Isotope best dating very old rocks
At any moment, the ratio between them is a measure of the time elapsed, as long as the system remains closed.
But if the hourglass were to break (become an open system), sand leaks out and the hourglass is no longer a reliable tool for telling time.
If, however, the rock is subjected to intense heat or pressure, some of the parent or daughter isotopes may be driven off.
Therefore, scientists perform radiometric dating only on rocks or minerals that have remained closed systems.
The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.
As magma cools, radioactive parent isotopes are separated from previously formed daughter isotopes by the crystallization process.
The discovery gave scientists a tool for dating rocks that contain radioactive elements.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.
Once scientists have determined the parent-daughter ratio, they can use this measurement along with half-life of the parent to calculate the age of a rock containing the radioactive isotope.
Radiometric dating has shown that very old rocks--3.5 billion years or older--occur on all the continents.
Therefore, their ages indicate when they were formed.