Compare and contrast relative dating and absolute dating
Relative dating is a scientific process of evaluation used to determine the relative order of past events, but does not determine the absolute age of an object.The circumstances of the object may allow one to say that one object is older than another without being able to assign a particular age to the objects.
Carbon dating is one example of radiometric dating.Dating the ash layers above and below a sedimentary rock layer to determine its age is called bracketing.Radiometric dating uses the decay of unstable isotopes -- atoms with specific electrical charges -- to calculate something's age.Before radiometric dating (or other methods of absolute dating like counting tree rings) it was difficult to determine the actual age of an object.Radiometric dating, based on known rates of decay of radioactive isotopes in objects, allows a specific age of an object to be determined to some degree of accuracy.Some rock layers are surrounded by volcanic debris, or tuff, in situ, meaning they weren't broken by igneous intrusions; rather, local volcanic activity simply blanketed an area with ash at various times.
These areas are the easiest to date because volcanic debris can usually be radiometrically dated with a high degree of accuracy.
Radioactive decay of uranium was first discovered in 1896 by Henry Becquerel, a French physicist.
By 1905, the British physicist Lord Rutherford made the first clear suggestion for using radioactivity as a tool for measuring geologic time directly; shortly thereafter, in 1907, Professor B. Boltwood, radiochemist of Yale Uniyersity, published a list of geologic ages based on radioactivity.
Very often historical evidence is found in layers and older layers are further down that the top layers.
For example: If an archaeologist is studying past civilizations, the archaeologist may be able to say that in a particular location the ruins of one civilization were found to have been built on another and so the layers unearthed in an excavation convey the sequence of historical occupations without revealing the actual dates.
If you know this rate and you know the proportion of potassium 40 to argon 40 in the surrounding ash, you can estimate the age of the surrounded rock layer.