Absolute Ages of Rocks

Isotopes

All atoms of a given element have the same number of protons. An element's atoms can have different numbers of neutrons. The different forms of an element's atoms are called isotopes.

Half-Life

An isotope's half-life is the time required for half of the parent isotopes to decay into daughter isotopes.

Isotopes

Atoms of the same element that have different numbers of neutrons.

Radiometric Ages

Because radioactive isotopes decay at a constant rate, they can be used like clocks to measure the age of the material that contains them. This process is called radiometric dating. Scientists measure the amount of parent isotope and daughter isotope in a sample of the material they want to date.

Radiocarbon Dating

One important radioactive isotope used for dating is an isotope of carbon called C-14. It is called this because there are 14 particle in its nucleus-six protons and eight neutrons. The half life of carbon-14 is 5,730 years,

Dating Igneous Rocks

One of the most common isotopes used in radiometric dating is uranium-235 or U-235. U-235 is often trapped in the minerals of igneous rocks that crystallize from hot, molten magma. As soon as U-235 is trapped in a mineral, it begins to decay to lead-207, or Pb-207.

Atoms

Each element is made up of atoms. An atom is the smallest part of an element that has all the properties of the element. Each atom contains smaller particles called protons, neutrons, and electrons.

Dating Sedimentary Rock

In order to be dated by radiometric means, a rock must contain U-235 or any other radioactive isotopes. The grains in many sedimentary rocks come from a variety of weathered rocks from different locations. The radioactive isotopes within these brains typically record the ages of the grains-not the time when the sedimentary rock was formed, making it difficult to date using radiometric dating.

Dating Rocks

Radiocarbon dating is useful only for dating organic material-material from once living organisms. These materials include bones, wood, parchment, and charcoal. Most rocks do not contain organic material-most fossils are not organic anymore.

Radioactive Decay

Stable isotopes don't change under normal conditions. Some isotopes are unstable. These isotopes are called radioactive isotopes. They decay, or change, over time.

Different Types of Isotopes

The half-life of U-235 is 704 million years. This makes it useful for dating rocks that are very old. Radioactive isotopes with short half-lives cannot be used for dating old rocks.

Absolute Age

The numerical age, in years, of a rock or object.

The Age of Earth

The oldest known rock formation dated by geologists using radiometric means is in Canada. It is estimated to be between 4.03 billion years old and 4.28 billion years old. Radiometric dating of rocks from the Moon and and meteorites indicate that Earth is 4.54 years old.

Radioactive Decay

The process by which an unstable element naturally changes into another element that is stable.

Half-Life

The rate of decay from parent isotopes into daughter isotopes is different for different radioactive elements. The rate of decay is constant for a given isotope. This rate is measured in time units called half-lives.

Absolute Ages of Rocks

Your numerical age is your age in numbers. Similarly, scientists can describe the ages of rocks numerically. The absolute age of a rock or object is its numerical age.