Chapter 13 Section 3 Absolute Ages of Rocks

Absolute Age Absolute age is the age in years of a rock or other object. Absolute age is determined by using the properties of atoms that make up materials.

Radioactive Decay Atoms of the same element that have different numbers are called isotopes. Isotopes are unstable and can break down. The process of this breaking down is called radioactive decay.

Half-life The half-life of an isotope is the time it takes for half of the atoms in the isotope to decay. The parent isotope goes through radioactive decay. The daughter product is produced from this decay. The time it takes half of the parent isotope to decay into its daughter product is the half-life of that isotope.

Radiometric Dating By measuring the ratio of parent isotope to daughter product, and by knowing the half-life of the parent, scientists can calculate the absolute age or a rock. This process is called radiometric dating. Carbon-14 dating is used to date bones, wood, and coal up to 75,000 years old. (half-life of carbon is 5,730 years.)

Radiometric Dating continued… Potassium-Aragon dating is used to date rocks millions of years old (half-life of potassium-40 is 1.25 billion years.) Dating is useful for igneous and metamorphic rocks. This dating is not useful for sedimentary rocks, because they are made of pieces of older rocks. The oldest rocks are 3.96 billion years old. The Earth is estimated to be 4.5 billion years old.