Radioactivity in geologic dating
The meteorites that fall to Earth today have orbited the Sun since that time, unchanged and undisturbed by the processes that have destroyed Earth's first rocks.Radiometric ages for these meteorites fall between 4.45 and 4.55 billion years old.Geologic time describes the immense span of time—billions of years—revealed in the complex rock surface of Earth.Geologists have devised a geologic time scale that divides Earth's history into units of time. A unit is defined in terms of the fossils or rock types found in it that makes it different from the other units.This method employs the natural process of radioactive decay.Every rock and mineral exists in the world as a mixture of elements. At the center of an atom is the positively charged nucleus made up of protons and neutrons.The two most recent periods are further subdivided into seven epochs.Before the eighteenth century, ideas about time and the history of Earth came mostly from religious theories.
They also believed that all the physical features of Earth—mountains, valleys, oceans, rivers, continents—were the same as they had always been.However, since they lacked sophisticated scientific measuring devices, they could only offer educated guesses.They compared the rock record from different parts of the world and estimated how long it would take natural processes to form all the rocks on Earth.The fossils of changing lifeforms found in the different strata also help geologists determine the long history of Earth.Determining the age of strata by looking at the fossils, position, grain size, minerals, color, and other physical properties contained within them is known as relative dating.
Radioactive decay: The predictable manner in which a population of atoms of a radioactive element spontaneously fall apart.