DOCSLIB.ORG

CAN EARTH-LIKE PLATE TECTONICS OCCUR in the OUTER ICE SHELLS of ICY SATELLITES? Samuel M

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
CAN EARTH-LIKE PLATE TECTONICS OCCUR in the OUTER ICE SHELLS of ICY SATELLITES? Samuel M 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1132.pdf CAN EARTH-LIKE PLATE TECTONICS OCCUR IN THE OUTER ICE SHELLS OF ICY SATELLITES? Samuel M. Howell1 and Robert T. Pappalardo1, 1Jet Propulsion Laboratory, California Institute of Technology ([email protected]). Introduction: The outer H2O ice shells of some Figure 1. icy satellites show evidence for divergent, strike-slip, Geometry of and convergent tectonics. Researches propose that conceptual Earth-like plate tectonic processes may occur in the ice model consid- shells of Europa and Enceladus [1-3], raising the pos- ered in the sibility of a buoyantly driven cycle of tectonic resurfac- force balance. ing and lithosphere recycling on icy satellites [1]. See text for On Earth, a cycle of spreading and subduction is parameter supported by buoyancy forces transmitted elastically descriptions. through the lithosphere [4]. The majority of tectonic forcing comes from “slab pull,” driven by the negative buoyancy of cold lithosphere “slabs” intruding into the warm asthenosphere. “Ridge push” accounts for only a tenth of these forces, and is driven by the cooling of strength of the lithosphere, FY, the force required to lithosphere with increasing age for 10s Myr and over elastically deform the lithosphere, FE, and basal trac- 1,000s km, causing warmer, more buoyant material to tions that resist sliding between the lithosphere and slide downhill away from the ridge axis. asthenosphere, FA. Thus, subduction is permitted when On Europa, proposed "subsumption zones" at in- FFFFF t .(1) ferred convergent margins may allow old lithosphere to STYEA be reincorporated into the ice shell and recycled [2]. The buoyancy force exerted on the lithosphere by a Johnson et al. [1] recently demonstrated that a subduct- penetrating slab per along-trench distance is ing ice I slab on Europa may remain negatively buoy- H FgS,(2) ant for appropriate values of salt content, salt distribu- Sla UU sin M tion, and porosity of the subducting plate, providing a mechanism for slab pull. Like mid-ocean ridges, exten- where ȡl and ȡa are respectively the densities of the sional bands may locally bring warm isotherms to shal- cold ice lithosphere and warm ice asthenosphere, g is low depths [e.g. 5]. However, their limited width (~10s the gravitational acceleration, S is the penetration depth km) [6] requires either a short lifespan or slow opening of the downwelling slab beneath the base level of unde- rate, inhibiting the development of the large gradients formed lithosphere, H is the thickness of the litho- in plate thickness persistent through geologic time that sphere, and ij is the dip of the subducting slab. drive ridge push. Using the simplifying assumption that tidal stresses, To explore whether Earth-like, convectively driven ıT, are uniformly distributed through the ice shell, the plate tectonics can occur in the outer shells of icy satel- depth-integrated tidal forcing is lites, we employ a simple force balance to determine FHYY V ,(3) whether the buoyancy forces associated with slab pull The depth-integrated Mohr-Coulomb yield strength can overcome the strength of the lithosphere to initiate depends on the lithosphere's cohesion, C, its coefficient or sustain subduction. We consider a range in litho- of internal friction, ȝ, and the lithostatic stress at its sphere cohesion and coefficient of friction for pristine base, and weakened lithosphere, salt content, and tempera- ture to determine the depth a slab must reach before 1 FHCYl PU gH.(4) sustaining subduction. As a feasibility test, this force 2 balance incorporates the most optimistic first-order We maximize driving forces by assuming a vertical assumptions, promoting Earth-like plate tectonics to slab, and minimize resisting forces by negating elastici- the greatest possible extent. ty and assuming that the lithosphere slides freely over Force Balance Framework: Fig. 1 shows the ge- the asthenosphere. Thus, Eq.1 becomes ometry of the conceptual model used in the force bal- ance. Self-driving plate tectonics requires the sum of 1 UUlatgS VT C PU l gH .(5) driving forces to exceed the sum of resisting forces [7, 2 8]. In icy satellites, potential driving forces include the To investigate the density anomaly required for sus- forces associated with a negatively buoyant slab, FS, tained subduction, we solve Eqs. 1-6 for ȡl, and explore and tidal interactions, FT. In order to sustain subduc- the surface temperature, salt content, and external tion, tidal forces must persist through geologic time. stresses required to sustain plate tectonics. Resisting forces include the depth-integrated yield 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1132.pdf Optimistic Assumptions: Continuing optimistic >70 kg/m3 if the lithosphere has somehow already pen- assumptions that promote slab pull-driven subduction, etrated to the base. For comparison, Johnson et al. [1] the slab is assumed to be perfectly insulated from the find that on Europa, maximum sustained density con- warm interior, retaining its initial temperature contrast. trasts may be just ~30 kg/m3. The porosity of the slab and interior are ignored to If a slab penetrates far into the ice shell in response maximize the buoyancy contrast. The convecting as- to external forcing, subduction can be maintained only thenosphere is assumed free of salt, preferentially parti- if the subduction zone has greatly reduced strength. For tioned into the melt and extracted to the ocean. Fur- example, if cohesion is reduced to ~100 kPa and the thermore, we assume no diffusion of salt from the coefficient of internal friction is reduced to 0.1, sub- downwelling slab to the convecting asthenosphere. duction can be buoyantly maintained on Ganymede for The ice shells are taken to be sufficiently thick so a density contrast of 30 kg/m3 once the slab has pene- that the maximum slab penetration can be large (25 km trated ~10% of the ice shell thickness. While the gravi- for Europa, 50 km for Ganymede, 145 km for Encela- ty of Enceladus is low, the ice shell may be very thick. dus), and the lithosphere thickness is minimized at 3 If subduction zone cohesion is reduced to 100 kPa and 2 km (FB increases with H, while FY increases with H ). the coefficient of friction remains 0.6, subduction be- This is appreciably thinner than lithosphere thicknesses comes self-sustaining when the slab has penetrated predicted by thermal and tectonic models [e.g. 5]. ~50% of the ice shell thickness (~70 km). Results and Discussion: With this simple force The external forces required to initiate subduction balance, we investigate whether it is plausible for slab in pristine ice are 1-2 MPa. For comparison, diurnal pull to initiate and/or maintain subduction and drive tidal forces on Europa are on the order 100 kPa and Earth-like plate tectonics in outer H2O ice shells. We reverse sign cyclically [9]. Thus, sustained secular tidal perform this analysis for Europa, Enceladus, and Gan- stresses, e.g. non-synchronous rotation or true polar ymede, spanning from the smallest to the largest icy wander, would be required to initiate subduction for satellites with observed tectonics. realistic density contrasts [1] and to sustain subduction While the large gravity of Ganymede and potential- unless ice shell strength is greatly reduced. ly thick ice shell of Enceladus are more conducive to Conclusions: In the outer ice shells of icy satel- subduction than the ice shell of Europa, we find that lites, the density contrast between subducting litho- negative slab buoyancy alone is unlikely to allow the sphere and warm asthenosphere is small compared to initiation of subduction in any of the broad range of icy the mechanical strength of the lithosphere, even under satellites considered (Fig. 2). For example, initiating the most favorable assumptions. Thus, slab pull is un- subduction on Europa in pristine lithosphere (ȝ =0.6, likely to initiate subduction in the outer H2O ice shells C = 1 MPa) requires a density contrast of >400 kg/m3 of icy satellites, and will maintain ongoing subduction when the slab has penetrated 10% of the ice shell, and only when the subduction zone is weakened signifi- ȝ = 0 ȝ = 0.1 ȝ = 0.6 cantly compared to pristine lithosphere. An Earth-like 100 cycle of convectively driven plate tectonics is therefore C = 1 MPa Enc. ] 80 unlikely on any icy satellite. -3 Eur. 60 C = 100 kPa Gan. If the strength of the ice shell is significantly lower 40 than considered here (i.e. if the cohesion and coeffi- [kg m ȡ cient of friction are orders of magnitude smaller), the ǻ 20 C = 0 negative buoyancy of the slab could plausibly drive 0 104 subduction. However, such a weak ice shell would have to be reconciled with the reliance of slab pull on 103 the elastic transmission of stresses, and inferences of 100 elastically supported topography from spacecraft ob- [kPa] T servations. ı 10 References: [1] B. C. Johnson et al. (2017, in 1 press) J. Geophys. Res. – Planets [2] S. A. Kattenhorn 0.01 0.1 1 0.01 0.1 1 0.01 0.1 1 Slab Penetration / Ice Shell Thickness and L. M. Prockter (2014) Nature Geosci., 10, 762-767 [3] M. T. Bland and W. B. McKinnon (2014), DPS Figure 2: Force balance predictions of the density Fall Meeting, Abstract #2817616. [4] D. Turcotte and contrast between the downwelling lithosphere slab and G. Schubert (2014), in Geodynamics, pp. 10-23. [5] S. warm asthenosphere (top) and regional driving stresses M. Howell and R. T. Pappalardo (2017) Europa Deep (bottom) required to satisfy Eq. 5. Colored lines show Dive I, Abstract #7002. [6] L. M. Prockter et al.
Load more

Recommended publications
  • Advanced Geodynamics: Fourier Transform Methods
    Advanced Geodynamics: Fourier Transform Methods David T. Sandwell January 13, 2021 To Susan, Katie, Melissa, Nick, and Cassie Eddie Would Go Preprint for publication by Cambridge University Press, October 16, 2020 Contents 1 Observations Related to Plate Tectonics 7 1.1 Global Maps . .7 1.2 Exercises . .9 2 Fourier Transform Methods in Geophysics 20 2.1 Introduction . 20 2.2 Definitions of Fourier Transforms . 21 2.3 Fourier Sine and Cosine Transforms . 22 2.4 Examples of Fourier Transforms . 23 2.5 Properties of Fourier transforms . 26 2.6 Solving a Linear PDE Using Fourier Methods and the Cauchy Residue Theorem . 29 2.7 Fourier Series . 32 2.8 Exercises . 33 3 Plate Kinematics 36 3.1 Plate Motions on a Flat Earth . 36 3.2 Triple Junction . 37 3.3 Plate Motions on a Sphere . 41 3.4 Velocity Azimuth . 44 3.5 Recipe for Computing Velocity Magnitude . 45 3.6 Triple Junctions on a Sphere . 45 3.7 Hot Spots and Absolute Plate Motions . 46 3.8 Exercises . 46 4 Marine Magnetic Anomalies 48 4.1 Introduction . 48 4.2 Crustal Magnetization at a Spreading Ridge . 48 4.3 Uniformly Magnetized Block . 52 4.4 Anomalies in the Earth’s Magnetic Field . 52 4.5 Magnetic Anomalies Due to Seafloor Spreading . 53 4.6 Discussion . 58 4.7 Exercises . 59 ii CONTENTS iii 5 Cooling of the Oceanic Lithosphere 61 5.1 Introduction . 61 5.2 Temperature versus Depth and Age . 65 5.3 Heat Flow versus Age . 66 5.4 Thermal Subsidence . 68 5.5 The Plate Cooling Model .
    [Show full text]
  • The Way the Earth Works: Plate Tectonics
    CHAPTER 2 The Way the Earth Works: Plate Tectonics Marshak_ch02_034-069hr.indd 34 9/18/12 2:58 PM Chapter Objectives By the end of this chapter you should know . > Wegener's evidence for continental drift. > how study of paleomagnetism proves that continents move. > how sea-floor spreading works, and how geologists can prove that it takes place. > that the Earth’s lithosphere is divided into about 20 plates that move relative to one another. > the three kinds of plate boundaries and the basis for recognizing them. > how fast plates move, and how we can measure the rate of movement. We are like a judge confronted by a defendant who declines to answer, and we must determine the truth from the circumstantial evidence. —Alfred Wegener (German scientist, 1880–1930; on the challenge of studying the Earth) 2.1 Introduction In September 1930, fifteen explorers led by a German meteo- rologist, Alfred Wegener, set out across the endless snowfields of Greenland to resupply two weather observers stranded at a remote camp. The observers had been planning to spend the long polar night recording wind speeds and temperatures on Greenland’s polar plateau. At the time, Wegener was well known, not only to researchers studying climate but also to geologists. Some fifteen years earlier, he had published a small book, The Origin of the Con- tinents and Oceans, in which he had dared to challenge geologists’ long-held assumption that the continents had remained fixed in position through all of Earth history. Wegener thought, instead, that the continents once fit together like pieces of a giant jigsaw puzzle, to make one vast supercontinent.
    [Show full text]
  • MAGNITUDE of DRIVING FORCES of PLATE MOTION Since the Plate
    J. Phys. Earth, 33, 369-389, 1985 THE MAGNITUDE OF DRIVING FORCES OF PLATE MOTION Shoji SEKIGUCHI Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, Japan (Received February 22, 1985; Revised July 25, 1985) The absolute magnitudes of a variety of driving forces that could contribute to the plate motion are evaluated, on the condition that all lithospheric plates are in dynamic equilibrium. The method adopted here is to solve the equations of torque balance of these forces for all plates, after having estimated the magnitudes of the ridge push and slab pull forces from known quantities. The former has been estimated from the age of ocean floors, the depth and thickness of oceanic plates and hence lateral density variations, and the latter from the density con- trast between the downgoing slab and the surrounding mantle, and the thickness and length of the slab. The results from the present calculations show that the magnitude of the slab pull forces is about five times larger than that of the ridge push forces, while the North American and South American plates, which have short and shallow slabs but long oceanic ridges, appear to be driven by the ridge push force. The magnitude of the slab pull force exerted on the Pacific plate exceeds to 40 % of the total slab pull forces, and that of the ridge push force working on the Pacific plate is the largest among the ridge push forces exerted on the plates. The high cor- relation that exists between the mantle drag force and the sum of the slab pull and ridge push forces makes it difficult to evaluate the absolute net driving forces.
    [Show full text]
  • An Evidence-Based Approach to Teaching Plate Tectonics in High School
    An evidence-based approach to teaching plate tectonics in high school COLIN PRICE ABSTRACT 5. relating the extreme age and stability of a large part of the Australian continent to its plate tectonic This article proposes an evidence-based and engaging history. approach to teaching the mechanisms driving the movement of tectonic plates that should lead high The problem with this list is the fourth elaboration, school students towards the prevalent theories because the idea that convection currents in the used in peer-reviewed science journals and taught mantle drive the movement of tectonic plates is a in universities. The methods presented replace the myth. This convection is presented as whole mantle inaccurate and outdated focus on mantle convection as or whole asthenosphere cells with hot material rising the driving mechanism for plate motion. Students frst under the Earth’s divergent plate boundaries and examine the relationship between the percentages of cooler material sinking at the convergent boundaries plate boundary types of the 14 largest plates with their with the lithosphere dragged along by the horizontal GPS-determined plate speeds to then evaluate the fow of the asthenosphere (Figure 1). This was the three possible driving mechanisms: mantle convection, preferred explanation for plate motion until the ridge push and slab pull. A classroom experiment early 1990s but it does not stand up to a frequent measuring the densities of igneous and metamorphic deduction made by Year 9 students: how can there be rocks associated with subduction zones then provides large-scale mantle convection if hotspots (like Hawaii) a plausible explanation for slab pull as the dominant don’t move? Most science teachers quickly cover driving mechanism.
    [Show full text]
  • The Relation Between Mantle Dynamics and Plate Tectonics
    The History and Dynamics of Global Plate Motions, GEOPHYSICAL MONOGRAPH 121, M. Richards, R. Gordon and R. van der Hilst, eds., American Geophysical Union, pp5–46, 2000 The Relation Between Mantle Dynamics and Plate Tectonics: A Primer David Bercovici , Yanick Ricard Laboratoire des Sciences de la Terre, Ecole Normale Superieure´ de Lyon, France Mark A. Richards Department of Geology and Geophysics, University of California, Berkeley Abstract. We present an overview of the relation between mantle dynam- ics and plate tectonics, adopting the perspective that the plates are the surface manifestation, i.e., the top thermal boundary layer, of mantle convection. We review how simple convection pertains to plate formation, regarding the aspect ratio of convection cells; the forces that drive convection; and how internal heating and temperature-dependent viscosity affect convection. We examine how well basic convection explains plate tectonics, arguing that basic plate forces, slab pull and ridge push, are convective forces; that sea-floor struc- ture is characteristic of thermal boundary layers; that slab-like downwellings are common in simple convective flow; and that slab and plume fluxes agree with models of internally heated convection. Temperature-dependent vis- cosity, or an internal resistive boundary (e.g., a viscosity jump and/or phase transition at 660km depth) can also lead to large, plate sized convection cells. Finally, we survey the aspects of plate tectonics that are poorly explained by simple convection theory, and the progress being made in accounting for them. We examine non-convective plate forces; dynamic topography; the deviations of seafloor structure from that of a thermal boundary layer; and abrupt plate- motion changes.
    [Show full text]
  • 19820014972.Pdf
    N O T I C E THIS DOCUMENT HAS BEEN REPRODUCED FROM MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED IN THE INTEREST OF MAKING AVAILABLE AS MUCH INFORMATION AS POSSIBLE N® i r F'B8;'- 137 118 Geodetic Monitoring of Tectonic Deformation Toward a Strategy (U.S.) National Research Council Washington, DC Prepared for Defense Mapping Agency Washington, DC !A Nov 81 U ft~ d O.ma ftllul ittfinf Ulwnrtion Service um REPORT 001lUrENTA710N IL 111110M N0. L !. ti ^koasa" N& PRRE 3 4. Two ad f► Vle L Ibwt Dab November 1981 Geod .`c Monitoring of Tectonic Deformation--Toward a Strategy ` 350046 7. AuUwW L Palo Ownbotwo wsot.we. Panel on Crustal Movement Measurements/Committee on Geodesy 9. P"NWJ rj Ogsalsalim NU a" Adkeft 1R Nsirtst/T*000wR Uelt W National Research Council Committee on Geodesy IL O'NabodO or Gnw*M H& 2101 Constitution. Avenue, NW Washington, D.C. 20418 tai 12. SOaasa b9 OrawguMs Iwma w d Addeaw is. rAw of NOW i Paned Cawmd Defense Mapping Agency, National Aeronautics and Space Admin., Final National Oceanic and Atmospheric Admin., U.S. Geological Surve y IL Suppm" .nt" Iratm I& Atrtfad (Weft 200 arad^) The report presents issues of interest and importance to society and science. The problems considered are of national concern; their solutions may contribute to a better understanding of tectonic deformation and earthquake hazards. The need for additional field data, the role of geodetic measurements, the importance of both ground and space techniques, and the need for advanced instrumentation development are discussed.-.-*-., 17.
    [Show full text]
  • This Dynamic Earth in January of 1992
    View of the planet Earth from the Apollo spacecraft. The Red Sea, which separates Saudi Arabia from the continent of Africa, is clearly visible at the top. (Photograph courtesy of NASA.) Contents Preface Historical perspective Developing the theory Understanding plate motions "Hotspots": Mantle Some unanswered questions Plate tectonics and people Endnotes thermal plumes 1 of 77 2002-01-01 11:52 This pdf-version was edited by Peter Lindeberg in December 2001. Any deviation from the original text is non-intentional. This book was originally published in paper form in February 1996 (design and coordination by Martha Kiger; illustrations and production by Jane Russell). It is for sale for $7 from: U.S. Government Printing Office Superintendent of Documents, Mail Stop SSOP Washington, DC 20402-9328 or it can be ordered directly from the U.S. Geological Survey: Call toll-free 1-888-ASK-USGS Or write to USGS Information Services Box 25286, Building 810 Denver Federal Center Denver, CO 80225 303-202-4700; Fax 303-202-4693 ISBN 0-16-048220-8 Version 1.08 The online edition contains all text from the original book in its entirety. Some figures have been modified to enhance legibility at screen resolutions. Many of the images in this book are available in high resolution from the USGS Media for Science page. USGS Home Page URL: http://pubs.usgs.gov/publications/text/dynamic.html Last updated: 01.29.01 Contact: [email protected] 2 of 77 2002-01-01 11:52 In the early 1960s, the emergence of the theory of plate tectonics started a revolution in the earth sciences.
    [Show full text]
  • Origin and Models of Oceanic Transform Faults
    Tectonophysics 522–523 (2012) 34–54 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Review Article Origin and models of oceanic transform faults Taras Gerya ⁎ Swiss Federal Institute of Technology Zurich, Department of Geosciences, Zurich, Switzerland article info abstract Article history: Mid-ocean ridges sectioned by transform faults represent prominent surface expressions of plate tectonics. A Received 22 March 2011 fundamental problem of plate tectonics is how this pattern has formed and why it is maintained. Gross-scale Received in revised form 30 June 2011 geometry of mid-ocean ridges is often inherited from respective rifted margins. Indeed, transform faults seem Accepted 7 July 2011 to nucleate after the beginning of the oceanic spreading and can spontaneously form at a single straight ridge. Available online 22 July 2011 Both analog and numerical models of transform faults were investigated since the 1970s. Two main groups of analog models were developed: thermomechanical (freezing wax) models with accreting and cooling plates Keywords: – Transform faults and mechanical models with non-accreting lithosphere. Freezing wax models reproduced ridge ridge Mid-ocean ridges transform faults, inactive fracture zones, rotating microplates, overlapping spreading centers and other Analog models features of oceanic ridges. However, these models often produced open spreading centers that are dissimilar Numerical models to nature. Mechanical models, on the other hand, do not accrete the lithosphere and their results are thus only applicable for relatively small amount of spreading. Three main types of numerical models were investigated: models of stress and displacement distribution around transforms, models of their thermal structure and crustal growth, and models of nucleation and evolution of ridge-transform fault patterns.
    [Show full text]
  • Lecture 13-15 Notes: Plate Tectonics
    Plate Tectonics T. Perron – 12.001 Overview: Today: o Lecture: History of ideas about plate tectonics o Lab: Scientific specialties Day 2 o Lecture: Rates and patterns of plate motions o Lab: Plate groups Day 3 o Lecture: Forces driving plate motions o Lab: Plate groups present, wrap-up discussion Definitions Earth Science’s Grand Unifying Theory (or at least, one that works very well for many things) From “tekton” (Greek, “builder”) Tectonics does not necessarily equal plate tectonics. For 2 centuries, ideas had been formulated about how mountains, ocean basins, etc. are formed. Textbooks explained these just like your textbooks explain plate tectonics, but they were mostly wrong. What was the series of ideas & events that led to the discovery of plate tectonics? Observations: what were geologists of the last 2 centuries trying to explain? [Your lab will focus one some of these observations] Topography [PPT: global shaded relief] o Mountains, ocean basins, continental basins o Some mountains are high and jagged, others low and subdued Global distributions of earthquakes, volcanoes [PPT: figures from Lab] Evidence that things were vastly different in the past o Some mountains made of marine sedimentary rocks. Fossils at the top of Mt. Everest (!) o Marine sediments covering the midwestern US (recall the Indiana Limestone of the main MIT buildings) Alternate theories Noah’s flood (The “Diluvian hypothesis”). Also works nicely for, e.g., Grand Canyon. Global contraction (motivated mainly by compressional mountain ranges) Global expansion (proposed later as an explanation for evidence of seafloor spreading) Sea level changes Geosynclines (Mom’s college geology textbook) 1 Continental Drift Wegener (German meteorologist), post-WWI, Origin of Oceans & Continents: Continents have moved! Evidence: Geographic match (jigsaw) [PPT: relief map] Rock type match (flood basalts in S.
    [Show full text]
  • Earth Science
    EARTH SCIENCE A Review of our Earth HISTORY OF THE EARTH The Geologic Time Scale HOW OLD IS THE EARTH? Geologists use fossil records to separate the history of the Earth The divisions are from largest to smallest length of time eon era period epoch 4.6 BILLION YEARS OLD Eon: measured in billions of years Era: measured in hundreds of millions of years and marked by differences in life-forms that are preserved in rocks. Period: measured in tens of millions of years and marked by life-forms that were abundant and then extinct Epoch: measured in millions of years and marked by different groups of organisms THE FOUR EONS Hadean (4.6-3.9 bya) Archeozoic (3.9-2.5 bya) Precambrian Proterozoic (2.5-0.54 bya) Time Phanerozoic (540 mya - present) WHAT ERA DO WE COME IN? Precambrian times: bacteria Paleozoic Era (540-248 mya): fish, reptiles, marine invertebrates, land plants and amphibians Mesozoic Era (248-65 mya): age of dinosaurs (triassic, jurassic & cretaceous) Cenozoic Era (65 mya - present): age of mammels WHAT PERIOD DO I HAVE HUMAN? Cenozoic Era Tertiary Period Paleocene Eopch Eocene Epoch Oligocene Epoch Miocene Epoch Pliocene Epoch Quaternary Period Pleistocene Epoch You are Holocene Epoch here RELATIVE AGE OF ROCKS The relative age of rocks can be determined by examining the different horizontal layers rocks fossils unconformities ABSOLUTE AGE DATING Radiometric Dating Half-life Thorium-232 (14.0 billion yrs) to Lead-208 Uranium-238 (4.5 billion yrs) to Lead-206 Uranium-235 (0.7 billion yrs) to Lead-207 Carbon-14 (only 5,730 yrs) to Nitrogen-14 GEOLOGIC TIMELINE ASSIGNMENT http://www.learner.org/vod/vod_window.html? pid=321 Complete the “Geologic Timeline” found on the “APES Assignment” page.
    [Show full text]
  • RESEARCH Ridge-Push Force and the State of Stress
    RESEARCH Ridge-push force and the state of stress in the Nubia- Somalia plate system Rezene Mahatsente1 and David Coblentz2 1DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF ALABAMA, TUSCALOOSA, ALABAMA 35487, USA 2EARTH AND ENVIRONMENTAL SCIENCES DIVISION, LOS ALAMOS NATIONAL LABORATORY, LOS ALAMOS, NEW MEXICO 87545, USA ABSTRACT We assessed the relative contribution of ridge-push forces to the stress state of the Nubia-Somalia plate system by comparing ridge-push forces with lithospheric strength in the oceanic part of the plate, based on estimates from plate cooling and rheological models. The ridge- push forces were derived from the thermal state of the oceanic lithosphere, seafloor depth, and crustal age data. The results of the compari- son show that the magnitude of the ridge-push forces is less than the integrated strength of the oceanic part of the plate. This implies that the oceanic part of the plate is very little deformed; thus, the ridge-push forces may be compensated by significant strain rates outside the oceanic parts of the plate. We used an elastic finite element analysis of geoid gradients of the upper mantle to evaluate stresses associated with the gravitational potential energy of the surrounding ridges and show that these stresses may be transmitted through the oceanic part of the plate, with little modulation in magnitude, before reaching the continental regions. We therefore conclude that the present-day stress fields in continental Africa can be viewed as the product of the gravitational potential energy of the ridge ensemble surrounding the plate in conjunction with lateral variations in lithospheric structure within the continent regions.
    [Show full text]
  • Tectonics, Earthquakes, & Volcanism
    The Geosphere: Earth’s Interior & Surface During the twentieth century, new research revolutionized our understanding of Earth sci- ence. These discoveries explained how volcanoes such as Ecuador’s Tungurahua were formed and how the continents and oceans came Tectonics, to their present arrangement. The theory of plate tectonics—the main subject of this chapter—provides a Earthquakes, unified explanation for the pat- terns of earthquakes, volcanoes, and mountain building that domi- & Volcanism nate our planet. Events such as the March 2011 earthquake and tsunami that devastated Japan turn world attention to the Earth sciences, including physical geogra- 8 phy, for explanations. SAMPLE CHAPTER M08_CHRI4744_01_SE_C08.indd 2 30/09/15 1:00 AM What Are Earth’s History, Interior Structure, How Does Plate Tectonics Explain Changes 1 & Materials? 2 in Earth’s Surface? 8.1 The Vast Span of Geologic Time 8.4 Plate Tectonics 8.2 Earth’s History & Interior 8.5 Seafloor Spreading & Subduction Zones 8.3 The Rock Cycle 8.6 Plate Boundaries How Do Plate Motions Affect 3 Earth’s Crust? 8.7 Deformation, Folding, & Faulting 8.8 Earthquakes 8.9 Volcanoes 8.10 Mountain Building Mount Tungurahua in Ecuador is one of many active volcanoes in the Andes mountain chain that extends along the western coast of South America. SAMPLE CHAPTER M08_CHRI4744_01_SE_C08.indd 3 30/09/15 1:00 AM 4 What Are Earth’s History, Interior Structure, & Materials? 8.1 The Vast Span of Geologic Time Key Learning Concepts ago” shown on the time scale. In contrast, relative time is ▶ Describe the geologic time scale.
    [Show full text]