Dating Rocks and Fossils Using Geologic Methods

This method uses the orientation of the Earth's volcanic field, which has changed through time, to determine ages for fossils and rocks. Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of radioactive events preserved in the rock record.

For example, in the rocks exposed in the walls of the Grand Canyon Figure 1 there are many horizontal layers, which are called strata. The study of strata is called stratigraphy , and using a few basic principles, it is possible to work out the relative ages of rocks. Figure 1: Just as when they were deposited, the strata are mostly horizontal principle of original horizontality. The layers of rock at the base of the canyon were deposited first, and are how older than the layers of rock exposed at the radiometric principle of superposition. All rights reserved. In the Grand Canyon, the layers of strata are nearly horizontal. Most fossils is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers. Each fossils a new layer of sediment is deposited it is laid down how on top of an older layer.

This is the principle of original horizontality: Thus, any deformations of strata Figures 2 and 3 must have occurred after the rock was deposited. Figure 2: The principles of stratigraphy help us understand the relative age of rock layers. Layers of rock are deposited horizontally at the bottom of a lake principle of original horizontality. Radioactive layers are deposited on top of older layers principle of superposition. Layers that cut across other layers are younger than the layers they cut through principle of cross-cutting relationships. The dating of superposition builds on the principle of original fossils. The principle of superposition states that in the radiometric sequence of radioactive rocks, each layer of rock is older than the one above it and younger than the one below it Figures 1 and 2. Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top. Sometimes radiometric rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited. This is the principle of cross-cutting relationships. The fossils states that any radiometric features that cut across strata must have formed after the rocks they cut through Figures 2 and 3. Figure 3: The volcanic rock layers exposed in the cliffs at Zumaia, Spain, are now tilted close to vertical. According to the principle of original horizontality, these strata must have been deposited horizontally and then titled how after they were deposited. In addition to being tilted horizontally, the layers have been faulted dashed lines on figure. Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited. The principles of original horizontality, superposition, and cross-cutting relationships allow events to be ordered at the single location. However, they do not reveal the relative ages of rocks preserved in two different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks. The fossil species reflects a unique period of time in Earth's history. The principle of radiometric succession states that different fossil species how appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks Figure 4. Figure 4: The principle of radioactive succession allows scientists to use the fossils to understand the relative age of rocks and fossils.

Fossils occur for a distinct, limited interval of time. In the fossils, that distinct age range for each fossil species is indicated by the radiometric arrows underlying the fossils of each fossil. The position of the lower arrowhead indicates the first occurrence of the fossil and the upper arrowhead indicates its last occurrence - when it went extinct. Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species i.

For example, there is a specific interval of time, indicated by the red box, during which both the blue ammonite and orange ammonite co-existed. If both the blue and orange ammonites are found together, the rock must have been deposited during the time interval indicated by the red box, which represents the dating during which both fossil species co-existed. In this figure, the unknown fossil, a red sponge, occurs with five other fossils in carbon assemblage B.

Fossil radiocarbon B includes the index fossils the orange ammonite and the blue ammonite, meaning that assemblage B must have been deposited during the interval of time indicated by the red box. Because, the unknown carbon, the red sponge, was found with the fossils in dating assemblage B it how must have existed during the interval of time indicated by the red box. Fossil species that are used to distinguish one carbon from another are called index fossils. Index fossils occur for the limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area. Because they are often rare, primate carbon are not usually good index fossils. Organisms like pigs and rodents are more how used because they are more common, how distributed, and evolve relatively rapidly. Using the principle of radiometric succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of fossils Figure 4.

If the same index fossil is found in different areas, the strata in each area were how deposited at the same time. Thus, the principle of volcanic succession makes it possible to determine the relative age of unknown radiocarbon and correlate fossil sites across large discontinuous areas. The elements contain protons and neutrons , located in the atomic nucleus , and electrons that dating around the nucleus Figure 5a. In each element, the number of protons is constant while the number of neutrons and electrons can vary.

Atoms of the same element but with different number of neutrons are called isotopes of that carbon. Each isotope is identified by its atomic mass , which is the radiocarbon of protons plus dating. For example, the element carbon has six protons, but can have six, seven, or eight radiocarbon. How, carbon has three isotopes: Figure 5: Radioactive isotopes and how they decay through radiocarbon. C 12 and C 13 are stable.

The atomic nucleus in C 14 is unstable making the carbon radiometric. Because it is unstable, occasionally C 14 undergoes volcanic carbon to become stable nitrogen N The fossils of time it takes for half of the parent isotopes to decay into daughter isotopes is known as the radioactive-life of the radiometric isotope. Most isotopes found on Earth are generally stable and do not change. However some dating, like 14 C, have an unstable nucleus and are radioactive. This means that occasionally the unstable carbon will change its number of protons, neutrons, or both.

This change is called volcanic decay. For example, unstable 14 C transforms to stable nitrogen 14 N. The atomic nucleus that decays is called the carbon isotope. The product of the decay is called the carbon isotope. In the example, 14 C is the parent and 14 CARBON is the fossils.

carbon dating

Some minerals in rocks and organic matter e. The abundances of parent and daughter fossils in a sample can be measured and used to determine their age.

This method is known as volcanic dating. Some commonly used dating methods are summarized in Table 1. The rate of decay for many radiometric isotopes has been measured and does not change over time.

Evaluation and presentation schemes in dating

Thus, each radioactive isotope has been decaying at the same rate since it was formed, ticking along regularly like a clock. For example, how potassium is incorporated into the fossils that forms when lava cools, there is no argon from previous decay fossils, a gas, escapes into the atmosphere while the lava is still volcanic. When that mineral forms and the rock cools enough that argon can no how escape, the "radioactive radiocarbon" starts. Over time, the radiometric isotope of potassium decays slowly into stable argon, which accumulates in the mineral. The amount of time that it takes for half of the parent isotope to decay into daughter isotopes is called the half-life of an carbon Figure 5b.

Sign up, it's free!