Absolute vs. Relative AgeRelative dating tells us which rocks are older by comparing positionit does not tell us age in actual years.Absolute dating assigns an age in years; it gives rocks and fossils birthdays!
Erosion Rates of RiversNiagara Falls erodes 1.3 meters per year.age is 9,900 years!Only works with younger features.
Deposition of SedimentRivers deposit sediment into oceans or lakes.Rates of deposition can be inconsistent over time and thus inaccurate.
Varve CountSediments are deposited yearly in glacial lakes.Spring melt brings coarse (large) sediment first which settles on bottom; fine (small) material lands on top.One year = one layer of big & small sediment.
Varve Countone band of dark & light = one year
Radioactive Decay
Radioactive DecayRadioactive elements act as geologic clocks.Radioactive decay: the loss of particles and energy from unstable (radioactive) elements.Radioactive elements (such as uranium) decay at their own unique, constant rate to form new elements
Radioactive DecayOnce that rate is known, scientists compare the amount of original radioactive material (parent) in a rock sample to the amount of newly created element present (daughter)and the age of the sample can be determined by plotting amount versus time on a logarithmic graph!
Half-LifeDefinition: the time it takes for half of a radioactive sample to decay to a different stable element.Example: it takes 10 g of U-238 4.5 b.y. to become 5 g of Pb-206 and 5 g of U-238.
Half-Life
Radioactive Parent
Stable Daughter
Half life
Potassium-40
Argon-40
1.25 billion yrs
Rubidium-87
Strontium-87
48.8 billion yrs
Thorium-232
Lead-208
14 billion years
Uranium-235
Lead-207
704 million years
Uranium-238
Lead-206
4.47 billion years
Carbon-14
Nitrogen-14
5730 years
Good to KnowThe number that appears with the element name (U-238) is the atomic mass number of the elementthe total of protons & neutrons found in the nucleus.During decay, an element that loses neutrons becomes an ISOTOPE of that element (like U-236).When that element loses protons, it becomes a different element!
Good to KnowRadioactive elements become concentrated in minerals found in igneous rocks (granite, gabbro) which make up continental crust.Soradiometric dating is not useful for determining the ages of sedimentary rock (you would simply be finding the birthday of the igneous rock that got changed into the sedimentary rock)
Carbon Dating
ProblemUranium, Potassium, & Rubidium work great with very old (more than 10 million year-old) ROCKSbut what about younger rocks and objects not containing radioactive elements (like once-living things)?
Carbon DatingCarbon dating is used for younger samples that contain carbon such as organic material and some sedimentary rocks.The half-life of C-14 is 5,730 years.Works for wood, bone, shells, early humans up to 50,000 years old.
