![]() This section discusses principles of relative time used in all of geology, but are especially useful in stratigraphy. Stratigraphy is the study of layered sedimentary rocks. James Hutton (see Chapter 1) realized geologic processes are slow and his ideas on uniformitarianism (i.e., “the present is the key to the past”) provided a basis for interpreting rocks of the Earth using scientific principles. The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the scientific revolution of the 17th and 18th centuries. 7.1 Relative DatingĪ Geologic Time Scale Relative dating is the process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages-i.e., how many years ago the object was formed. The specific events within Earth history are discussed in Chapter 8. Using this time scale, geologists can place all events of Earth history in order without ever knowing their numerical ages. The figure of this geologic time scale shows the names of the units and subunits. Eventually he produced the first national geologic map of Britain, becoming known as “the Father of English Geology.” Nineteenth-century scientists developed a relative time scale using Steno’s principles, with names derived from the characteristics of the rocks in those areas. William Smith (1769-1839), working with the strata of English coal mines, noticed that strata and their sequence were consistent throughout the region. Nicolas Steno (1638-1686) introduced basic principles of stratigraphy, the study of layered rocks, in 1669. Working out Earth’s history depended on realizing some key principles of relative time. The geologic time scale and basic outline of Earth’s history were worked out long before we had any scientific means of assigning numerical age units, like years, to events of Earth history. Explain the relationship between time units and corresponding rock units-chronostratigraphy versus lithostratigraphy.List the eons, eras, and periods of the geologic time scale and explain the purpose behind the divisions.Outline how natural selection takes place as a mechanism of evolution.Define a fossil and describe types of fossils preservation.Explain how sedimentary sequences can be dated using radioisotopes and other techniques.Explain how scientists know the numeric age of the Earth and other events in Earth history.Explain how carbon-14 forms in the atmosphere and how it is used in dating recent events.Describe how radioisotopic dating is accomplished and list the four key isotopes used.Define an isotope, and explain alpha decay, beta decay, and electron capture as mechanisms of radioactive decay. ![]() Apply relative dating principles to a block diagram and interpret the sequence of geologic events.Describe the five principles of stratigraphy. ![]() Explain the difference between relative time and numeric time. ![]() To suggest that a vast pile of strata had not simply been called into existence but had developed over a very long period of time was a new paradigm.Perhaps no place on Earth better exemplifies the principles geologists use to determine the ages of rocks than Arizona’s Grand Canyon National Park. Although this might appear to be somewhat obvious it is important to remember that this principle was being introduced to a world that believed that everything around us had been created over a period of days. The Principle of Superposition states that in layered strata (sedimentary rocks or lava flows), the oldest layer will be at the bottom of the exposed strata and the youngest at the top. However, this principle basically states that " tilted or folded layers used to be flat”. After they have been transformed into rock, strata may become tilted or folded by various tectonic movements. Īccording to the Principle of Original Horizontality, sediments are deposited horizontally. For a 4:13 minutes introduction to these concepts see. ![]()
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