Geologic History of the Western US: Reconstructing the Geologic Past
We often wonder: What will the places we live in look like a millennium from now? A vision of “what might be” is critical for making the big decisions that will shape our future and that of our descendants. But to look forward, we have to first look back in time.
The oldest rocks found on Earth are 4.3-billion-year-old greenstone beds found along the eastern shore of Hudson Bay in northern Quebec. The oldest known materials are 4.4-billion-year-old zircons from Western Australia.
The shape and position of North America has changed dramatically over the last billion years, and geologic processes continue these changes today. The Earth is estimated to be approximately 4.6 billion years old. The oldest rocks known are located in northern Quebec and date to 4.3 billion years ago. These are part of Canada’s Precambrian shield, the ancient core of the North American continental landmass. Rocks more than 3.5 billion years old are found on every continent.
Figure 1.1: The layers of the Earth include the rigid crust of the lithosphere, which is constantly moving over the plastically flowing asthenosphere.
The Earth is dynamic, consisting of constantly moving plates that are made of rigid continental and oceanic lithosphere overlying a churning, plastically flowing asthenosphere (Figure 1.1). These plates pull apart, collide, or slide past one another with great force, creating strings of volcanic islands, new ocean floor, earthquakes, and mountains. The continents likewise continuously shift position because they are part of the moving plates. This not only shapes the land over time, but it also affects the distribution of rocks and minerals, natural resources, climate, and life.
Reconstructing the past is a lot like solving a mystery. Geologists use scraps of evidence to piece together events they have not personally observed, but to do so they must contend with two major complications. First, the overwhelming majority of geologic history occurred long before there were any human witnesses. Second, much of the evidence for the older events is highly fragmented.
See Chapter 2: Rocks to learn more about different rocks found in the West.
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 of 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.
See Chapter 3: Fossils for more information about the West’s prehistoric life.
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.
See Chapter 4: Topography for more detail about the landscapes found in the Western States.
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.
Sedimentary rocks often reveal the type of environment in which they formed by the presence of structures within the rock. Sedimentary structures include ripple marks, cross-beds, mud cracks, and even raindrop impressions. Consider the type of environments in which you see these sedimentary structures today in the world around you.
Ripple marks suggest the presence of moving water (though wind can also create ripples and even dunes). Mud cracks indicate that the sediment was wet but exposed to the air so that it dried and cracked.
Cross-beds form as flowing water or wind pushes sediment downcurrent, creating thin beds that slope gently in the direction of the flow as migrating ripples. The downstream slope of the ripple may be preserved as a thin layer dipping in the direction of the current, across the natural flat-lying repose of the beds. Another migrating ripple will form an additional layer on top of the previous one.
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, 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.
Organizing and Presenting the Case
There are two important tools that geologists use to portray the history of the Earth: the geologic time scale and paleogeographic maps.
The geologic time scale is 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 typically divided 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 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 (more about this later).
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!
How did geologists come up with the timeline for the history of the Earth? Over the course of many years and through the combined work of geologists around the world, the geologic time scale was developed (Figure 1.2). No rock record in any one place contains the complete sequence of rocks from Precambrian to present. Geology as a science grew as geologists studied individual sections of rock. Gradually, evolutionary successions of fossils were discovered that helped geologists determine the relative ages of groups of rocks. Rock units were then correlated with similarly aged rock units from around the world. The names you see for the different periods on the geologic time scale have diverse origins. Time periods were named after dominant rock types, geography, mountain ranges, and even ancient tribes like the Silurese of England and Wales, from which the Silurian period was derived.
Paleogeographic maps portray the probable ancient geography of the Earth. They often appear in sequences designed to show the geologic development of a region. The process of constructing these maps is based on first looking at the rocks and landscapes to develop the geologic history of a specific area. Then, by comparing the histories of neighboring areas, geologists can determine how much of an area experienced a similar history. Because an enormous amount of data is required to construct even a small paleogeographic map, completed sections often show only general details and are frequently subject to considerable debate. Maps of the distant past are particularly difficult to piece together since the important clues are so fragmented.
Evidence for Pangaea
How do we know that Pangaea existed 250 million years ago? Fossil evidence and mountain belts provide some of the clues. For example, the Permian-age fossil plant Glossopteris had seeds too heavy to be blown across an ocean. Yet Glossopteris fossils are found in South America, Africa, Australia, India, and Antarctica! The mountain belts along the margins of North America, Africa, and Europe line up as well and have similar rock types, an indication that the continents at one time were joined as Pangaea. Despite the discovery of Glossopteris and other geologic evidence, the theory of continental drift was not accepted for decades, until the mechanisms of continental movement were discovered and reformulated under the modern theory of plate tectonics. The supercontinent Pangaea existed for approximately 100 million years, reaching its largest size during the Triassic period. During the Jurassic, the landmass began to fragment into the modern continents, slowly moving toward their present-day positions over the following 150 million years.