Castor River Pink Granite Shut-Ins, Amidon State Conservation Area

The History of the Geological Development of the Midwest

All of geology has come through three major stages as mere humans have tried to grasp the formation of the world beneath their feet.

The first, divine creation exactly as it looks today, was dismissed three hundred years ago in Europe, though it dies hard and still gasps among more fundamentalist religious sects today. As recently as the turn of the twentieth century, official geological publications in Missouri still nodded towards that belief, while at the same time seeking to carve a system based more on actual observation and analysis of data by geological principles.

The second stage, that of geomorphological and geosynclinal development, combined basic observations such as weathering, erosion, transport and deposition with known geological principles such as original depositional horizontality (all things being calm, sediments will deposit in flat layers), superposition (things on the bottom of an undisturbed layering will be older) and cross-cutting relationships (rock layers that cut across other layers have to be younger than those they cut across), known geological processes (volcanoes, earthquakes, floods, glaciers) and the geomorphological theory of William Morris Davis, a geographer who explained landscape development in terms of youth, maturity, old age, and rejuvenation.

In second stage theory, vertical movement drove surface topograhy. First came mountain building. Mountain building occured as a result of the crust being uplifted in one place after being pushed down in another, sort of like sitting on a waterbed. Volcanoes and earthquakes happened during mountain building. Next, came weathering and erosion, as wind and water physically and chemically wore down the mountains, sculpted valleys, and moved sediment to the sea, where it accumulated in structures called geosynclines. After so much sediment accumulated in one place, sheer pressure would cause diagenesis or lithification of the sediments, as well as cause the resulting basin to either subside. Eventually, the old land surface would be worn to a flat peneplain (think of flat like the prairie). At the same time, pressure on the ocean basins would cause uplift of old peneplains, metamorphism of sediments, and emergence of new land as the crust adjusted to balance the relative weights of old land and new. Upwarping and downwarping of the crust caused by these pressures and releases created rock structures such as regional uplifts, basins and arches.

The number of geosyncline types multiplied astonishingly, until few could keep them straight. Since the point of a theory is to find a mechanism which covers many examples, geosynclinal theory failed. Though geosynclinal theory fell into disrepute in the 1960's, many of the terms, such as craton, basins, and continental arch persist in the new tectonic scheme.

Current geological theory relies on plate tectonics. In plate tectonics, horizontal movement, plate collisions and spreading centers create, compress, stress, stretch and even recycle two kinds of crust--continental and oceanic. Plate movement is driven by the internal heat of the earth, like slabs on an endless conveyor belt, and geological processes such as mountain building, earthquakes, the opening and closing of oceans are driven this 'prime mover', instead of being the causing agents themselves. The study of plate tectonics is primarily the study of the oceans, and continental margins--even though slow by human scales, process rates in these areas are rapid on a geological scale and their effects can be easily monitored by modern equipment.

Early plate tectonic theory ignored the interiors of continents as being "stable" and therefore relatively uninteresting. It has only been recently that geosyncline explanations for continental structures have begun to be reinterpreted in terms of plate tectonics. Continents have been recogized as accumulations of sub-continents, and island arcs, and as being subject to the same sorts of stresses expressed more dramatically along coastal collision or subduction zones. The use of on-site and satellite technology capable of detecting minute earth movements has greatly aided this study in places as firmly continental, yet tectonically active as Yellowstone National Park. Seismic advances, both in subsurface imaging and in detecting movement, and better rock dating techniques have helped gather information where it is quite subtle.

So what does this mean for Missouri geology? Quite frankly, we don't entirely know. So far, much study has been made of our Precambrian exposures, and the lowest Cambrian times--there are economic incentives driving both sorts of study. The New Madrid Rift Zone, along the Missouri Bootheel, is also subject to intense scrutiny, largely because of the great potential for damage and loss of human life should this area shift, causing disastrous earthquakes similar to those of 1811-1812. Some geologists have investigated possible effects of the Ouachita orogeny (the uplifting of the Ouachita mountains in Arkansas and Oklahoma), and possible effects (dolomitizing our limestones, and placing galena deposits) on southern Missouri.

But the translation of geosynclinal arches and basins, (evidence of crustal movements over the last 500 million years) into plate tectonics terms is still in its early stages. With so much time and thousands of meters of deposited carbonates, it is clear that karstification, the processes of chemical and physical weathering on limestone and dolomite, has played an important part, but it cannot account for crustal warping.

It is clear that if "nothing happened" except seas coming and going, and weathering until the Quaternary glaciers, Missouri would long ago have become utterly structurally featureless. Since this is not so, the quest for the story continues.

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