Determining absolute ages by radiometric dating
In other words, you can use superposition to tell you that one rock layer is older than another.
But determining the absolute age of a substance (its age in years) is a much greater challenge.
During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques.
For example, geologists measured how fast streams deposited sediment, in order to try to calculate how long the stream had been in existence.
By dating these surrounding layers, they can figure out the youngest and oldest that the fossil might be; this is known as "bracketing" the age of the sedimentary layer in which the fossils occur.
Teach your students about absolute dating: Determining age of rocks and fossils, a classroom activity for grades 9-12.
In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter.
The longest cores have helped to form a record of polar climate stretching hundreds of thousands of years back.
Another example of yearly layers is the deposition of sediments in lakes, especially the lakes that are located at the end of glaciers.
The universe is full of naturally occurring radioactive elements.
Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms." When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.