%There are several methods of directly dating some types of rock.
Radiometric dating
Measures the degree of decay of various isotopes (different forms of the same chemical element) that are contained in particular minerals in the rock.
Paleomagnetic dating
Scientists measure the magnetism left within a rock.
Fission track dating
Scientists examine the effects of uranium breakdown in zircon crystals.
More on Radiometric dating
Two pairs of elements are compared. The most widely used pair is potassium 40 and argon 40.
Potassium is a common element in minerals such as feldspars, which are frequently present in basalt and other igneous (vcolcanic) rocks. A small proportion of naturally occurring potassium includes potassium 40, which decays at a steady, and known, rate to produce argon 40 (a gas).
When a rock is molten, any accumulated argon gas can escape, and the "rock clock" is reset. Once the rock solidifies, the argon gas starts to build up again, trapped inside the rock. The longer the time since the rock solidified, the greater the amount of argon gas relative to the amount of potassium 40 in a rock gives an accurate measure of how much time has elapsed since the rock solidified from its molten state.
Other pairs are rubidium/strontium, uranium/lead, and samarium/neodymium.
Carbon 14 dating is perhaps the most famous dating term.
When an organism is alive, it takes in carbon 14, along with the rest of the carbon it needs from the environment in order to live. When that organism dies, it stops taking in carbon and the C14 that has accumulated inside begins to decay.
As in other radiometric techniques, getting a C14 date involves measuring how much of the isotope is left in an organism. However, carbon 14 decays fairly rapidly and is only useful for dating once-living material less than 50,000 years old. It is oif no use in dating dinosaur fossils.
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