GEOLOGY 101 1st Edition Lecture 17 Outline of Last Lecture I. Principles of Relative Datinga. Absolute DatingOutline of Current Lecture I. Radioactive methods on Rocksa. Carbon-14II. Geologic Time Scalea. FossilsCurrent LectureWhat kinds of rocks can be dated with radioactive methods?- In igneous rocks, the clock begins when the molten rock cools – get age of crystallization (formation).o No parent or daughter atoms added or removed once started the radioactive decay.o No resetting with metamorphism (or get time of metamorphism).o Best rock to age-date!- For sedimentary rocks, radiometric dating, you would get the average age of all the grains which form the sedimentary rock. - For metamorphic rocks, radiometric dating gives the age of metamorphic event (may be more than one) rather than the initial age of crystallization (formation).What about Carbon-14?- What it is:o If Carbon-12 is carried to the upper atmosphere, high energy photons from the sun will damage it, creating Carbon-14 [in the form of CO2]o Organisms are made of Carbon, so they will consume the Carbon-14.o The Carbon-14 is radioactive, and decays to Nitrogen-14. What’s the problem? We measure the amount of Carbon-14 left in the organism and compare it to the amount originally around in the atmosphere to determine the age.- Why don’t we use Carbon-14?These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Most rocks do not contain carbon.o The half-life is only 5720 years, so you can’t date anything older than about 70,000 years.o Carbon-14 can be used to determine: Ages of recent lava flows Ages of recent ash and pumice falls Ages of recent landslides, laharsGeological Time Scale:- Can be thought of as a relative-age calendar of the Earth’s Geologic history.o Time is divided into Eons, Eras, Periods, and Epochso The time between each division is NOT the same.- A key component to constructing the geologic timescale is the correlation of fossils between outcrops. o Geologic Time Scale: Precambrian: from birth of Earth up to before complex life forms developed (greater than 543 m.y.) Paleozoic: Ancient life Mesozoic: Age of middle life (dinosaurs) Cenozoic: Age of recent life (mammals and humans)- What is a fossil?o Fossils are evidence of past life forms RAPID BURIAL:- Soft mud, Flood deposits, Volcanic Ash Skeletons or shells preserve the best (hard parts)- Soft body parts don’t preserve well- Hard parts usually undergo chemical replacement Petrified Wood: plants have been replaced by quartz. Most bones have been replaced by minerals.o Trace Fossils: Evidence that the organism was there, although the organism is gone.  Burrows: the burrowing animals are gone but their burrows remain.o Index Fossil: Used to identify the relative age of a rock unit Used in stratigraphic correlation of rock units What makes a good index fossil?- Unique and easy to identify- Lived for a short period of