Using relative dating the fossil is compared to something for which an age is already known.For example if you have a fossil trilobite and it was found in the Wheeler Formation.Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy.Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events.The universe is full of naturally occurring radioactive elements.Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms." When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.Particular isotopes are suitable for different applications due to the type of atoms present in the mineral or other material and its approximate age.For example, techniques based on isotopes with half lives in the thousands of years, such as Carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.
Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium.
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.
The majority of the time fossils are dated using relative dating techniques.
In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter.
This pattern of growth results in alternating bands of light-colored, low density "early wood" and dark, high density "late wood".
Droughts and other variations in the climate make the tree grow slower or faster than normal, which shows up in the widths of the tree rings.