DOCSLIB.ORG

Geologic Time a Major Difference Between Geologists and Most Other Scientists Is Their Attitude About Time

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text

Load more

Exam 1 GG 101 Mean Score 61.4 (SD 13.1) 6 5 4 3 No of obs 2 1 0 0 20 32.8125 40 60 80 Score, % Class Grades • Drop lowest 3 Chapter Test homework grades – 25% based on 21 of 24 on-line Chapter Tests • Drop lowest Exam grade – 75% of your grade is based on the average of your two best Exam grades 1 Relative Age Dating Absolute Age Dating Age Dating Using Magnetic Records The Age of the Earth Geologic Time A major difference between geologists and most other scientists is their attitude about time. A "long" time may not be important unless it is > 1 million years. 2 Earth History Two ways to date geologic events 1) relative dating (fossils, structure) 2) absolute dating (isotopic, tree rings, etc.) 3 Relative vs Absolute Age • Usually geologists first establish relative ages then try to get absolute age dates • Determining relative age relies on a number of geologic principles that were developed during the 17th to early 19th centuries Steno's Laws Nicolaus Steno (1669) • Principle of Superposition • Principle of Original Horizontality • Principle of Lateral Continuity Laws apply to both sedimentary and volcanic rocks. 4 Principle of Superposition In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom. Principle of Superposition Sedimentary rocks are deposited in a layer-cake fashion: Layer 4 Layer 3 Layer 2 Layer 1 Each layer is older than the one above and younger than the one below 5 Principle of Superposition Youngest rocks Oldest rocks Principle of Original Horizontality Layered strata are deposited horizontal or nearly horizontal or nearly parallel to the Earth’s surface. 6 Principles of original horizontality and superposition 7 Principle of Lateral Continuity Layered rocks are deposited in continuous contact. Principle of Lateral Continuity Map view 8 Principle of Lateral Continuity Map view Principle of Lateral Continuity Map view 9 Correlation • Process used to tie separated strata together • Based on matching physical features such as – Physical continuity - trace of rock unit – Similar rock types - marker beds, coal seams, rare minerals, odd color Correlation • Within sedimentary layers there are often the remains of small animals (fossils) • Fossils are quite useful for correlating between two sections that are not laterally continuous 10 Ammonite Fossils Petrified Wood Correlation • Fossils represent living creatures that have evolved through time, so when we find a fossil of the same type in two different areas, we are pretty sure that the rocks are about the same age 11 CorrelatingCorrelating bedsbeds usingusing indexindex fossilsfossils Using Fossils to Correlate Rocks 12 Unconformity A buried surface of erosion 13 Layers of rock are said to be conformable when they are found to have been deposited essentially without interruption Unconformity represents missing time in the geologic record Sedimentation of Beds A-D Beneath the Sea 14 Uplift and Exposure of D to Erosion Continued Erosion Removes D and Exposes C to Erosion 15 Subsidence and Sedimentation of E over C Unconformity: a buried surface of erosion Formation of a Disconformity 16 First type of unconformity South rim of the Grand Canyon 17 South rim of the Grand Canyon 250 million years old Paleozoic Strata 550 million years old 1.7 billion years old Precambrian South rim of the Grand Canyon 250 million years old 550 million years old 1.7 billion years old Nonconformity 18 Nonconformity in the Grand Canyon Nonconformity in the Grand Canyon Tapeats Sandstone (~550 million years old) Vishnu Schist (~1700 million years old) 19 Second type of unconformity Siccar Point, Scotland 20 Siccar Point, Scotland Buried and tilted erosional surface Sedimentation of Beds A-D Beneath the Sea 21 Deformation and Erosion During Mountain Building Erosional Surface Cuts Across Deformed Rocks 22 Subsidence and Subsequent Deposition Buries Erosional Surface Angular Unconformity Formation of an Angular Unconformity 23 Third type of unconformity Principle of Cross-Cutting Relationships • A rock unit must always be older than any feature that cuts or disrupts it – If a rock unit is cut by a fracture • The rock itself is older than the fracture that cuts across it 24 Cross-cutting Relationships Relative Geologic Dating 25 The Geologic time scale • Divisions in the worldwide stratigraphic column based on variations in preserved fossils • Built using a combination of stratigraphic relationships, cross- cutting relationships, and absolute (isotopic) ages TheThe GeologicGeologic TimeTime ScaleScale 26 Absolute geochronology • Add numbers to the stratigraphic column based on fossils. • Based on the regular radioactive decay of some chemical elements. Isotopes Different forms of the same element containing the same number of protons, but varying numbers of neutrons. i.e.: 235U, 238U 87Sr, 86Sr 14C, 12C 27 NaturallyNaturally OccurringOccurring IsotopesIsotopes ofof CarbonCarbon Half-life The half-life of a radioactive isotope is defined as the time required for half of it to decay. 28 Proportion of Parent Atoms Remaining as a Function of Time Isotopic dating • Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters). • The rate at which this decay occurs is constant and known. • Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding. 29 Radioactivity and Absolute Time 30 31 Radioactive Decay of Rubidium to Strontium by β emission Production and Decay of Radiocarbon 32 of Radiocarbon of Radiocarbon Production and Decay Production and Decay Major Radioactive Elements Used in Isotopic Dating 33 Geologically Useful Decay Schemes Parent Daughter Half-life (years) 235U 207Pb 4.5 x 109 238U 206Pb 0.71 x 109 40K 40Ar 1.25 x 109 87Rb 87Sr 47 x 109 14C 14N 5730 Radiometric Dating • We can calculate geologic age if – The half-life of a radioactive isotope is known – The parent/daughter ratio can be measured – There is no loss of an isotopes from the system • e.g., 222Rn is an intermediate daughter product in the 238U decay series to 208Pb 34 Oldest rocks and minerals on Earth Narryer Gneiss, Western Australia • Zircons in a metamorphosed sandstone dated at 4.35 to 4.40 Ga Acasta Gneiss, Northwestern Canada and the Nuvvuagittuq greenstone belt, Northern Quebec • Rocks dated at 3.80 to 4.28 Ga Several other regions dated at 3.8 Ga by various methods including Minnesota, Wyoming, Greenland, South Africa, and Antarctica. Age of the Earth Although the oldest rocks found on Earth are 4.4 Ga, we believe that the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.4 Ga have been destroyed (the rock cycle) or are presently covered by younger rocks. 35 Age of the Earth This is based on the age of rocks brought back from the Moon (4.4 Ga), and meteorites (4.6 Ga), that are thought to be good representatives of the early solar system. These data suggest that the present chemical composition of the crust must have evolved for more than 4.5 Ga. The geologic timescale and absolute ages Isotopic dating of intebedded volcanic rocks allows assignment of an absolute age for fossil transitions 36 The big assumption The half-lives of radioactive isotopes are the same as they were billions of years ago. Test of the assumption Meteorites and Moon rocks (that are thought to have had a very simple history since they formed), have been dated by up to 10 independent isotopic systems all of which have given the same answer. However, scientists continue to critically evaluate this data. 37 Bracketing ages Radiometric dates provide absolute ages to the Geologic Column 38 Earth’s Magnetic Field Magnetostratigraphy • Technique that works best in volcanic rocks • Time scale based on polarity reversal of Earth's magnetic field • Major problem is that Earth's magnetic field has been constant for the past 700,000 yrs (no reversals), so this doesn't work for very young rocks 39 Magnetization of Magnetite Lavas record magnetic reversals 40 MagneticMagnetic reversalsreversals overover thethe pastpast 2020 millionmillion yearsyears The Geologic time scale • Divisions in the worldwide stratigraphic column based on variations in preserved fossils • Built using a combination of stratigraphic relationships, cross- cutting relationships, and absolute (isotopic) ages 41 The Geologic Column and Time Scale 42.
Recommended publications
  • Geologic Time Two Ways to Date Geologic Events Steno's Laws

    Geologic Time Two Ways to Date Geologic Events Steno's Laws

    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.
  • Lab 7: Relative Dating and Geological Time

    Lab 7: Relative Dating and Geological Time

    LAB 7: RELATIVE DATING AND GEOLOGICAL TIME Lab Structure Synchronous lab work Yes – virtual office hours available Asynchronous lab work Yes Lab group meeting No Quiz None – Test 2 this week Recommended additional work None Required materials Pencil Learning Objectives After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to: • Apply basic geological principles to the determination of the relative ages of rocks. • Explain the difference between relative and absolute age-dating techniques. • Summarize the history of the geological time scale and the relationships between eons, eras, periods, and epochs. • Understand the importance and significance of unconformities. • Explain why an understanding of geological time is critical to both geologists and the general public. Key Terms • Eon • Original horizontality • Era • Cross-cutting • Period • Inclusions • Relative dating • Faunal succession • Absolute dating • Unconformity • Isotopic dating • Angular unconformity • Stratigraphy • Disconformity • Strata • Nonconformity • Superposition • Paraconformity Time is the dimension that sets geology apart from most other sciences. Geological time is vast, and Earth has changed enough over that time that some of the rock types that formed in the past could not form Lab 7: Relative Dating and Geological Time | 181 today. Furthermore, as we’ve discussed, even though most geological processes are very, very slow, the vast amount of time that has passed has allowed for the formation of extraordinary geological features, as shown in Figure 7.0.1. Figure 7.0.1: Arizona’s Grand Canyon is an icon for geological time; 1,450 million years are represented by this photo.
  • Short-Lived Increase in Erosion During the African Humid Period: Evidence from the Northern Kenya Rift ∗ Yannick Garcin A, , Taylor F

    Short-Lived Increase in Erosion During the African Humid Period: Evidence from the Northern Kenya Rift ∗ Yannick Garcin A, , Taylor F

    Earth and Planetary Science Letters 459 (2017) 58–69 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Short-lived increase in erosion during the African Humid Period: Evidence from the northern Kenya Rift ∗ Yannick Garcin a, , Taylor F. Schildgen a,b, Verónica Torres Acosta a, Daniel Melnick a,c, Julien Guillemoteau a, Jane Willenbring b,d, Manfred R. Strecker a a Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Germany b Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Telegrafenberg Potsdam, Germany c Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Casilla 567, Valdivia, Chile d Scripps Institution of Oceanography – Earth Division, University of California, San Diego, La Jolla, USA a r t i c l e i n f o a b s t r a c t Article history: The African Humid Period (AHP) between ∼15 and 5.5 cal. kyr BP caused major environmental change Received 2 June 2016 in East Africa, including filling of the Suguta Valley in the northern Kenya Rift with an extensive Received in revised form 4 November 2016 (∼2150 km2), deep (∼300 m) lake. Interfingering fluvio-lacustrine deposits of the Baragoi paleo-delta Accepted 8 November 2016 provide insights into the lake-level history and how erosion rates changed during this time, as revealed Available online 30 November 2016 by delta-volume estimates and the concentration of cosmogenic 10Be in fluvial sand. Erosion rates derived Editor: A. Yin −1 10 from delta-volume estimates range from 0.019 to 0.03 mm yr . Be-derived paleo-erosion rates at −1 Keywords: ∼11.8 cal.
  • Deformation Characteristics of the Shear Zone and Movement of Block Stones in Soil–Rock Mixtures Based on Large-Sized Shear Test

    Deformation Characteristics of the Shear Zone and Movement of Block Stones in Soil–Rock Mixtures Based on Large-Sized Shear Test

    applied sciences Article Deformation Characteristics of the Shear Zone and Movement of Block Stones in Soil–Rock Mixtures Based on Large-Sized Shear Test Zhiqing Li 1,2,3,*, Feng Hu 1,2,3, Shengwen Qi 1,2,3, Ruilin Hu 1,2,3, Yingxin Zhou 4 and Yawei Bai 5 1 Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; [email protected] (F.H.); [email protected] (S.Q.); [email protected] (R.H.) 2 College of Earth and Planetary Science, University of Chinese Academy of Sciences, Beijing 100049, China 3 Innovation Academy of Earth Science, Chinese Academy of Sciences, Beijing 100029, China 4 Yunnan Chuyao Expressway Construction Headquarters, Chuxiong 675000, China; [email protected] 5 Henan Yaoluanxi Expressway Construction Co. LTD, Luanchuan 471521, China; [email protected] * Correspondence: [email protected] or [email protected]; Tel.: +86-13671264387 Received: 27 July 2020; Accepted: 15 September 2020; Published: 17 September 2020 Abstract: Soil–rock mixtures (SRM) have the characteristics of distinct heterogeneity and an obvious structural effect, which make their physical and mechanical properties very complex. This study aimed to investigate the deformation properties and failure mode of the shear zone as well as the movement of block stones in SRM experimentally, not only considering SRM shear strength. The particle composition and proportion of specimens were based on field samples from an SRM slope along national highway 318 in Xigaze, Tibet. Shear zone deformation tests were carried out using an SRM-1000 large-sized geotechnical apparatus controlled by a motor servo, considering the effects of different stone contents by mass (0, 30%, 50%, 70%), vertical pressures (50, 100, 200, 300, and 400 kPa), and block stone sizes (9.5–19.0, 19.0–31.5, and 31.5–53.0 mm).
  • Evaluation of the Depositional Environment of the Eagle Ford

    Evaluation of the Depositional Environment of the Eagle Ford

    Louisiana State University LSU Digital Commons LSU Master's Theses Graduate School 2012 Evaluation of the depositional environment of the Eagle Ford Formation using well log, seismic, and core data in the Hawkville Trough, LaSalle and McMullen counties, south Texas Zachary Paul Hendershott Louisiana State University and Agricultural and Mechanical College, [email protected] Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_theses Part of the Earth Sciences Commons Recommended Citation Hendershott, Zachary Paul, "Evaluation of the depositional environment of the Eagle Ford Formation using well log, seismic, and core data in the Hawkville Trough, LaSalle and McMullen counties, south Texas" (2012). LSU Master's Theses. 863. https://digitalcommons.lsu.edu/gradschool_theses/863 This Thesis is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Master's Theses by an authorized graduate school editor of LSU Digital Commons. For more information, please contact [email protected]. EVALUATION OF THE DEPOSITIONAL ENVIRONMENT OF THE EAGLE FORD FORMATION USING WELL LOG, SEISMIC, AND CORE DATA IN THE HAWKVILLE TROUGH, LASALLE AND MCMULLEN COUNTIES, SOUTH TEXAS A Thesis Submitted to the Graduate Faculty of the Louisiana State University Agricultural and Mechanical College in partial fulfillment of the requirements for degree of Master of Science in The Department of Geology and Geophysics by Zachary Paul Hendershott B.S., University of the South – Sewanee, 2009 December 2012 ACKNOWLEDGEMENTS I would like to thank my committee chair and advisor, Dr. Jeffrey Nunn, for his constant guidance and support during my academic career at LSU.
  • The Geohistorical Time Arrow: from Steno's Stratigraphic Principles To

    The Geohistorical Time Arrow: from Steno's Stratigraphic Principles To

    JOURNAL OF GEOSCIENCE EDUCATION 62, 691–700 (2014) The Geohistorical Time Arrow: From Steno’s Stratigraphic Principles to Boltzmann’s Past Hypothesis Gadi Kravitz1,a ABSTRACT Geologists have always embraced the time arrow in order to reconstruct the past geology of Earth, thus turning geology into a historical science. The covert assumption regarding the direction of time from past to present appears in Nicolas Steno’s principles of stratigraphy. The intuitive–metaphysical nature of Steno’s assumption was based on a biblical narrative; therefore, he never attempted to justify it in any way. In this article, I intend to show that contrary to Steno’s principles, the theoretical status of modern geohistory is much better from a scientific point of view. The uniformity principle enables modern geohistory to establish the time arrow on the basis of the second law of thermodynamics, i.e., on a physical law, on the one hand, and on a historical law, on the other. In other words, we can say that modern actualism is based on the uniformity principle. This principle is essentially based on the principle of causality, which in turn obtains its justification from the second law of thermodynamics. I will argue that despite this advantage, the shadow that metaphysics has cast on geohistory has not disappeared completely, since the thermodynamic time arrow is based on a metaphysical assumption—Boltzmann’s past hypothesis. All professors engaged in geological education should know these philosophical–theoretical arguments and include them in the curriculum of studies dealing with the basic assumptions of geoscience in general and the uniformity principle and deep time in particular.
  • Weathering, Erosion, and Susceptibility to Weathering Henri Robert George Kenneth Hack

    Weathering, Erosion, and Susceptibility to Weathering Henri Robert George Kenneth Hack

    Weathering, erosion, and susceptibility to weathering Henri Robert George Kenneth Hack To cite this version: Henri Robert George Kenneth Hack. Weathering, erosion, and susceptibility to weathering. Kanji, Milton; He, Manchao; Ribeira e Sousa, Luis. Soft Rock Mechanics and Engineering, Springer Inter- national Publishing, pp.291-333, 2020, 9783030294779. 10.1007/978-3-030-29477-9. hal-03096505 HAL Id: hal-03096505 https://hal.archives-ouvertes.fr/hal-03096505 Submitted on 5 Jan 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Published in: Hack, H.R.G.K., 2020. Weathering, erosion and susceptibility to weathering. 1 In: Kanji, M., He, M., Ribeira E Sousa, L. (Eds), Soft Rock Mechanics and Engineering, 1 ed, Ch. 11. Springer Nature Switzerland AG, Cham, Switzerland. ISBN: 9783030294779. DOI: 10.1007/978303029477-9_11. pp. 291-333. Weathering, erosion, and susceptibility to weathering H. Robert G.K. Hack Engineering Geology, ESA, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente Enschede, The Netherlands e-mail: [email protected] phone: +31624505442 Abstract: Soft grounds are often the result of weathering. Weathering is the chemical and physical change in time of ground under influence of atmosphere, hydrosphere, cryosphere, biosphere, and nuclear radiation (temperature, rain, circulating groundwater, vegetation, etc.).
  • Correlating Foreland Basin Subsidence with Eclogite Metamorphism

    Correlating Foreland Basin Subsidence with Eclogite Metamorphism

    ATI..ANTic GEOLOGY 79 Loading the Laurentian margin: correlating foreland basin subsidence with eclogite metamorphism 2 3 4 John WF. Waldron•, R.A. Jamieson , G.S. Stockmal and L.A. Quinn 1Geology Department, Saint Mary '.S' University, Halifax, Nova Scotia B3H 3C3, Canada 'Department ofEarth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada 1Geological Survey of Canada (Calgary), 3303-33rd Street Northwest, Calgary, Alberta T2L 2A 7, Canada 'Department ofGeology, Brandon University, Brandon, Manitoba R7A 6A9, Canada Paleozoic loading of the former Laurentian continental de Lys Supergroup) are exposed in the Baie Verte Peninsula margin is recorded both in the subsidence history of the Ap­ and elsewhere. These units record Barrovian metamorphism palachian foreland basin and in metamorphic rocks now ex­ with peak temperatures around 700 to 750°C at 7 to 9 kbar; humed in internal parts of the Newfoundland Humber zone. isotopic data indicate that peak temperatures were reached in The Cambrian-Ordovician passive margin ofLaurentia Early Silurian time ('Salinian orogeny'), followed by rapid ex­ underwent a transition to a foreland basin setting beginning humation. Amphibolite facies metamorphism overprints an in Early Ordovician time. Middle Ordovician ('Taconian ') foreland earlier eclogite facies assemblage, for which minimum pres­ basin sediments (Table Head and Goose Tickle groups), in sures of 1.2 GPa at 500°C require burial of the Laurentian part derived from the Humber Arm Allochthon, are relatively margin beneath at least 40 km of overburden, which may have thin (ca. 250 m in offshore industry seismic data, thinning to included thrust sheets of continental margin rocks and the west).
  • Metamorphism Associated with Extensional Rifting of Gondwana

    Metamorphism Associated with Extensional Rifting of Gondwana

    Geological Society, London, Special Publications Basement and cover rock history in western Tethys: HT-LP metamorphism associated with extensional rifting of Gondwana Robert Hall Geological Society, London, Special Publications 1988; v. 37; p. 41-50 doi:10.1144/GSL.SP.1988.037.01.04 Email alerting click here to receive free email alerts when new articles cite this article service Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by Robert Hall on 26 November 2007 © 1988 Geological Society of London Basement and cover rock history in western Tethys: HT-LP metamorphism associated with extensional rifting of Gondwana Robert Hall ABSTRACT: High-temperature-low-pressure metamorphism of the deep crust is probable during continental lithospheric extension. The 75-65 Ma cooling ages of metamorphic and magmatic rocks preserved in overthrust crystalline slices in the southern Aegean are most plausibly explained by late Cretaceous extension of the Apulian continental margin, rather than by subduction-related magmatism. Metamorphism and magmatism at depth can be correlated with those stratigraphic features of the cover sequences that indicate extension. In particular, the puzzling 'premier flysch' of the region is interpreted as one consequence of peripheral uplift associated with stretching. Introduction observed in overthrust terrains, but evidence of the extensional history of the margin may also be discernible from interpretation of the sedimen- Many recent models for the evolution of passive tary sequences deposited within the former con- continental margins imply the possibility of a tinental margin.
  • Geologic Models and Evaluation of Undiscovered Conventional and Continuous Oil and Gas Resources— Upper Cretaceous Austin Chalk, U.S

    Geologic Models and Evaluation of Undiscovered Conventional and Continuous Oil and Gas Resources— Upper Cretaceous Austin Chalk, U.S

    Geologic Models and Evaluation of Undiscovered Conventional and Continuous Oil and Gas Resources— Upper Cretaceous Austin Chalk, U.S. Gulf Coast Scientific Investigations Report 2012–5159 U.S. Department of the Interior U.S. Geological Survey Front Cover. Photos taken by Krystal Pearson, U.S. Geological Survey, near the old Sprinkle Road bridge on Little Walnut Creek, Travis County, Texas. Geologic Models and Evaluation of Undiscovered Conventional and Continuous Oil and Gas Resources—Upper Cretaceous Austin Chalk, U.S. Gulf Coast By Krystal Pearson Scientific Investigations Report 2012–5159 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Pearson, Krystal, 2012, Geologic models and evaluation of undiscovered conventional and continuous oil and gas resources—Upper Cretaceous Austin Chalk, U.S.
  • A Nonlinear Response of Sahel Rainfall to Atlantic Warming

    A Nonlinear Response of Sahel Rainfall to Atlantic Warming

    7080 JOURNAL OF CLIMATE VOLUME 26 A Nonlinear Response of Sahel Rainfall to Atlantic Warming NARESH NEUPANE AND KERRY H. COOK Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas (Manuscript received 24 July 2012, in final form 2 February 2013) ABSTRACT The response over West Africa to uniform warming of the Atlantic Ocean is analyzed using idealized simulations with a regional climate model. With warming of 1 and 1.5 K, rainfall rates increase by 30%–50% over most of West Africa. With Atlantic warming of 2 K and higher, coastal precipitation increases but Sahel rainfall decreases substantially. This nonlinear response in Sahel rainfall is the focus of this analysis. Atlantic warming is accompanied by decreases in low-level geopotential heights in the Gulf of Guinea and in the large- scale meridional geopotential height gradient. This leads to easterly wind anomalies in the central Sahel. With Atlantic warming below 2 K, these easterly anomalies support moisture transport from the Gulf of Guinea and precipitation increases. With Atlantic warming over 2 K, the easterly anomalies reverse the westerly flow over the Sahel. The resulting dry air advection into the Sahel reduces precipitation. Increased low-level moisture provides moist static energy to initiate convection with Atlantic warming at 1.5 K and below, while decreased moisture and stable thermal profiles suppress convection with additional warming. In all simula- tions, the southerly monsoon flow onto the Guinean coast is maintained and precipitation in that region increases. The relevance of these results to the global warming problem is limited by the focus on Atlantic warming alone.
  • 6. Relative and Absolute Dating

    6. Relative and Absolute Dating

    6. Relative and Absolute Dating Adapted by Sean W. Lacey & Joyce M. McBeth (2018) University of Saskatchewan from Deline B, Harris R, & Tefend K. (2015) "Laboratory Manual for Introductory Geology". First Edition. Chapter 1 "Relative and Absolute Dating" by Bradley Deline, CC BY-SA 4.0. View Source. 6.1 INTRODUCTION To develop a history of how geologic events have acted on the Earth through time, we need to understand what and when geological processes have occurred through Earth's history. Geologists learn about what processes occur on Earth through studying the rock record and observing geologic processes in modern environments. To understand when these processes have acted during Earth's geologic time, geologists make observations about the relationships of rocks to one another in the rock record, using a process called relative dating. Geologists use this information to construct models for how these relationships developed. For example, if the rock record in an area contains sedimentary rocks that are folded, a model to explain those relationships would start with a region where sediments were deposited, followed by lithification of the sediments to form rock, then the rocks would be subjected to tectonic pressures that folded the rocks. Using relative dating techniques, we know those events occurred in that order, but not when they occurred precisely in time. To add specific dates for the events in the model, geologists can use absolute dating techniques to date the rocks (determine their age). Geologists develop models such as this at locations all across Canada, North America, and around the globe. Each location geologists study may only provide information on Earth history from a short window in time; collectively, however, the information in these models can be used to develop our understanding of processes that have acted on Earth since it first formed.