Frank Press • Raymond Siever • John Grotzinger • Thomas H. Jordan Understanding Earth Fourth Edition Chapter 10: The Rock Record and the Geologic Time Scale Lecture Slides prepared by Peter Copeland • Bill Dupré Copyright © 2004 by W. H. Freeman & Company Geologic Time Two Ways to Date Geologic Events A major difference between geologists and most other 1) relative dating (fossils, structure, cross- scientists is their concept of time. cutting relationships): how old a rock is compared to surrounding rocks A "long" time may not be important unless it is greater than 1 million years 2) absolute dating (isotopic, tree rings, etc.): actual number of years since the rock was formed Steno's Laws Principle of Superposition Nicholas Steno (1669) In a sequence of undisturbed • Principle of Superposition layered rocks, the oldest rocks are • Principle of Original on the bottom. Horizontality These laws apply to both sedimentary and volcanic rocks. Principle of Original Horizontality Layered strata are deposited horizontal or nearly horizontal or nearly parallel to the Earth’s surface. Fig. 10.3 Paleontology • The study of life in the past based on the fossil of plants and animals. Fossil: evidence of past life • Fossils that are preserved in sedimentary rocks are used to determine: 1) relative age 2) the environment of deposition Fig. 10.5 Unconformity A buried surface of erosion Fig. 10.6 Cross-cutting Relationships • Geometry of rocks that allows geologists to place rock unit in relative chronological order. • Used for relative dating. Fig. 10.8 Fig. 10.9 Fig. 10.9 Fig. 10.9 Fig. Story 10.11 Fig. Story 10.11 Fig. Story 10.11 Fig. Story 10.11 Fig. Story 10.11 Fig. Story 10.11 Fig. Story 10.11 The Geologic Timescale Divisions in the worldwide stratigraphic column based on variations in preserved fossils Fig. 10.12 Fig. 10.7 Fig. 10.7 Earth Issues 10.1 Earth Issues 10.1 Absolute Geochronology • Add numbers to the stratigraphic column based on fossils • Based on the regular radioactive decay of some chemical elements Earth Issues 10.1 Isotopic Dating Isotopes • Radioactive elements (parents) decay Atoms of elements with the same to stable, non-radioactive elements number of protons and varying (daughters) numbers of neutrons • The rate at which this decay occurs is constant and known Examples: • If we know the rate of decay and the 235U, 238U 87Sr, 86Sr 14C, 12C amount present of parent and daughter we can calculate how long this reaction has been occurring. Requirements for Isotopic Dating Half-life • Closed system The half-life of a radioactive • decay rate constant isotope is defined as the time • Initial concentration of daughter required for half of it to decay. is known (zero is best) Fig. 10.14 Direct Measurement of the Rates of Geologic Processes The precision now available through the Global Positioning System (GPS) allows measurements of processes, such as plate motion, to within ± 1 mm/year..