3 Two types of datingRelative Age Dating – approximately how old rocks areFound by using the age of things around itAbsolute Age Dating – exactly how old rocks areFound using radiometric dating
4 Laws of Relative Age Dating Relative dating – used to determine the order of events and the relative age by examining the position of rocks in a sequence.Example: if bedrock is broken by a fault, then the fault must be younger because it cuts through the rocks
5 Relative Age DatingThis is called the Law of Cross Cutting Relationships – If something cuts across a layer of rock it must be younger than the rock.
6 Laws of Relative Age Dating The Law of Superposition –In an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks become younger toward the top.Sediments are deposited in horizontal layers, so the oldest one would be on the bottomSometimes layers flip, so geologists must use other clues to determine if this has happened.
7 Laws of Relative Age Dating Principle of Original Horizontality – rocks are deposited in horizontal layers
8 Laws of Relative Age Dating Unconformities – gaps in the rock record caused by missing rock layersForm when layers are eroded awayAlso when time goes by without any new deposition
9 Principle of Uniformitarianism – earth processes that are occurring today have been occurring since the beginning of the earth.
10 Three types of Unconformities Angular Unconformity – horizontal layers of sedimentary rocks are tilted and uplifted, where they are eroded and weathered. Younger sediments are then deposited horizontally on top of the tilted layers
11 Three types of Unconformities Disconformity – layers missing between two beds were an erosional surface is present
12 Three types of Unconformities Nonconformity – sedimentary rock layers above igneous or metamorphic rocks
13 Correlating layers – finding the same rock layer at two separate locations and matching them up. How?Index fossils – similar, rare fossils found in one layer at multiple locationsRock correlation – matching similar rock types
14 Absolute DatingAbsolute Dating – process that uses the properties of radioactive decay in rocks to determine their exact age
15 Absolute Dating Radioactive decay – unstable isotopes break down A neutron breaks down into a proton and an electron: the electron is given off as a radioactive particleThe number of neutrons & protons are changed, so a new element is formedUranium-238 is the parentLead-206 is the daughter
16 Absolute DatingEach parent has a specific rate of decay – this is the half-life of the elementThe time it takes for half of the atoms to decayCarbon-14 has a half-life of 5730 years
17 Absolute DatingRadiometric dating – measuring the amounts of parent and daughter materials in a rock and knowing the half-life, a geologist calculates the age of a rockGeologists must decide what isotope to useMust have an appropriate half-life
18 Absolute DatingRadiocarbon dating – using carbon-14 to date fossils, bones and wood up to 50,000 years oldMust be a formerly living thing to have Carbon