UNIT 3 Geologic Time and Fossils Lesson 2 Absolute Dating Fossils

ABSOLUTE DATING Absolute dating – determining the age of objects or events in terms of years of age. Relative dating is in relation to other events, not in relation to years in the past. Example: Relative dating : Mr.McGovern is older than his students. Absolute dating: Mr.McGovern was born 43 years ago.

Radioactive Decay Radioactive decay – the process in which an unstable isotope (radioisotope) decays (breaks down) into a stable isotope of another element. Isotopes – varieties of atoms of a given element that all have the same # of protons but different # of neutrons (different atomic mass) Example: Carbon (C-12, C-14)

Rates of Decay Decay occurs at a predictable rate for each radioisotope in nature. The decay rate is exponential and slows down as the radioactive “parent material” disappears. It never completely disappears. The stable material that the parent decays into is called the “daughter material”. Decay rates for radioactive isotopes are expressed in terms of their “half-life”. Half-life – the amount of time that is needed for half of the remaining parent material to decay into its stable daughter material.

half-lives passed

Radiometric Dating Radiometric dating – using radioactive decay of isotopes present in some minerals as a means of determining the absolute age of rocks containing those minerals. When magma cools and hardens, the “clock” starts running on radioactive decay. The more daughter material present compared to parent material, the older the rock. Igneous intrusions and volcanic eruptions help establish the ages of sedimentary strata.

Commonly used methods of radiometric dating Potassium (K)-argon (Ar) K-40 (present in many rocks) decays into Argon; H.L.= 1.3 billion years; used to date rocks over 100,000 years old. Uranium (U) – Lead (Pb) U-238 (present in some rocks, most meteorites) decays into Pb-206; H.L.= 4.5 billion years; used to date rocks over 10 million years old.

The Carbon-14 Method Used to date biological remains of plants and animals that lived within the last 50,000 years. C-14 decays with a half-life of 5,730 years and is present in the carbon that plants and animals naturally contain.

Animations / Interactives decay applet