Fossils themselves, and the sedimentary rocks they are found in, are very difficult to date directly. Instead, other methods are used to work out a fossil’s age. These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.
Two major dating methods applied to artifacts and fossils are stratagraphic dating (based upon the particular layer of rock of sediment in which the object is found) or radiom … etric dating (which is based on the decay rates of certain radioactive isotopes). Answer by NerdAlert. Confidence votes 104. Among many methods, carbon dating is most commonly used to date fossils. In carbon dating, scientists look at how much carbon is left in the fossil, look at the half-life period, and use that to see when it was from. By comparing the ratio of parent isotopes to daughter isotopes, scientists are able to determine the age of the fossil and the rock. Isotopes that have long half-lives are necessary to date fossils because many fossils are millions of years old. Uranium-235 is used quite often in the absolute dating of fossils. Uranium-235 has a half-life of 704 million years. Therefore, Uranium-235 can be used to accurately date rocks that are hundreds of million years old. Many people are under the misconception that carbon-14 is used for dating of fossils. Carbon-14 has a half life of 5,730 years, which is too short to be useful in dating fossils. Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they formed. Potassium-40 has a half-life of 1.3 billion years, and so this method is applicable to the oldest rocks. Radioactive potassium-40 is common in micas, feldspars, and hornblendes, though the closure temperature is fairly low in these materials, about 350 °C (mica) to 500 °C (hornblende). This scheme is used to date old igneous and metamorphic rocks, and has also been used to date lunar samples. Closure temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample. Potassium-40 has a half-life of 1.3 billion years, and so this method is applicable to the oldest rocks. Radioactive potassium-40 is common in micas, feldspars, and hornblendes, though the closure temperature is fairly low in these materials, about 350 °C (mica) to 500 °C (hornblende). Fossils themselves, and the sedimentary rocks they are found in, are very difficult to date directly. Instead, other methods are used to work out a fossil’s age. These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages. There are two main methods to date a fossil. These are: 1  absolute dating methods that tell us the actual age (in years) of an object. 2  There are many absolute dating methods. 3  Nearly all of these methods make use of radioactive elements that occur naturally in various types of minerals and organic matter. The Age of Dinosaurs was so many millions of years ago that it is very difficult to date exactly. Scientists use two kinds of dating techniques to work out the age of rocks and fossils. The first method is called relative dating. This considers the positions of the different rocks in sequence (in relation to each other) and the different types of fossil that are found in them. Index fossils are fossils that can be used to date the rock in which they are found. The best examples are fossils of animals or plants that lived for a very short period of time and were found in a lot of places. Ammonites, shelled relatives of today’s octopus, make ideal index fossils. Older fossils cannot be dated by carbon-14 methods and require radiometric dating. Scientists can use different chemicals for absolute dating: 1  The best-known absolute dating technique is carbon-14 dating, which archaeologists prefer to use. 2  However, the half-life of carbon-14 is only 5730 years, so the method cannot be used for materials older than about 70,000 years. Fossil dating is accurate since the method follows strict scientific guidelines: 1  the age of rocks around a fossil can be considered. 2  mathematical calculations are used. 3  the state of decay, carbon-14, and isotopes figure in calculations. 4  tree of life relationships often help sort the dates.
http://www.answers.com/Q/What_methods_are_used_to_date_fossils http://www.ehow.com/way_5422879_methods-dating-fossils.html https://en.wikipedia.org/wiki/Radiometric_dating https://en.wikipedia.org/wiki/Radiometric_dating http://australianmuseum.net.au/dating-dinosaurs-and-other-fossils http://www.factmonster.com/dk/science/dinosaurs/dating-fossils.html http://www.actionbioscience.org/evolution/benton.html
what methods are used to date fossils and rocks