University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Papers in Natural Resources Natural Resources, School of April 1991 Plate Tectonics, Space, Geologic Time, and the Great Plains: A Primer for Non-Geologists Robert F. Diffendal University of Nebraska - Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/natrespapers Part of the Natural Resources and Conservation Commons Diffendal, Robert F., "Plate Tectonics, Space, Geologic Time, and the Great Plains: A Primer for Non- Geologists" (1991). Papers in Natural Resources. 138. https://digitalcommons.unl.edu/natrespapers/138 This Article is brought to you for free and open access by the Natural Resources, School of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Papers in Natural Resources by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. PLATE TECTONICS, SPACE, GEOLOGIC TIME, AND THE GREAT PLAINS A PRIMER FOR NON-GEOLOGISTS R. F. DIFFENDAL, JR. For most Americans, "The Great Plains" evokes of the first human technologies to affect the images of grasslands, dust storms, prairie fires, Plains, and it favored the development of some Indians on horseback, cowboys and wheat lands, types of vegetation, particularly grasses, over and perhaps flat valleys crossed by braided rivers others, such as woody plants. Farming and carrying a heavy load of sand and gravel, ex- ranching have had vast impacts on the vege- tremes of weather, and a climate typified by an tative cover, on the soils, and on wildlife dis- alternation of droughts and wetter periods. Ge- tribution. And dams, built across rivers to ologists picture such general images, too, but control flooding, to produce hydroelectric power, they also see radical changes in the landscape or to divert water for irrigation or other uses, over periods expressed in millions rather than have produced major changes in the behavior hundreds of years. Geologically speaking, hu- of the rivers. man activities on the Great Plains are too re- While historians look at the past on a human cent to have much of a place in the broad time scale, for geologists human time is simply geologic history of the region, but they have the most recent frame in an unfinished movie. certainly influenced both the use and the ap- Each frame is composed of a mosaic of world- pearance of the region today and hence the wide snapshots. Pieces of the film are missing, terms in which we understand it. Fire was one but what is left is usually in a chronological sequence. What geologists seek to do is to trace their way back through time as it is recorded on this broken film to explain the development R. F. Dijj'endal, Jr., is professor and research ge- of a particular place, a region, or even the entire ologist at the University of Nebraska~Lincoln.A earth. The geologic history of any region is tied Felbw of the Geological Society of America, he has up with the history of earth as a whole, because spent the last fifteen years studying the Cenozoic events happening far away can leave an indel- geology of Nebraska, on which he has published ible impression on the region under consider- many articles. ation. Recent volcanic eruptions, for example, have injected into the atmosphere dust particles [GPQ 11 (Spring 1991): 83-1021 that have altered the weather or that have been 84 GREAT PLAINS QUARTERLY, SPRING 1991 deposited as ash for great distances downwind weathering rates of rocks and sediments have from their sources. The geologist's holistic view changed in response to the actions of wind and of the earth should include the overall conse- rain and also of plants, and thus the elements quences of evolving life as well as purely geo- have laid down various kinds of soils in places logical forces. Beginning in 1972, J. E. Lovelock and thicknesses that changed over time. Or- hypothesized that all life makes up "a single ganisms have evolved, emigrated or immi- living entity, capable of manipulating the Earth's grated, and changed in abundance. Yet for the atmosphere to suit its overall needs and en- past 66.5 million years, since the uplift of the dowed with faculties and powers far beyond those Rocky Mountains and the withdrawal of the of its constituent parts." Lovelock suggested that shallow seas from the North American mid- living organisms might influence not only cli- continent, this region has maintained the basic mate change but also volcanism, mountain for- identity that we associate with the Great Plains. mation, and the movement of the earth's lithospheric plates. Climate change is also at least partly attributable to the solar cycle of variations in the earth's orbital eccentricity, ax- In order to understand what it is that geol- ial tilt, and precession of the equinoxes due to ogists know about the Great Plains, it is nec- variation in the earth's motion and position essary to understand a little bit about the history relative to other planets.' I. E. Oliver linked of the science of geology and of how geologists these plate movements, differences in the understand what it is that they are doing. In amount of radiation coming from the sun, and the 1700s geologists began describing a chro- other factors to make a "compromise theory" of nological sequence of layers of rock, upsetting climatic change. earlier ideas of a relatively stable and recently My purpose in this paper is to explain for the created world. Geologists recognized that the non-geologist what some of these factors are, layers at the bottom of the sequence were the to give a history of how geologists have come oldest and that the layers were usually deposited to understand them, and then to go beyond horizontally. A third idea, that any feature cut- Oliver's general model to focus on the relation- ting across or deforming several strata had to ship between climate change and the geological be younger than any of the affected strata, also development of the Great Plains as it has been helped in constructing a time frame. When sim- influenced by major geological events both out- ilar types of fossils appeared in rocks great dis- side and within the region. These events in- tances apart, they served as indexes to match clude movements of the earth's rigid lithosphere up or correlate the strata. By the latter part of plates with attendant mountain formation and the nineteenth century, when the concept of volcanism, cyclic changes in sea level and gla- evolution was sweeping the biological sciences, ciation, broad uplifts and downwarps of the con- evolutionary succession of organisms allowed tinents, the evolution of new environment- geologists to establish a fairly precise worldwide changing organisms, and meteorite impacts. chronological sequence of rock strata. In the While most of these affect an area far greater twentieth century, with the discovery of the than just the Great Plains, local events also principles of radioactive dating, geologists ten- produce regional change. For example, the al- tatively established time spans for formation of titude of the Plains was once considerably lower rock sequences and increasingly refined these than it is at present. Folding, faulting, and ig- dates to the point that many correlations are neous activity have thrust up mountains in parts now e~cellent.~ of the region. Rivers have cut valleys and then Precambrian rocks are older than 5 70 million filled them back in, wholly or partially, with years before present (mybp); the oldest of these sediments. The climate has been wetter and contain no known fossils, while somewhat dryer, hotter and colder, than it is now. Natural younger Precambrian rocks have been found so 86 GREAT PLAINS QUARTERLY, SPRING 1991 FIG. 2. Changes in the positions of land mses over the last 60 my. (Modified after Smith and Briden, note 7, courtesy of Cambridge University Press.) PRIMER FOR NON-GEOLOGISTS 87 far to contain only primitive invertebrate ani- the geology and paleontology along this fit, and mals, algae, bacteria, and viruses (Fig. 1). The the evidence of past climate changes that seemed remaining younger rocks, including those that explicable only in terms of continental drift.4 contain more advanced fossils, were formed dur- Other geophysicists, however, insisted that the ing parts of three major time spans or eras, the continents could not float about like rafts on Paleozoic (5 70-250 mybp) , Mesozoic (250-66.5 the seas of a denser mass of solid rock. Geol- mybp), and Cenozoic (66.5 mybp to present). ogists studied spinoffs of Wegener's ideas or pro- The fossils from the rocks formed during these posed other explanations for the similarities, eras are the preserved remains of life that more such as the development and later disappear- and more resembles the organisms alive today ance of land bridges across the oceans, to avoid as one progresses from the oldest to the youngest the problems involved in the drift ~oncept.~ rocks in the sequence. Geologists define the By the early 1960s, however, oceanographers systems of rocks formed during periods by major and other earth scientists produced data that changes in the types of fossils and types of rock showed that the entire crust of the earth is bro- strata. Subdivisions are made according to def- ken into larger and smaller pieces that move inite but less dramatic changes in the rock for- with respect to each other.6 New crustal ma- mations and their fossils. In this paper I will terial is generated along submarine oceanic ridges deal primarily with the geologic history of the and consumed under oceanic trenches. Within Great Plains during the part of the Cenozoic a decade of the production of these new data era from 37 mybp, when perhaps the oldest that satisfactorily explained the basic mechan- Cenozoic rocks in western Nebraska were de- ics, the idea of sea floor spreading and plate posited, to 1.65 mybp, the time of the depo- tectonics had produced the major conceptual sition of the youngest Pliocene sediments.