Historical geology is the study of changes in Earth and its life forms over time. It includes sub-disciplines such as paleontology, paleoclimatology, and paleoseismology. In addition to providing a scientific basis for understanding the evolution of Earth over time, historical geology provides important information about ancient climate changes, volcanic eruptions, and earthquakes that can be used to anticipate the sizes and frequencies of future events.
Scientific interpretation of Earth's history requires an understanding of currently operating geologic processes. According to the doctrine of actualism, most geologic processes operating today are similar to those that operated in the past. The rates at which the processes occur, however, may be different. By studying modern geologic processes and their products, geologists can interpret rocks that are the products of past geologic processes and events. For example, the layering and distribution of different grain sizes within a sandstone layer may be similar to those in a modern beach, leading geologists to infer that the sandstone was deposited in an ancient beach environment. There have been some past geologic events, however, that are beyond the range of human experience. Evidence of catastrophic events such asteroid impacts on Earth has led geologists to abandon the doctrine of uniformitarianism, which holds that all of the geologic past could be explained in terms of currently observable processes, in favor of actualism.
Rocks preserve evidence of the events that formed them and the environments in which they were formed. Fossils are an especially useful type of biological evidence preserved in sedimentary rocks (they generally do not occur in igneous or metamorphic rocks). Organisms thrive only in those conditions to which they have become adapted over time. Therefore, the presence of particular fossils in a rock provides paleontologists with insights into the environment in which the fossilized organisms lived. Sediments and sedimentary rocks also preserve a variety of tracks, trails, burrows, and footprints known as trace fossils. Information about tree ring widths and changes in the isotopic composition of some sedimentary rocks and glacial ice over time have been used to reconstruct patterns of past climate changes over millennial time scales. These patterns, in turn, provide important information about the magnitude and frequency of future climate changes.
Any study of Earth's history involves the element of time. Relative geologic time considers only the sequence in which geologic events occurred. For example, rock A is older than rock B, but younger than rock C. Relative geologic time is based largely on the presence or absence of index fossils that are known to have existed over limited ranges of geologic time. Using the concept of relative geologic time, geologists in the nineteenth century correlated rocks around the world and developed an elaborate time scale consisting of eons, eras, periods, and epochs. The development of radiometric dating techniques during the second half of the twentieth century allowed geologists to determine the absolute ages of rocks in terms of years and assign specific dates to the relative time boundaries, which had previously been defined on the basis of changes in fossil content.
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