2/5/17. #7 Geologic Time #7 Geologic Time

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1 #7 #7 = all of time since Earth formed (4.6 billion years ago, b.y.a.). Geologists study rocks, fossils, geologic structures (faults, folds) to characterize Earth's history. Natural disasters involve present-day. Why should we care about Earth's geologic history? Study ancient volcanoes, faults, landslides, etc. to understand present and future risks Why should we care about Earth's geologic history? Study ancient climates of Earth to understand present and future climate change Natural disasters involve present-day. Why should we care about Earth's geologic history? Study mass extinctions in geologic past to understand present and future mass extinctions 1

2 Web Exercise 1 Due today 1:00 PM 2-point penalty for every 24-hour period that assignment is late. No Web Exercise #1 is accepted after 1 pm on Friday After ~5PM Friday, Web Exercise #1 scores will be visible in IL Compass Gradebook; at the same time, feedback, including your answers and the correct answers, will be available in IL Compass Questions? Contact the TA Absolute Age = age of rock (or geologic event) in years. Need constant process as well as record of process. Web Exercise #2 - Starts Friday 2/10; due 2/17 Web Exercise #2 (Earthquakes) will be available on the GEOL 118 IL Compass Web site on Friday Web Exercise #2 will include questions based on information in Hazard City software (MasteringGeology Web Site) and the Internet. You must buy access to Mastering Geology and Register (see class web pages for instructions) I) Absolute Age = Constant process in rocks is radioactive decay Parent (radioactive) isotope Daughter (stable) isotope + Energy e.g., Uranium Lead Isotope = different form of atom with same number of protons, different number of neutrons Rate of radioactive decay is given by half-life (time needed for half of parent isotopes to change to daughter isotopes). Note: atoms must not move around after. Age of rock is determined by knowing half-life of reaction and amount of parent and daughter isotopes (radiometric age dating) Igneous rocks yield most reliable radiometric ages of formation 2

3 II) Relative Age = age of one rock with respect to another (sequence of events from oldest to youngest) A) Principle of Uniformitarianism = "Present is key to past" current physical processes also operated in geologic past (James Hutton in 1700's). Geologists use this principle to interpret Earth history. Examples? Modern lava flow Ancient lava flow (Giant s Causeway) A) Principle of Uniformitarianism = Examples? A) Principle of Uniformitarianism = For natural disasters, uniformitarianism used to predict future events. Examples? Modern mudcracks Ancient mudcracks Which rock layer is oldest (bottom, middle or top)? C) Principle of Cross-Cutting Relations = If igneous intrusion (or fault or river valley) cuts across rock layers, intrusion is younger than rock layers B) Principle of Superposition = moving upward through sequence of sedimentary rocks, age becomes younger 3

4 C) Principle of Cross-Cutting Relations = Put into correct sequence: dark host rock, thick intrusion, thin intrusion D) Principle of Fossil Succession = When fossils in sedimentary rocks are arranged by relative age, groups of fossil plants + animals have changed form (evolved) over geologic time in definite + recognizable order. D) Principle of Fossil Succession = Groups of fossils in rock can represent interval of geologic time. Scale Using fossils + radiometric ages of rocks from all over world resulted in Scale - table of ages represents Earth's geologic history Age of Earth = 4.6 b.y. (from age of meteorites, moon rocks) Age-dated meteorite Age-dated moon rock 4

5 Oldest rocks from Earth (continental rock) = b.y. Why not 4.6 b.y.? Earth is dynamic; melting and metamorphism reset radiometric ages of rocks from Earth PreCambrian = Largest block of time (88% of Earth history); life evolved at ~3-3.5 b.y.a. (bacteria). Only soft-bodied organisms lived, so few fossils are preserved. Extreme volcanism - Early Precambrian 3.5 b.y. old stromatolites (cyanobacteria) Iron Formation - Middle Precambrian PreCambrian/Paleozoic boundary = Appearance of abundant animals with shells, fossils now abundant. Paleozoic ("Old life") Era = Trilobites, crinoids, coral reefs, snails; Appearance of fish, plants, amphibians, insects Abundant new life in Early Paleozoic Abundant ocean life in Paleozoic Paleozoic ("Old life") Era = Carboniferous Period involved extensive forests, swamps (later became coal). Paleozoic/Mesozoic boundary = Mass extinction. Half of all families of organisms suddenly disappeared. Carboniferous Forest and Swamp 5

6 Mesozoic ("Middle life") Era = Dominance of reptiles (dinosaurs), First birds. Mesozoic/Cenozoic boundary = Mass extinction of 1/4 of all families of organisms. Extinction of all dinosaurs. Dinosaurs of Mesozoic Era Mesozoic/Cenozoic Mass Extinction Cenozoic ("Recent life") Era = Dominance of mammals, Appearance of humans, currently experiencing mass extinction. Mammals of Cenozoic 6