Relative Dating of Rocks Layers Age of Earth • Early 1900s: 6000 years old • Today: 4.6 billion years old How did scientist come up with this number? 1) Relative Age Dating 2) Absolute Age Dating James Hutton (1700s) –Developed the principle of uniformitarianism •processes occurring today have been occurring since Earth formed. 1) Relative Age Dating - Comparing one rock layer to another. - Sequences the order of events - No exact dates - Easy comparison of fossil ages Geologic Principles a) Original Horizontality – sedimentary rocks are deposited in horizontal layers. b)Superposition – in an undisturbed rock sequence: - Oldest rock layers are at the bottom - Youngest layers are on top Law of Superposition L M U X • • • • S Which layer is the oldest? Which layer is the youngest? Would the fossils in rock layer “U” be older or younger the fossils in rock layer “X”? How do you know?