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2.3 distinguish between and describe some relative and absolute techniques used for dating fossils
distinguish between and describe some relative and absolute techniques used for dating fossils
- Once a fossil is discovered it is important to be able to place the fossil in the history of life. This is called dating the fossil. There are two different methods of dating fossils, relative dating and absolute dating.
- Relative dating ages fossils by comparing the fossil with other known fossils. The date that is reached is relative to the date of another fossil. Examples of this type of dating are stratigraphy, palaeomagnetism and faunal dating.
- Stratigraphy is the study of rock layers or strata. Fossils found in lower rock levels are thought to be older than fossils found in higher layers. This is called the principle of stratigraphic superposition. Some fossils that are particularly useful for stratigraphic dating are called index fossils.
- Palaeomagnetism is based on the periodic reversals of the magnetic field that Earth experiences. As rocks solidify from lava they record the magnetic field of the Earth at the time of solidification. By comparing the patterns of magnetic fields in rocks it is possible to find deposits that were formed at the same time.
- Faunal dating or biostratigraphy uses common fossils to date other fossils. For example in Africa the fossil pig record can be used to date the fossils of early humans.
Relative dating
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- Absolute dating seeks to place an actual age on a fossil. Examples of absolute dating techniques include dendrochronology, radiocarbon dating, thermoluminescence and potassium-argon dating.
- Dendrochronology is the study of tree rings. Deciduous trees loose their leaves during the winter. At this time growth stops, so for each winter that a tree experiences a ring in the cross section of the stump can be seen. When there are periods of drought or good growing seasons then the size of the ring changes. Some trees such as the Giant Sequoia live for thousands of years so by comparing fossilised tree stumps it is possible to use dendrochronology to absolute age as far back as 6500 years ago.
Dendrochronology
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- Radiocarbon dating using the amount of radioactive carbon that is found in fossils such as charcoal or bones. Most naturally occur carbon is carbon -12. Carbon -14 is produced in the upper atmosphere by cosmic radiation bombarding nitrogen -14. The carbon joins with oxygen to form carbon dioxide. Plants take in this carbon dioxide during photosynthesis and it becomes part of the body structure. It moves through the food chain when an animal eats the plant. The half-life of carbon-14 is 5730 years. This means that after 5730 years half of the original material will be available. When an organism dies the carbon-14 that has been taken in during the life times decay. The original amount of carbon-14 is assumed and then compared with the remaining C-14 in the fossil. The older the fossil the less C-14 that remains.
Virtual dating exercise
Geology Labs Online.
- Thermoluminescence uses the property of minerals such as quartz, feldspar and calcite to emit light when they are heated. The amount of light shows when the object was last heated. A piece of pottery when fired has its thermoluminescence clock set to zero. Over time natural radiation causes the amount of thermoluminescence to increase. This technique is useful for inorganic items up to 200,000 years of age. It is used to date campfire stones, pottery and figurines.
Thermoluminesence
Wikipedia
- Potassium argon is another radiometric measuring system that is similar to carbon dating. Naturally occurring radioactive potassium decays to argon and calcium over time. The higher the potassium to argon ratio the older the sample. It is used to age volcanic flows that lie between fossil beds.