Absolute dating is more exact, but dates in geologic time are spans of time, not specific dates. Determining absolute age is done with radiometric dating, the scientific process of using the half-life of radioactive elements to establish the absolute age of a fossil. Rocks contain trace amounts of radioactive elements that decay at specific rates, starting from when the rock was formed. Radiometric dating is like a clock that measures thousands or millions of years, depending on the type of isotope being used for analysis. Carbon isotopes allow for accurate dating of remains up to 40,000 years old. Older fossils require the use of isotopes with long half-life periods, such as uranium-253 and potassium-40.
Radioactive material decays from the parent isotope to the daughter isotope. Its rate of decay is measured by the amount of time it takes for a parent isotope to decay so that the isotopes measured are 50% parent and 50% daughter, called a half-life. Decay continues until, after six half-life periods, the parent isotope is completely replaced by the daughter isotope. As an example: A bone is dated using carbon-14. The amount of carbon-14 (parent) present compared to the amount of nitrogen-14 (daughter) present is 25% carbon-14 and 75% nitrogen-14. The half-life of carbon-14 is 5,730 years. The carbon isotope has decayed through two half-life periods, or 11,460 years.
Using isotopes for dating is not always successful. Mollusks get the carbon in their shells from aged carbonate. Their shells form with decayed carbon-14, so dating the half-life of carbon-14 in such a shell gives a false result. Plants get carbon from carbon dioxide while they are living; animals that eat plants get carbon that has already started the decay process. Sediments that surround a fossil during formation may be from rock formed at different ages, so determining the fossil's absolute age is more complex and less accurate.
Radiometric Dating Isotopes
|Isotope||Daughter Isotope||Half-life||Dated Material|
|Uranium-238||Lead-206||4.5 billion years||Zircon|
|Uranium-235||Lead-207||713 million years||Zircon|
|Potassium-40||Argon-40||1.3 billion years||Biotite, muscovite, volcanic rock|
|Carbon-14||Nitrogen-14||5,730 years||Plant fossils, bone, wood|
Major Mass Extinctions
|Extinction Event||Age (million years ago)||Percentage of Species Extinct|
|Ordovician||439 mya||Up to 85%|
The Ordovician mass extinction, which occurred 439 million years ago, may have been caused by rapid plate tectonic shifting combined with an ice age that reduced sea levels. Up to 60% of marine genera died in this extinction, and up to 86% of all species became extinct.
The Devonian extinction may have been a single catastrophic event, such as an asteroid striking Earth, or it may have been the result of another ice age. In this extinction, which occurred 367 million years ago, ocean and sea life suffered the greatest losses, but up to 80% of species disappeared.
The Permian episode, which occurred 245 million years ago, had the greatest impact on living organisms, destroying 95% of all marine species as well as trilobites, large land insects, and seed ferns; 96% of all species became extinct during this event. It was most likely caused by several smaller events that resulted in a complete collapse of the environment.
The Triassic has been a frustrating mystery to scientists because there appears to be no single cause and extinctions cascaded over a period of 10,000 years. This extinction occurred 208 million years ago, and 76% of species became extinct.
The most recent, the Cretaceous extinction, brought about the end of the dinosaurs as well as many reptiles, birds, and mammals. It is possible that the Cretaceous extinction was caused by an asteroid striking near Mexico's Yucatan peninsula and a simultaneous climate cooling period, possibly due to mass volcanic activity in Siberia. This extinction, which occurred 65 million years ago, resulted in 76% of species becoming extinct.Current extinction rates are such that scientists are concerned that Earth may soon experience a sixth mass extinction. The combination of human factors and global climate change may produce a cascading event in which extinctions continue to occur exponentially. How this will, ultimately, impact humans and the Earth is unclear.