Absolute dates must agree with dates from other relative methods in order to be valid.
The most widely used and accepted form of absolute dating is radioactive decay dating. Radioactive decay refers to the process in which a radioactive form of an element is converted into a nonradioactive product at a regular rate.
Steno's idea that fossils are older than the rock in which they are found hints at this principle, but Hutton is most often given credit for this principle.states that fossil organisms succeed one another in a definite, irreversible, and determinable order.
This law was independently discovered by William Smith (1769-1839), a British engineer, while working on excavations for canals in England (Winchester, 2002 p.
Since certain species of animals existed on Earth at specific times in history, the fossils or remains of such animals embedded within those successive layers of rock also help scientists determine the age of the layers.
Similarly, pollen grains released by seed-bearing plants became fossilized in rock layers.
Cosmic rays: Invisible, high-energy particles that constantly bombard Earth from all directions in space.
The nucleus of every radioactive element (such as radium and uranium) spontaneously disintegrates over time, transforming itself into the nucleus of an atom of a different element.
In the process of disintegration, the atom gives off radiation (energy emitted in the form of waves). Each element decays at its own rate, unaffected by external physical conditions.
Once Steno recognized that the fossils he was contemplating (sharks teeth and sea shells) were formed in the sediments of oceans he was able to work out the basic rules of stratigraphy.
Steno formalized the laws of superposition, original horizontality, original continuity and inclusions in his publication entitled states that any inclusion is older than the rock that contains it.
Playfair later commented that, "the mind seemed to grow giddy by looking so far into the abyss of time." Mc Phee (1998) points out that Hutton removed humans from a specious place in time just as Copernicus had removed humans from a specious position in the universe (p. Hutton gives us three more laws to consider when seeking relative dates for rock layers, one of which, the law of inclusions was described earlier.states any feature that cuts across a rock or sediment must be younger than the rock or sediment through which it cuts.