Relative dating which rock layer formed first
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.
Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.
This follows due to the fact that sedimentary rock is produced from the gradual accumulation of sediment on the surface.
Therefore newer sediment is continually deposited on top of previously deposited or older sediment.
On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1.26 billion years.
The first principle is the Principle of Superposition which states that in an undisturbed succession of sedimentary rock, the oldest layers are on the bottom.
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.
Chart of a few different isotope half lifes: In reality, geologists tend to mix and match relative and absolute age dates to piece together a geologic history.
If a rock has been partially melted, or otherwise metamorphosed, that causes complications for radiometric (absolute) age dating as well.