GEOL 101: Geologic Time Part II: Lecture 15
GEOL 101: Geologic Time Part II: Lecture 15 101-017
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This 3 page Class Notes was uploaded by Natalee Stanton on Wednesday February 17, 2016. The Class Notes belongs to 101-017 at University of South Carolina taught by in Winter 2016. Since its upload, it has received 28 views. For similar materials see Geology 101-017 in Geology at University of South Carolina.
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Date Created: 02/17/16
GOEL 101: Clocks in Rocks: Timing the Geologic Record Part II Sequence stratigraphy Analysis in which the major geologic units are bound by unconformities Unconformities are caused by changes in sea level Used widely with seismic date, especially in petroleum exportation Geologic timescale Division in the worldwide stratigraphic column based on variations in preserved fossils Examples : o Cambrian – Cambria, England o Devonian – Devon, England o Jurassic – Jura mountains, Switzerland Absolute Geochronology Add numbers to the stratigraphic column which was originally based on fossils and correlation Based on the regular radioactive decay of some chemical elements o These elements are unstable Don’t Forget! Isotopes – are atoms of the same element with different number of neutrons Ions – when an atom loses or gains an electron to or from another atom Anion – negatively charged ions (gain of electron) Cation – positively charged ions (loss of electron) Isotopes – atoms of elements with the same number of protons and varying number of neutrons Ex: 235U, 238U 87Sr, 86Sr 14C, 12C Isotopic dating radioactive elements (parents) decay to stable, nonradioactive elements (daughters) o Only happens to unstable radioactive decay The rate at which this decay occurs is constant and known If we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been occurring o This is how we determine absolute time Types of radioactive decay 1) Alpha decay – loss of a 4He (2n,2p) o Helium (H) shooting out of the nucleus o Biggest energy producer 2) Beta decay – convert a neutron to a proton o Electron goes shooting out of the nucleus 3) Electron capture – convert a proton to a neutron o Absorb an electron Halflife – the time required for half of it (radioactive element) to decay to the daughter product The halflife is constant and does not vary with time, temperature, pressure Never get rid of the parent Fission tracks – damage trails Radioactive decay causes damage to the crystal structure of minerals Alpha decay Direct Measurement of the Rates of Geologic Processes Addition of absolute ages to the geologic time scale provides a great insight to Earth evolution Precision of dating techniques decreases with increasing geologic time The precision now available through the Global Positioning System (GPS) allows measurements of processes o Such as plate motion, to within 1 mm/year Sample Exam Questions Which of the following is the best statement of the principle of original horizontality? a. Igneous intrusions form horizontal layers b. Metamorphic isograds are horizontal before deformation c. Sediments are deposited as horizontal layers d. Most igneous, metamorphic, and sedimentary rocks in the earth’s crust form horizontal layers The study of the layers in sedimentary rock is known as ___. a. Stratigraphy b. Paleontology c. Sedimentation d. Geochronology Based on the concept of halflife in radioactive decay, after three halflives, the amount of a parent isotope remaining will be ____ of the original amount. a. ½ b. 1/3 c. ¼ d. 1/8 The principle of superposition states that _____. a. A fault is younger than the rocks it cuts b. Undisturbed sedimentary layers get progressively younger form bottom to top c. The present is the key to the past d. Sediments are deposited as essentially horizontal layers
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