What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.
You might have noticed that many of the oldest age dates come from a mineral called zircon.
Carbon-14 dating is a way of determining the age of certain archeological artifacts of a biological origin up to about 50,000 years old.
It is used in dating things such as bone, cloth, wood and plant fibers that were created in the relatively recent past by human activities.
But the most accurate forms of absolute age dating are radiometric methods. Sedimentary rocks in particular are notoriously radioactive-free zones.
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.
A child mummy is found high in the Andes and the archaeologist says the child lived more than 2,000 years ago.
How do scientists know how old an object or human remains are?
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.