Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Absolute Dating with Radioactivity What is Radioactivity? • When the forces binding protons and neutrons together are not strong enough, and the nuclei spontaneously break apart (decay) • Isotopes—elements with the same number of protons, but different numbers of neutrons Common Types of Radioactive Decay • Alpha Particles—α particles – Composed of 2 protons and 2 neutrons – Emitted from the nucleus – Mass number of the isotope is reduced by four – The atomic number is decreased by two Common Types of Radioactive Decay • Beta Particles—β particles – Composed of one electron – Comes from a neutron – Increases protons by one – Atomic number increases by one Common Types of Radioactive Decay • Electron Capture – An electron is captured by the nucleus – It combines with a proton and forms a neutron – Mass number remains unchanged – One less proton; atomic number decreases by one Half-Life • The time required for one-half of the nuclei in a sample to decay • A common way of expressing the rate of radioactive disintegration Radiometric Dating • Radioactivity (unstable nuclei) is a reliable means of calculating the ages of rocks and minerals • The rates of decay for many isotopes have been precisely measured • A correct date is not possible unless the mineral remained a closed system during the entire period since its formation Carbon-14 • Half-life is 5730 years old • Used to date recent events in history • While an organism is alive, the proportion of carbon-14 to carbon-12 is constant • When the organism dies, carbon-14 decays to nitrogen-14 by beta emission • Scientists compare the proportions of carbon-14 and carbon-12 Importance of Radiometric Dating • Have produced thousands of dates for events in Earth’s history • Some rocks have been dated at more than three billion years • Proven that geologic time is immense Difficulties in Dating the Geologic Time Scale • Not all rocks can be dated radiometrically • Most sedimentary rock cannot be accurately dated because the particles that formed it are much older than the rock in which they occur Difficulties in Dating the Geologic Time Scale • Metamorphic rock is difficult to date because the age of a particular mineral in the rock is not necessarily the same as when the rock initially formed • Use cross-cutting relationships