GEOL102.24p-Topic6_Geologic_Time.pdf
Transcript
Source: Bernard Spragg (2007) Public domain Topic 6: Geologic Time Reconstructing Geologic History geologists reconstruct Earth’s geologic history by studying the rock record - fossils - rock layers and structures - rock chemistry determining ages within t...
Source: Bernard Spragg (2007) Public domain Topic 6: Geologic Time Reconstructing Geologic History geologists reconstruct Earth’s geologic history by studying the rock record - fossils - rock layers and structures - rock chemistry determining ages within the rock record is key to reconstructing Earth history relative ages absolute ages How do geologists measure time? relative dating is a system of reasoning that is used to determine the relative chronological sequence or order of a series of geologic events (i.e. Source: Fletcher et al (2014) Intro. to Physical Geology, Wiley. with respect to each other) absolute (radiometric) dating uses the natural decay of radioactive isotopes within minerals to calculate their actual Source: science.howstuffworks.com (absolute) chronological age (age in years) Relative Ages and Geologic Processes five types of geologic processes used to determine relative ages - intrusion - erosion - deposition - faulting - rock deformation which processes are evident in this slide? these processes form the basis of stratigraphy - description and interpretation of the sequence of deposition of rock layers Some Key Principles of Stratigraphy there are many fundamental principles used to determine relative ages. Four important ones are: 1. Principle of original horizontality 2. Principle of lateral continuity 3. Principle of superposition 4. Principle of cross-cutting Source: San Diego Archeological Center relationships Principles of Stratigraphy Source: Nature Education Principle of original horizontality - sediments are deposited under the influence of gravity as nearly horizontal layers ("strata") Principles of Stratigraphy Principle of original horizontality - subsequent earth movements (deformation) can cause folding or faulting of sedimentary strata Source: Fletcher et al (2014) Intro. to Physical Geology, Wiley. Principles of Stratigraphy Principle of lateral continuity - initial layers were laterally continuous, but later separated by erosional features (e.g. Grand Canyon) Source: Fletcher et al (2014) Intro. to Physical Geology, Wiley. Principles of Stratigraphy Principle of superposition - each layer of a sedimentary sequence is younger than the one beneath and older than the one above - strata are vertically ordered in time from oldest (at bottom) to youngest (at top) Source: Fletcher et al (2014) Intro. to Physical Geology, Wiley. Practice Exercise (crime sleuthing!) Source: Saskatoon Star-Phoenix (Sat. Nov. 13, 2021) Cassandra Cat claims that a burglar broke into her house through the sliding glass door, stole her designer handbags, and dropped her wallet while fleeing. What evidence and geological principle did Slylock Fox use that suggests this "burglary" was staged? Principles of Stratigraphy Principle of cross-cutting relationships - geologic features that cut across pre- existing rocks must be younger than the rocks they cut across - such geologic features could be faults or dikes Source: University College Dublin Source: Fletcher et al (2014) Intro. to Physical Geology, Wiley. Source: Ontario Ministry of Northern Development and Mines Absolute Dating & The Age of the Earth application of relative-dating techniques in 1700’s implied earth was much older than previously thought (10-100's of millions of years) - fossils - sedimentation rates - cooling of Earth radiometric dating - based on natural radioactive decay of certain elements (isotopes) radiometric dating places the age of the earth as ~4.54 Ga - a = "annum" (year) - G = SI prefix "giga" (109) - M = SI prefix "mega" (106) Source: Fletcher et al (2014) Intro. to Physical Geology, Wiley. Review: Atoms and Isotopes atoms Example: Carbon atom - composed of nucleus (protons + neutrons) and cloud of electrons atomic number: - # of protons in an atom - defines the element mass number: - # of protons plus neutrons in the atom isotopes - isotopes of an element have the same atomic number (# of protons constant) but different mass number (# of neutrons different), e.g. 12C vs. 14C Isotopes of a Chemical Element element determined by # of protons, e.g. H has 1 proton but # of neutrons can vary, e.g. H has 3 isotopes Source: science.howstuffworks.com Radioactive Decay Change in atomic and mass units: neutron atomic #: -2 mass #: -4 proton atomic #: +1 mass #: 0 atomic #: 0 mass #: 0 Source: http://ib.bioninja.com.au/standard- level/topic-5-evolution-and-biodi/51-evidence- for-evolution/radioactive-dating.html rad decay of parent daughter may be a new element some commonly used geological "clocks": 238U-206Pb, 40K-40Ar, 87Rb-87Sr Example: Uranium-238 (parent) – Lead-206 (daughter) decay in a rock (not to scale!) Modified after http://vertpaleo.org/ Radiometric Dating need to determine ratio of radioactive parent isotope to stable daughter isotope because decay rate is constant, can calculate time since the mineral crystalized "half-life" (t1/2) - time required for 50% of the original parent atoms have decayed need to use isotopes with a half- life that is comparable to the rock's age http://earthsci.org/ Example: 238U - 206Pb decay (t1/2 ≈ 4.5 Ga) (not to scale!) http://vertpaleo.org/ Oldest Earth Materials oldest rock: Acasta Gneiss - Northwest Territories - oldest known rock outcrop - age of ~4.0 Ga from U-Pb dating of zircon oldest mineral: 4.4 Ga zircon from Jack Hills, Australia Source: Wikimedia user “Pedroalexandrade” Source: Wilde et al. (2001) Nature 409: 175-178. (2011) CC-BY 3.0 Geological Time Scale subdivisions - eon, era, period, and epoch two major eons - Precambrian (4543 - 541 Ma): includes 88% of earth history, earliest life (Archean) - Phanerozoic (541 Ma - present): increased O2 in atmosphere, rapid "explosion" of diverse life boundaries characterized by significant geologic events, e.g. extinction of the dinosaurs at the Cretaceous-Paleogene boundary Modified from Wikipedia Commons Camels and the Geological Time Scale "Camels Often Sit Down Mostly Pretty Painfully. Their Joints Creak. Perhaps Early Oiling Might Prevent Permanently Hazardous Ageing." What does "Ageing" stand for? Modified from Wikipedia Commons Earth History Summary – Geologic Time determining the ages of rock is essential in reconstructing earth history - relative dating and absolute dating relative dating: key principles of stratigraphy - original horizontality - superposition - lateral continuity - cross-cutting relationships Summary – Geologic Time absolute dating - radioactive decay of parent isotope to daughter isotope - age is calculated from ratio of parent / daughter - concept of half-life - Geological Time Scale - subdivisions and boundaries - history of the Earth Note: This course will not cover all of the material in Suggested Readings each chapter of the textbook. These reading recommendations highlight Textbook material relevant to this course. Chapter 19 Introduction Section 19.1: The Geologic Time Scale Section 19.2: Relative Dating Methods Section 19.4: Isotopic Dating Methods Section 19.6: Understanding Geological Time Workbook Chapter 19 - vocabulary and review questions Additional Materials: Geologic Time The Science of Dates and Rates - a brief overview of the role of geochronology in the earth sciences, and some recent advances in the field The World's Oldest Rock Fragment - how the world's oldest crystal (~4.4 Ga!) was found, and the steps needed to prove that the age was reliable Additional Materials: Geologic Time The Moon is Older Than We Thought - state-of-the art geochronological (rock dating) techniques discover that the moon is 40 million years older than we previously thought!
