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Comparative Dating
Comparative or relative dating infers the age of a fossil by comparing the specimen to the rock found around it, as well as any fossils found above or below. Because of how sedimentary rocks form, the younger layers overlie older strata; therefore, the oldest fossils exist in the deeper layers. Paleontologists compared similar strata from different locations, and discovered the layers also contained the same sequence of fossils. Scientists matched the interrelated sequences revealing a historical progression of rock types and fossils, known as "general stratigraphic history." Consequently, the age of the sedimentary layer infers the age of the fossils it contains.
Index Fossils
Some organisms are extremely widespread as fossils with recognizable characteristic changes, making these sequences useful for dating. These well-known fossils are known as index or dating fossils, and help to pinpoint the time of more obscure fossils found with them. Common examples include shelled marine animals such as trilobites, graptolites, ammonites and foraminiferans.
Absolute Dating
Absolute dating or chronometric dating is based primarily on the physical process of radioactive decay. Many minerals contain pure, radioactive, chemical elements. These elements emit radioactivity at a regular rate, eventually changing from one element to another. Scientists measure the relative proportions of different elements in rocks, to deduce when the mineral formed.
Palaeomagnetism
As rocks formed, the Earth's magnetic field affected the small iron-rich particles within the rock, causing the particles to line up. The Earth's magnetic field varies through time, both in strength and orientation. Certain rocks, usually volcanic in origin, preserve these magnetic changes. Paleontologists have dated these sequence changes; therefore, the age of fossils found is directly related to the age of the volcanic layers above or below the specimen.
Radioactive Decay
Each element decays at its own fixed rate. Since carbon decay is relatively fast, it is used in radiocarbon dating for specimens up to 70,000 years old; potassium-argon decay is used for time spans greater the 100,000 years. Uranium decays very slowly, making it effective in dating rocks billions of years old.
Compass-Clinometer
Paleontologists use a device known as a compass-clinometer to measure the incline, or dip, of the boundary separating one layer of rock from the layers above and below at various magnetic compass orientations. Paleontologists assess the nature, formation and movement of each rock layer with the exposed fossil, enabling them to approximate the fossil's age.