GEOLOGIC TIME Debates About the Age of the Earth Bishop Usher Lord Kelvin Discovery of Radioactive Decay Most geologists believe that the Earth formed about 4.6 billion years ago The continental crust of the Earth is ~4.0 b.y. The oceanic crust is much younger Relative Time Order events from oldest to youngest Principle of Superposition Principle of Original Horizontality Principle of Lateral Continuity Principle of Cross-Cutting Relationships Principle of Inclusions Principle of Faunal Succession Geologic Time Scale Absolute Time Methods relying on event in the geological record with very strong annual cyclicity: tree growth rings coral growth cycles varves (annual clay sediment layers) Methods relying on the decay of naturally occurring radiogenic isotopes Half-life : time it takes for half of the Parent to decay (change) to the Daughter Uranium-235 -> Lead-207 Uranium-238 -> Lead-206 Thorium-232 -> Lead-208 Rubidium-87 -> Strontium-87 Potassium-40 -> Argon-40 Carbon-14 -> Nitrogen-14 Decay Schemes During the decay Heat is given off - this is an important source of energy to produce temperatures necessary for partial melting. alpha decay - two protons and two neutrons are emitted from the nucleus. This reduces the atomic number of the parent by 2 and the mass number of the parent by 4. Uranium to Lead Schemes Electron capture - occurs when a proton captures an electron and changes into a neutron. The atomic number of the parent element is decreased by 1 but the mass number is unchanged : Beta decay - an electron is emitted from a neutron in the nucleus changing the neutron to a proton. This increases the atomic number of the parent element by 1 but does not change the atomic mass number : Parent Daughter Half-Life C-14 N-14 5,730 U-235 Pb-207 710,000,000 K-40 A-40 1,300,000,000 U-238 Pb-206 4,500,000,000 Th-232 Pb-208 15,000,000,000 Rb-87 Sr-87 47,000,000,000 Implications for heat flow due to radioactive decay during Earth history Geologic Time Scale