Dating Fossils

Superposition Suggests that younger rocks lie above older rocks in undisturbed sequences

Relative Dating Used to determine whether something is younger/older than something else. Used for events and objects

Geologic Column Unconformities occur due to changes such as faults, folds, tilts, and intrusions. Unconformities disrupt superposition, so scientists must rely on the geologic column to see where rock is missing and to tell what should be there

Absolute Dating Determining the exact age of an event or an object using radioactive isotopes Uses isotopes (most are unstable) If you know the rate of decay of an isotope, you can see how much of an element is in a fossil and compare that to the half-life of the element’s isotope

Half-life The time it takes for ½ of a radioactive sample to decay After every half-life, the radioactive sample decays by ½

Isotopes Uranium 238 (decays to Lead 206) Half-life is 4.5 billion years Used to date rocks more than 10 my old Other isotopes: Potassium 40 has a half-life of 1.3 by Carbon 14 has a half-life of 5,730 years

Faults Breaks in the Earth’s crust Causes blocks of crust to slide

Folds Rock layers fold and buckle from Earth’s internal forces

Tilting Occurs when internal forces in the Earth slant rock layers without folding them

Intrusions Molten rock and squeezes upward into the existing rock and cools.