Age Dating of Rocks

Question: How do we know how old rocks are?

Two types of dating Relative Age Dating – approximately how old rocks are Found by using the age of things around it Absolute Age Dating – exactly how old rocks are Found using radiometric dating

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

Relative Age Dating This is called the Law of Cross Cutting Relationships – If something cuts across a layer of rock it must be younger than the rock.

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 bottom Sometimes layers flip, so geologists must use other clues to determine if this has happened.

Laws of Relative Age Dating Principle of Original Horizontality – rocks are deposited in horizontal layers

Laws of Relative Age Dating Unconformities – gaps in the rock record caused by missing rock layers Form when layers are eroded away Also when time goes by without any new deposition

Principle of Uniformitarianism – earth processes that are occurring today have been occurring since the beginning of the earth.

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

Three types of Unconformities Disconformity – layers missing between two beds were an erosional surface is present

Three types of Unconformities Nonconformity – sedimentary rock layers above igneous or metamorphic rocks

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 locations Rock correlation – matching similar rock types

Absolute Dating Absolute Dating – process that uses the properties of radioactive decay in rocks to determine their exact age

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 particle The number of neutrons & protons are changed, so a new element is formed Uranium-238 is the parent Lead-206 is the daughter

Absolute Dating Each parent has a specific rate of decay – this is the half-life of the element The time it takes for half of the atoms to decay Carbon-14 has a half-life of 5730 years

Absolute Dating Radiometric dating – measuring the amounts of parent and daughter materials in a rock and knowing the half-life, a geologist calculates the age of a rock Geologists must decide what isotope to use Must have an appropriate half-life

Absolute Dating Radiocarbon dating – using carbon-14 to date fossils, bones and wood up to 50,000 years old Must be a formerly living thing to have Carbon