Absolute dating numerical age geologic time
This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.
Unlike people, you can’t really guess the age of a rock from looking at it.In addition, like any good scientific measurement, every dated boundary has an uncertainty associated with it, expressed as " - X millions of years".These can not be included in the diagram for practical reasons, but can be found in Harland , 1990, along with a detailed description of the history of earlier-proposed time scales and the terminology, methodology and data involved in constructing this geological time scale.Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.Relative age dating also means paying attention to crosscutting relationships.Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.
In a way this field, called geochronology, is some of the purest detective work earth scientists do.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
Pretty obvious that the dike came after the rocks it cuts through, right?
This geological time scale is based upon Harland , 1990, but with the Precambrian/Cambrian boundary modified according to the most recently-published radiometric dates on that interval, revising the boundary from 570 -15 million years to 543 -1 million years ago (Grotzinger , 1995).
Other changes have been proposed since 1990 (e.g., revision of the Cretaceous by Obradovich, 1993), but are not incorporated because they are relatively small.
It is important to realize that with new information about subdivision or correlation of relative time, or new measurements of absolute time, the dates applied to the time scale can and do change.