Absolute and relative dating methods
Absolute and relative dating methods are essential techniques used by geologists to determine the age of rocks and to arrange geological events in a sequential order. Absolute dating provides the actual age of rocks in years, primarily through radiometric methods that measure the decay of radioactive isotopes such as potassium-40 and uranium-238. By analyzing the ratio of parent isotopes to their daughter products, scientists can calculate the age of a rock based on the known half-lives of these isotopes.
In contrast, relative dating establishes the sequence of geological events without determining their exact ages. This method relies on principles of stratigraphy, which examines rock layers (strata) to infer the order in which geological processes occurred. Key principles, such as the Principle of Superposition and the Principle of Faunal Succession, allow scientists to correlate and compare rock layers and fossils from different locations. By integrating both absolute and relative dating methods, geologists can construct a more comprehensive understanding of Earth's history and the timing of past events. This dual approach enriches the study of geology, providing insights into the dynamic changes that have shaped our planet over millions of years.
Absolute and relative dating methods
Absolute and relative dating are the two basic methods used to date rocks and arrange geological events in sequential order. Absolute dating determines the actual age of rocks while relative dating uses rocks and fossils to put geological events in a sequence. Scientists often use radiometric methods for absolute dating. These methods involve measuring radioactive elements and their isotopes, which are found in rocks. After measuring the isotopes, a scientist can use the known half-life of the isotopes to determine the actual age of a rock. In relative dating, scientists use the fundamental principles of stratigraphy, as well as a process called correlation, to determine a sequence of geological events. Scientists often make correlations between rocks and fossils in different locations to determine the succession of events.
Overview
Scientists use absolute dating to determine a rock’s actual age in years. Specifically, they typically use radiometric methods to obtain the age. Radiometric methods measure radioactive elements, which are found in rocks. These radioactive elements have different forms, or isotopes. In a process called radioactive decay, the isotope of the radioactive element, or parent, eventually breaks down, and the radioactive element transforms into a different element, or daughter. This decay happens at a constant rate for each radioactive element. The rate is measured by the half-life of the isotope, which is the time it takes for half of the parent to decay into the daughter. For example, potassium-40 has a half-life of 1.25 billion years and decays into argon-40. This means that it takes 1.25 billion years for half of the potassium-40 to become argon-40. Scientists can use this information to determine the age of rocks. A scientist would test a sample of rock to determine the ratio of potassium-40 to argon-40 in the rock. The scientist would then use the half-life to figure out the rock’s exact age. For example, if the rock contained three parts potassium-40 to one part argon-40, the rock would be 625 million years old. Besides potassium-40, other isotopes are commonly used for dating rocks. They include the following:
- Uranium-235, which decays into lead-207 with a half-life of 704 million years
- Uranium-238, which decays into lead-206 with a half-life of 4.5 billion years
- Thorium-232, which decays into lead-208 with a half-life of 14 billion years
- Lutetium-176, which decays into hafnium-176 with a half-life of 35.9 billion years
- Rubidium-87, which decays into strontium-87 with a half-life of 48.8 billion years
- Samarium-147, which decays into neodymium-143 with a half-life of 106 billion years
Many scientists measure these isotopes using a mass spectrometer to date igneous and some metamorphic rock. Igneous rock is formed when lava or magma cools. Metamorphic rock is formed from other rock through heat and pressure.
While absolute dating provides the actual age of rocks, relative dating does not. Instead, relative dating uses rocks and fossils to provide the sequential order of geological events. Relative dating is primarily used in the branch of geology called stratigraphy, which studies rock layers, or strata. Rock layers occur in sedimentary rock, which is made up of particles of other rock. In other words, sedimentary rock forms in layers.
Stratigraphy has four fundamental principles—the Principle of Original Horizontality, the Principle of Lateral Continuity, the Principle of Superposition, and the Principle of Faunal Succession. The Principle of Original Horizontality states that sediments are deposited on Earth’s surface in horizontal layers. Although these layers may later tilt or fold, they are normally laid down horizontally. The Principle of Lateral Continuity maintains that layers of rock extend for distances ranging from a few meters to hundreds of kilometers. The Principle of Superposition declares that older layers of rock lie beneath younger layers. Lastly, the Principle of Faunal Succession states that fossils found in layers of rock change in vertical succession.
Scientists can use the four fundamental principles of stratigraphy to determine a sequence of geological events based on relative time. Derived from the Principle of Superposition, relative dating of rocks and fossils can help put these geological events in sequential order. A process called correlation is important in determining the succession of events. Correlation is the process of matching rocks or fossils in one location to those in another location. Making correlations between rocks in different areas uses the Principle of Lateral Continuity while making correlations between fossils in different areas uses the Principle of Faunal Succession. The basic idea behind correlation is as follows: Most species live for only a few million years. They then evolve into other species or become extinct. Therefore, if rocks in different locations contain fossils of the same species, then the rocks were likely both formed just a few million years apart.
Scientists can incorporate both absolute and relative dating. They can determine the actual ages of rocks using absolute dating and then incorporate those rocks into a succession of rock layers using relative dating. In this sense, the ages of rocks can be inferred using correlation even though the ages cannot be directly dated.
Bibliography
Department of Paleobiology, National Museum of Natural History, Smithsonian Institution. “Absolute Dating.” Department of Paleobiology, National Museum of Natural History, Smithsonian Institution. Smithsonian Institution. Web. 5 Nov. 2014. <http://paleobiology.si.edu/geotime/main/foundation‗dating3.html>
Department of Paleobiology, National Museum of Natural History, Smithsonian Institution. “Relative Dating.” Department of Paleobiology, National Museum of Natural History, Smithsonian Institution. Smithsonian Institution. Web. 5 Nov. 2014. <http://paleobiology.si.edu/geotime/main/foundation‗dating2.html>
Science Learning Hub. “Absolute Dating.” Science Learning Hub. University of Waikato. 20 May 2011. Web. 5 Nov. 2014. <http://www.sciencelearn.org.nz/Contexts/Dating-the-Past/Science-Ideas-and-Concepts/Absolute-dating>
Science Learning Hub. “Relative Dating.” Science Learning Hub. University of Waikato. 18 May 2011. Web. 5 Nov. 2014. <http://www.sciencelearn.org.nz/Contexts/Dating-the-Past/Science-Ideas-and-Concepts/Relative-dating>