ABSOLUTE AGE IS EXPRESSED IN SPECIFIC UNITS, SUCH AS YEARS BEFORE PRESENT. ONE METHOD OF ABSOLUTE DATING IS COUNTING THE ANNUAL LAYERS OF GLACIAL LAKE SEDIMENTS (SIMILAR TO TREE RING DATING). THE PRIMARY METHOD (FIRST USED) FOR ABSOLUTE DATING IS ISOTOPES AND RADIOACTIVE DECAY Absolute Dating of Rocks – Using Radioactive Decay

Isotopes and Radioactive Decay An element is a substance consisting of atoms that are chemically alike. Most elements exist in several varieties called isotopes. The difference between one isotope of an element and another is in the mass of its atoms. (Carbon – mass is 12, Carbon-14 – the mass is 14) Almost all the mass of an atom is concentrated in the center region called the nucleus. When an atom is unstable, they emit particles and electromagnetic energy called radioactive decay. Radioactive decay is when an atom changes into atoms of other isotopes until the atom becomes stable. Ex: Carbon-14 is unstable and decays into Nitrogen with a mass of 14 (Nitrogen-14).

Half-Life (Front Cover in ESRT) The decay of any individual atomic nucleus is random and can occur at any time. However, a certain definite fraction will decay in a given time. The time required for half of the atoms in a given mass of an isotope to decay is called the half-life. In other words, in a given sample, half of the original atoms will have decayed to other isotopes at the end of one half-life, the other half will remain unchanged. This halving process continues indefinitely (like the energizer bunny)

Radioactive Dating The half-life of a radioactive isotope can be used, along with the ratio between the amount of the original isotope (mother isotope) and the amount of its decay-product (daughter isotope), to estimate the absolute age. This method is called radioactive dating. Radioactive Decay Data Radioactive IsotopeDisintegrationHalf-life in Years Carbon-14C 14 N x 10 3 = 5700 Pottasium-40K 40 Ar 40 Ca x 10 9 = 1,300,000,000 Uranium-238U 238 Pb x 10 9 = 4,500,000,000 Rubidium-87Rb 87 Sr x = 49,000,000,000

Uranium 238 One of the most important radioactive isotope that is used in dating rocks. The nuclei of its atoms pass through a series of radioactive decays, eventually producing atoms of lead-206, a stable isotope of the element lead.

Carbon-14 This radioactive isotope is useful for dating rocks and organic remains of recent origin, such as bones. The half-life of carbon-14 is 5,700 years.