For example, about 1.5 percent of a quantity of Uranium 238 will decay to lead every 100 million years.
By measuring the ratio of lead to uranium in a rock sample, its age can be determined.
Geologist Ralph Harvey and historian Mott Greene explain the principles of radiometric dating and its application in determining the age of Earth.
As the uranium in rocks decays, it emits subatomic particles and turns into lead at a constant rate.
Hutton's theories were short on evidence at first, but by 1830 most scientists concurred that Noah's ark was more allegory than reality as they documented geological layering.
The worldwide average natural dose to humans is about 2.4 millisieverts (m Sv) per year.
This is four times more than the worldwide average artificial radiation exposure, which in the year 2008 amounted to about 0.6 m Sv per year.
This radiation interacts with atoms in the atmosphere to create an air shower of secondary radiation, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons.
The immediate dose from cosmic radiation is largely from muons, neutrons, and electrons, and this dose varies in different parts of the world based on the geomagnetic field and altitude.