GLG 121 1st Edition Lecture 4Relative Geologic Time- Allows us to determine the chronological order in which geologic events occur- Identify the relative ages of rock formations- What are the rocks trying to tell us?o Note the layers on rock formations (grand canyon)- Original horizontalityo If the layers are slanted we can assume a force moved them after they were deposited - Lateral continuity- Superpositiono An undisturbed layer of rocks at the bottom can be assumed to be deposited first- Crosscutting relationshipso Cutting rocks are younger than the ones being cut- Inclusiono Rock inclusions are older than the host rocks- Unconformitieso A surface of discontinuity representing an interruption in sediment over a significant amount of geologic time Disconformity- When layers are missing Angular discontinuity - A layer that has been deformed with new clean layers on top of it Nonconformity - Sedimentary rocks overlay metamorphic or igneous rocks- Fossilso Shells/boneso Mineralizedo Carbon imprints of soft tissueo Trackso Plant remainso MicrofossilsNumerical Geologic Time- Isotopes can be used to give specific dateso Number of protons determines the number of neutronso Isotopes are atoms are the same element with different numbers of neutrons- We can measure the amount of neutrons which helps us determine decay rateso Alpha decay Atomic mass decreaseso Beta decay Atomic number increases by one- All decay processes occur at a constant rate- Half lifeo The length it takes for something to decay- Daughter increases while father decreases- We can only measure about 100,000 years back in tome with carbon because we can only measure 12 half lives.o Picking the right element combination to study will allow you to look back further- Dating rocks is expensive so geologists only date a few layers and approximate the rest- Dating eartho Failed methods Counting biblical generations takes us back to 4004 B.C. Total thickness of sediments 75 million yrs Salinity of the oceans 100 million yrs Sun’s luminosity 20-40 million years Cooling time for iron 30 million yearso Radioactive dating of meteorites have been the most successful way todate the earth and our solar system Asteroids and comets in orbit are also used to bate