Little is known about this time period since very few fossils or unaltered rocks have survived. The first of these, the Precambrian, extends from about 4.6 billion years ago to 541 million years ago. It spans Earth’s entire history and is typically divided into four principle divisions. ![]() The geologic time scale is a standard timeline used to describe the age of rocks and fossils, and the events that formed them. There are two important tools that geologists use to portray the history of the Earth: the geologic time scale and paleogeographic maps. ![]() 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. Because younger environments retain more evidence than older environments, the Earth’s recent history is better known than its ancient past. Ultimately, geologists rely upon the preserved clues of ancient geologic processes to understand Earth’s history. Another migrating ripple will form an additional layer on top of the previous one. 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. 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.Ĭross-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 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. 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. 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. 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. In general, igneous rocks, created through tectonic activity, reflect past volcanism. Rocks and sediments are indicators of past geologic processes and the environments in which those processes took place. See Chapter 2: Rocks to learn more about different rocks found in the West. ![]() Second, much of the evidence for the older events is highly fragmented. First, the overwhelming majority of geologic history occurred long before there were any human witnesses. 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. Reconstructing the past is a lot like solving a mystery. This not only shapes the land over time, but it also affects the distribution of rocks and minerals, natural resources, climate, and life. The continents likewise continuously shift position because they are part of the moving plates. 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 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). ![]() Figure 1.1: The layers of the Earth include the rigid crust of the lithosphere, which is constantly moving over the plastically flowing asthenosphere.
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