Geologic History of the South Central US: The Big Picture

Geologic history is the key to this guide and to understanding the story recorded in the rocks of the South Central. By knowing more about the geologic history of our area, you can better understand the type of rocks that are in your backyard and why they are there. We will look at the history of the South Central as it unfolds: as a series of major events that created and shaped the area over the past one billion years. These events will act as the framework for the topics to follow and will shed light on why our region looks the way it does!

Figure 1.1: The Geologic Time Scale (spacing of units not to scale).

The geologic time scale (Figure 1.1) is an important tool used to represent the history of the Earth—a standard timeline used to describe the age of rocks and fossils, and the events that formed them. It spans Earth’s entire history and is separated into four principle divisions.

The first of these, the Precambrian, extends from about 4.6 billion years ago to 541 million years ago. Little is known about this time period since very few fossils or unaltered rocks have survived. What few clues do exist indicate that life first appeared on the planet some 3.9 billion years ago in the form of single-celled organisms.

The second division, the Paleozoic, extends from 541 to 252 million years ago. Fossil evidence shows that during this time period, life evolved in the oceans and gradually colonized the land.

The third division, the Mesozoic (from 252 to 66 million years ago), is also called the “age of reptiles” since dinosaurs and other reptiles dominated both marine and terrestrial ecosystems. It is also noteworthy that during this time the last of the Earth’s major supercontinents, Pangaea, formed and later broke up, producing the Earth’s current geography.

The last and current division, the Cenozoic, extends from the extinction of the dinosaurs, nearly 66 million years ago, to the present. With the demise of the dinosaurs, mammals became dominant and, subsequently, more diverse and highly developed. We humans don’t come into the picture until the last two million years. To get some perspective on this, if the entire geologic time scale were reduced to 24 hours, we wouldn’t come onto the stage until two seconds before midnight!

The Earth is dynamic, consisting of constantly moving plates that are made of a rigid continental and oceanic lithosphere overlying a churning, plastically flowing asthenosphere (Figure 1.2). These plates are slowly pulling apart, colliding, or sliding past one another with great force, creating strings of volcanic islands, new ocean floor, mountains, and earthquakes. The continents are likewise continuously shifting position relative to each other. This not only shapes the land, but also affects the distribution of rocks and minerals, natural resources, climate, and life.

Rocks and sediments are indicators of past geologic processes and the environments in which those processes took place. In general, igneous rocks, created through tectonic activity, reflect past volcanism. By looking at both their texture and chemistry we can determine the tectonic setting and whether or not the rocks formed at the surface or deep underground. Likewise, metamorphic rocks, created when sediment is subjected to intense heat and pressure, provide important clues to past mountain-building events, and geologists often use them to map the extent of now-vanished mountain ranges. Sedimentary rocks tell perhaps the most comprehensive story of the Earth’s history, as they record characteristics of far-away mountain ranges, river systems that transported the sediments, and the final environment in which the sediments accumulated and lithified. The size and shape of sediments in sedimentary rocks, as well as the presence of fossils and the architecture of sedimentary rock layers (sedimentary structures), can help us infer how the sediments were transported and where they were finally deposited. However, because rocks are often reformed into different rock types, ancient information is lost as the rocks cycle through the igneous, metamorphic, and sedimentary stages.

Figure 1.2: The layers of the Earth include the rigid crust of the lithosphere, which is constantly moving over the plastically flowing asthenosphere.

Continental and Oceanic Crust

The lithosphere has two types of crust: continental and oceanic. Continental crust is less dense but significantly thicker than oceanic crust. The higher density of the oceanic crust means that when continental crust collides with oceanic crust, the more dense oceanic crust will be dragged (or subducted) under the buoyant continental crust. Although mountains are created at these oceanic/ continental crust collisions due to the compression of the two plates, much taller ranges are produced by continental/ continental collisions. When two buoyant continental crusts collide, there is nowhere for the crust to go but up! The modern Himalayas, at the collision site of the Asian and Indian plates, are a good example of very tall mountains formed by a collision between two continental crusts.

Fossils indicate both the type of life that once flourished in an area and the kind of climate in which that life existed. Paleontologists use groups of fossils found in the same place to construct pictures of entire ecosystems. These ecosystems of the past are matched to similar present-day ecosystems, whose climate conditions are then used to infer what sort of climate the fossilized organisms lived in. Unfortunately, few organisms are easily preserved as fossils, and many environments also do not lend themselves to preserving organisms as fossils. As a result, the clues that fossils give provide only glimpses of the ancient world, with many important details missing.

Landscapes and geologic structures are also indicators of past geologic processes and the environments in which they occurred. For instance, the shape of a valley reflects the forces that carved it. Valleys with V-shaped profiles tend to be the products of stream erosion, whereas U-shaped valleys are more likely to have been carved by glaciers. Layers of intensely folded rock indicate a violent past of tectonic plate collisions and mountain building. Sedimentary structures, such as ripple marks or cross-bedding, can demonstrate the direction and energy level of the water that moved the sediment. Although landscapes tell us much about the geologic processes that created them, they inevitably change over time, and information from the distant past is overwhelmed by the forces of the more recent past.

Ultimately, geologists rely upon the preserved clues of ancient geologic processes to understand Earth’s history. Because younger environments retain more evidence than older environments do, the Earth’s recent history is better known than its ancient past. Although preserved geologic clues are indeed fragmented, geologists have become increasingly skilled at interpreting them and constructing ever more detailed pictures of the Earth’s past.