History of radiometric dating

Layers that cut across other layers are younger than the layers they cut through (principle of cross-cutting relationships).The principle of superposition builds on the principle of original horizontality.These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.In addition to being tilted horizontally, the layers have been faulted (dashed lines on figure).Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited.

There are three general approaches that allow scientists to date geological materials and answer the question: "How old is this fossil?

Third, magnetism in rocks can be used to estimate the age of a fossil site.

This method uses the orientation of the Earth's magnetic field, which has changed through time, to determine ages for fossils and rocks.

Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of geological events preserved in the rock record.

For example, in the rocks exposed in the walls of the Grand Canyon (Figure 1) there are many horizontal layers, which are called strata.