called St. Peter Sandstone. This sandstone is unique in that the sand grains are thought to have been carried by the wind from shoreline sand dunes from eroded Proterozoic hills, as the grains are pitted which occurs when sediment collides with other grains, as would happen as the grains were airborne blown by the wind. Additionally, as a volcanic chain of islands collided with eastern North America, volcanic ash was carried into Missouri and deposited as clay called bentonite (Spencer 2011).
Early in Silurian time, a sea partially covered the Northwest section of the state, but then expanded and eventually covered the whole state by the Devonian period. More carbonate sediment was deposited of varying thickness across this sea. Also in early Silurian time, the tectonic collision in Eastern North America with a volcanic island chain came to a climax, which caused many vo
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