Historical Geology
All areas of geologic study are subdisciplines of either historical geology, which focuses on the chemical, physical, and biological history of Earth, or physical geology, which is the study of Earth materials and processes. Historical geology uses theory, observation, and facts derived from studying rocks and fossils to learn about the evolution of Earth and its inhabitants.
According to the principle of uniformitarianism, most physical and chemical processes occurring today are very similar to those that operated in the geologic past, although their rates may be different. Therefore, by studying modern geologic activities and their products, geologists can understand how these activities produced the ancient rock record. In other words, the present is the key to the past. The principle of uniformitarianism has been very useful in deciphering much of the rock record.
Studies in historical geology rely on the rock record for factual information about Earth's past. As geologists collect data, they develop hypotheses to explain phenomena they observe. Geologists test hypotheses by making further observations of rocks and the fossils they contain. If this and other research supports a hypothesis, eventually it will be accepted as a theory explaining how Earth, and the life on it, evolved through time.
Rocks preserve a record of the events that formed them. The trained observer can examine the physical, chemical, and biological characteristics of a rock and interpret its origin. Fossils are an especially useful type of biological evidence preserved in sedimentary rocks (they 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 very specific insights into the environment that formed that rock.
In addition to body fossils, sediments also preserve a variety of tracks and trails (for example, footprints, burrows, etc.). These biological impressions preserve traces of the daily activities of organisms, rather than their bodies, and so are called trace fossils. These too provide important clues to certain aspects of Earth history.
Through studies of rocks and fossils, geologists have produced what is called the geologic time scale. This is a convenient way of representing the vast amounts of time and the numerous details of historical geology in a way that is easily expressed and understood. The geologic time scale consists of the dates of major events in Earth's history, placed in chronological order. These events, primarily major extinctions and episodes of organic evolution, separate the scale into distinct time units. From largest to smallest, these units are the geologic eon, era, period, and epoch. The age of each boundary event is determined by radiometric dating of rocks associated with the time unit boundary. Radiometric dating uses the rates of atomic decay for radioactive elements to determine the age of geologic materials.
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