These techniques can be grouped as numerical, relative dating, and correlation.Numerical techniques are best, but datable materials are often lacking, and in these cases age estimation must be made using relative-dating or correlation techniques.Relative-dating techniques are nearly always applicable but are not precise and require calibration.Correlation techniques are locally useful and depend on recognition of an event whose age is known, such as a volcanic eruption or a paleomagnetic reversal.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.
About 26 dating techniques can be applied to dating deposits and deformation of late Cenozoic age (past few million years).
Over time the uranium-238 very slowly decays into uranium-234 (half-life = 248,000 ).
Radium-226 in the soil exhibits the same level of radioactivity as uranium-238 from which it was originally derived, because of a natural phenomenon called secular equilibrium.
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.