Relative age dating also means paying attention to crosscutting relationships.Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.In a way this field, called geochronology, is some of the purest detective work earth scientists do.There are two basic approaches: relative age dating, and absolute age dating.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.One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains.Unlike people, you can’t really guess the age of a rock from looking at it.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 the 1860's, English physicist Lord Kelvin disagreed with Charles Lyells proposition that the earth behaves in a uniform, unchanging manner.Strata Thickness- In the late 1800s, a British geologist estimated that 75 million years has lapsed since the beginning of the Cambrian.This estimate was based upon the maximum known thickness of strata (from Cambrian to present) divided by the average rate of sedimentation in modern environments. Joly used the salinity of ocean water to determine the age of the earth.In geology, an absolute age is a quantitative measurement of how old something is, or how long ago it occurred, usually expressed in terms of years.Most absolute age determinations in geology rely on radiometric methods. The most useful methods for measuring the ages of geologic materials are the radiometric methods-the ones that make use of radioactive parent isotopes and their stable daughter products, as preserved in rocks, minerals, or other geologic materials.