Geologic Time and the Age of the Earth
Geologic Time Scale (Relative Age Dating)
Based on Stratigraphic prinicples and UNIFORMITARIANISM
Original Horizontality
Superposition
Inclusions and Cross cutting relations
Lateral Continuity
Faunal Succession and Fossils
UNCONFORMITIES- Gaps in the rock record. Know what types of rocks are on either side of
the unconformity surface and what these breaks represent.
Angular Unconformity
Disconformity
Nonconformity
Relative ages all over the world are based mainly on fossils
Index Fossils: Remains of Past organisms that are easily recognizable, have a short species age,
and are found in a wide variety of locations on earth
Geologic Time Scale: Know the ERAS of the geologic time scale (Precambrian, Paleozoic,
Mesozoic, Cenozoic) and the ages (in Ma) of the boundaries
Absolute Age Dating and Radiometic Ages
Radioactive isotopes of elements-unstable atoms which spontaneously decay
Easiest way to thing about the time involved is through the concept of
Isotopes of atoms have the same number of protons (that is what gives them the properties of
that element) but a different number of neutrons, and hence a different atomic mass
HALF-LIFE-time it takes for 50% of the remaining parent isotopes to decay to the stable
daughter isotope.
Each geologically important isotopic system has a half-life that allows us to us the ratio of parent
to daughter isotopes in a rock to determine the age and the number of half-lives that have
elapsed.
We want to find minerals that incorporate the Parent isotope in the structure, but exclude the
Daughter product, this means that any of the daughter isotope in the mineral when we examine it
will be due to radioactive decay.
After one half life Parent: Daughter is 50:50 or 1:1
After two half lives Parent:Daughter is 25:75 or 1:3 (half of the original half is 25%)
Radiometric dating is mainly used on igneous rocks, while relative age dating is best used on
sedimentary rocks