While people are most familiar with carbon dating, carbon dating is rarely applicable to fossils. What can a scientist do with these two outcrops? An index fossil is a fossil representing a plant or animal that existed for a relatively short duration of time. Let's find out how scientists deal with this common problem by using the fossils inside the rocks. We don't want to use fossils belonging to species that lived for too long; these fossils would show up in more than one rock layer. Therefore, we can use the succession of fossil assemblages to establish the relative ages of rocks. Fossils that are found in many rock layers, therefore living long periods of time, do not qualify as index fossils. Smith was the first person to understand the principle of fossil succession. Review of Relative Dating In previous lessons, we talked about the Geologic Time Scale and how scientists use it to piece together the history of the earth. Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it. This uses radioactive minerals that occur in rocks and fossils almost like a geological clock.
William Smith collected fossils from his work sites and, over time, he learned to recognize which fossils tended to show up in which rock strata. Remember that some species of animals and plants lived for a very long time, while others existed only for a short period of time. Fossil Succession Back in , there lived a land surveyor named William Smith. Some of the scientists' favorite index fossils are trilobites, ammonites and scallop shells. Watch this video to find out how we use index fossils to establish the relative ages of rocks. One outcrop shows layers from one geologic time period, while the other outcrop represents a different time. Absolute Dating Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods. These are the fossils that we want to use for relative dating. This makes it ideal for dating much older rocks and fossils. He worked in Southern England, and he got to see all kinds of different rock strata that were exposed in outcrops and canals. We'd want to use a more short-lived fossil, like the dodo bird. Law of Superposition Knowing the relative age of index fossils in one location helps scientists infer the relative age of fossils and rock layers in other locations. These isotopes break down at a constant rate over time through radioactive decay. What can a scientist do with these two outcrops? Relative dating can be used only when the rock layers have been preserved in their original sequence. The same rock formation also contains a type of trilobite that was known to live to million years ago. The relative age of rocks and fossils can be determined using two basic methods: If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer must be younger in age. So, the relative age of the rock or fossil in the rock or fossil in the rock is older if it is farther down in the rock layers. An index fossil is a fossil representing a plant or animal that existed for a relatively short duration of time. So, how do we know how old a fossil is? He began to identify rock layers by the fossils they contained, and he even noticed that the general order of strata was identical over many different parts of the country. While people are most familiar with carbon dating, carbon dating is rarely applicable to fossils. We want fossils of plants and animals that lived for a relatively short amount of time, like a few hundred thousand years or so. We also want our index fossils to be common, widely-distributed species that are easy for scientists to identify. Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
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