2/13/17. Geologic Time - Relative Age Determination. Geologic Time - Relative Age Determination. Geologic Time - Relative Age Determination

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1 A) Principle of Uniformitarianism = For natural disasters, uniformitarianism used to predict future events. Examples? Which rock layer is oldest (bottom, middle or top)? B) Principle of Superposition = moving upward through sequence of sedimentary rocks, age becomes younger C) Principle of Cross-Cutting Relations = If igneous intrusion (or fault or river valley) cuts across rock layers, it is younger than rock layers Clicker question When did the fault on the right side of the diagram occur? A. After layer G was laid down but before layer J was laid down B. After igneous rock M intruded, but before layer K was laid down. C. After layer K was laid down, but before layer F was laid down. 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. 1

2 D) Principle of Fossil Succession = Groups of fossils in rock can represent interval of geologic time. Geologic Time Scale Using fossils + radiometric ages of rocks from all over world resulted in Geologic Time 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 4,400 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. PreCambrian/Paleozoic boundary = Appearance of abundant animals with shells; fossils common from this time and later Extreme volcanism - Early Precambrian 3.5 b.y. old stromatolites (cyanobacteria) Iron Formation - Middle Precambrian Abundant new life in Early Paleozoic 2

3 Paleozoic ("Old life") Era = Trilobites, crinoids, coral reefs, snails; Appearance of fish, plants, amphibians, insects Paleozoic ("Old life") Era = Carboniferous Period involved extensive forests, swamps (later became coal). Abundant ocean life in Paleozoic Carboniferous Forest and Swamp Paleozoic/Mesozoic boundary = Mass extinction. Half of all families of organisms suddenly disappeared. Mesozoic ("Middle life") Era = Dominance of reptiles (dinosaurs), First birds. Dinosaurs of Mesozoic Era Mesozoic/Cenozoic boundary = Mass extinction of 1/4 of all families of organisms. Extinction of all dinosaurs. Cenozoic ("Recent life") Era = Dominance of mammals, Appearance of humans, currently experiencing mass extinction. Mammals of Cenozoic Mesozoic/Cenozoic Mass Extinction 3

4 Exam #1 Monday February 27 th 7:15 PM 141 Loomis Hall for A - N 151 Loomis Hall for O Z Web Exercise 1 Due two days ago 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 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) #8 Plate Tectonics VJNnoZ4 Start at 2:00 Plate Tectonics Plate Tectonics theory = outer portion of Earth consists of number of thick slabs of rock (tectonic plates) that slowly move around globe; In map view, tectonic plate can consist entirely of oceanic crust or both oceanic + continental crust. Plate Tectonics In cross-section, tectonic plate = lithosphere - ~cool, outer rigid layer where rocks can break, ~100 km thick, includes entire crust + outer part of mantle; below lithosphere is asthenosphere - layer of ~hot, weak rock that flows, from ~100 - ~300 km in depth, only mantle rock. 4

5 Plate Tectonics The asthenosphere is NOT a layer of liquid Tectonic plate boundaries are defined by zones of frequent earthquakes. Volcanoes and mountain ranges are also common at plate boundaries. Three types of plate boundaries: Divergent, Convergent, Transform I) Divergent boundary - where plates move apart. I) Divergent boundary - where plates move apart. Defined by oceanic ridge - undersea volcanic mountain range of non-explosive volcanoes where oceanic crust (basalt) is produced due to rising molten rock (which generates minor EQ's only); cooled rock moves laterally away from ridge afloorspreading.html 5

6 - Divergent Divergent boundary/oceanic ridge wraps around globe - Divergent Examples = Mid-Atlantic ridge, Iceland Better viewed in Google Earth Oceanic ridge shown in red II) Convergent boundary - where plates come together; defined by subduction zone, where old ocean lithosphere is subducted (descends back into mantle + melts) + destroyed - Convergent Oceanic trench (deep linear trough) = where subduction begins. As crust + mantle melt (partially), produces explosive volcanic eruptions + volcanic mountains - Convergent Many earthquakes (EQs, major and minor) as rocks grind past each other - Convergent Oceanic trench (deep linear trough) = where subduction begins. As crust + mantle melt (partially), produces explosive volcanic eruptions + volcanic mountains See Google Earth trenches 6

7 - Convergent Examples = Andes mountains (ocean/continent convergence), Japan (ocean/ocean convergence) - Convergent Margin of Pacific Ocean = "Ring of Fire" (lots of EQs + volcanoes), there is convergent boundary at most locations - Convergent If all ocean lithosphere is subducted, there is continent/continent collision; continents can't subduct (~low density), creating tremendous mountain range (and major + minor EQs) but no volcanoes (e.g., Himalayas). - Convergent Continent/Continent Collision Zone Example = Himalayas - Transform Where two plates grind past each other with horizontal motion; defined by major fault (transform fault); not creating new crust or destroying old crust (no rock melts), so no mountains or volcanoes; grinding two blocks of crust past each other, so lots of EQ's (major + minor). - Transform Example = San Andreas Fault, CA. 7

8 Clicker question Which type of plate boundary usually generates explosive volcanoes? A. Divergent plate boundary B. Convergent plate boundary C. Transform plate boundary D. All of the above E. None of the above Clicker question Which type of plate boundary does not often generate large, damaging earthquakes? A. Divergent plate boundary B. Convergent plate boundary C. Transform plate boundary Plate Tectonic Motion - Speed Plates move at speeds of 2-10 cm/yr (measured by age of ocean crust and by accurate positions of locations on Earth from laser-satellite data) Skipped next 4 slides Plate Tectonic Motion - Over Geologic Time Plates move at speeds of 2-10 cm/yr; Over geologic time, Earth's surface is constantly changing place Plate Tectonic Motion - Future Projections Continent positions 50 m.y. in future 8

9 Plate Tectonic Motion - Future Projections California won t fall into Pacific Ocean Plate Tectonic Motion - Future Projections California Plate Tectonic Theory - Importance Plate Tectonic Theory - Importance Why is Plate Tectonics theory relevant to Natural Disasters? Why is Plate Tectonics theory relevant to Natural Disasters? 9