NOTES: GEOLOGIC DATING How are the age of rocks determined? How old is planet earth?

Geologic Time Scale A system of chronological dating that relates geologic land forms (strata) to time. Shows sequence of major evolutionary events. Plate motions and mountain building events can be placed on the geologic time scale Geologists have divided Earth’s history into time units based on the fossil record

Geologic Dating Two Types Relative (qualitative): Used to determine if one thing is younger or older than another. Ex: Law of Superposition, use of index fossils, correlation of rock layers. Absolute (quantitative): Determines how many years old something is. Ex: radiometric Dating

Relative Dating Law of Superposition Younger rocks are on top, older rocks are on bottom. Lower layers must be in place before younger rocks can be deposited on top of them Problems: Layers can shift. Does not give exact age.

Absolute Dating Earth is estimated to be 4.6 billion years old How did we measure that? Radiometric Dating = calculating the age of an object by measuring proportions of radioactive isotopes

Radiometric Dating Radioactive Isotope = an unstable form of an element, decays into stable element, gives off energy (radiation) Ex. Carbon-14 decays into Nitrogen Ex. Potassium-40 decays into Argon-40

Radiometric Dating Half-life = the time it takes for half of a radioactive isotope to decay Ex. K-40 half life is 1.3 billion years # of Half-lives Fraction of radioactive isotype Remains 1 1/2 50% 2 1/4 25% 3 1/8 12.5% 4 1/16 6.25%

Radiometric Dating Practice RADIOMETRIC DATING EQUATION: (# of half-lives)(known half-life of radioactive isotope) = Age of object Practice The half life of carbon-14 is 5,730 years. If only 25% of the original amount of carbon-14 is left in an fossil, how hold it the fossil? Determine how many half lives have elapsed. 25% or ¼ = 2 half lives Multiply this by the known half life. Carbon half life = 5,730 years 2(5,730 yrs) = 11,460 years

# of Half-lives Fraction of radioactive isotype Remains 1 1/2 50% 2 1/4 25% 3 1/8 12.5% 4 1/16 6.25% Radiometric Dating Practice If a sample contains 20g of an isotope that has a half life of 1000 years. How much will be left after 2000years? After 2,000 years, how many half-lives would have passed? 2 What percent of the radioactive isotopes would be left? 25% What is 25% of 20g? 5g