How do geologists date rocks? Radiometric dating!
Radioactive elements were incorporated into the Earth when the Solar System formed. All rocks and minerals contain tiny amounts of these radioactive elements. Radioactive elements are unstable; they breakdown spontaneously into more stable atoms over time, a process known as radioactive decay. Radioactive decay occurs at a constant rate, specific to each radioactive isotope. Since the 1950s, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks.
Radiometric clocks are "set" when each rock forms. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools off. It's this resetting process that gives us the ability to date rocks that formed at different times in earth history.
A commonly used radiometric dating technique relies on the breakdown of potassium (40K) to argon (40Ar). In igneous rocks, the potassium-argon "clock" is set the moment the rock first crystallizes from magma. Precise measurements of the amount of 40K relative to 40Ar in an igneous rock can tell us the amount of time that has passed since the rock crystallized. If an igneous or other rock is metamorphosed, its radiometric clock is reset, and potassium-argon measurements can be used to tell the number of years that has passed since metamorphism.
Carbon-14 is a method used for young (less than 50,000 year old) sedimentary rocks. This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things. When living things die, they stop taking in carbon-14, and the radioactive clock is "set"! Any dead material incorporated with sedimentary deposits is a possible candidate for carbon-14 dating.
Radiometric dating has been used to determine the ages of the Earth, Moon, meteorites, ages of fossils, including early man, timing of glaciations, ages of mineral deposits, recurrence rates of earthquakes and volcanic eruptions, the history of reversals of Earths magnetic field, and many of other geological events and processes.