Mantle Discontinuities

Total Page:16

File Type:pdf, Size:1020Kb

Mantle Discontinuities REVIEWS OF GEOPHYSICS, SUPPLEMENT, PAGES 783-793, APRIL 1991 U.S. NATIONAL REPORT TO INTERNATIONAL UNION OF GEODESY AND GEOPHYSICS 1987-1990 Mantle Discontinuities CRAIG R. BINA Dept. of Geological Sciences, Northwestern University, Evamton, Illinois INTRODUCT?ON mantle discontinuities which have taken place during the period 1987-1990, as well as briefly discussing some The nature of the discontinuities evident in seismic implications for mantle thermal structure and dynamics. velocity profiles of the Earth's mantle has been a topic of While I have endeavored to compile as complete a refer- active research since the work of Birch in the 1950s. ence list as possible, the lesser number of references 'The chemical, mineralogical, and thermal properties of directly discussed in the text necessarily reflects my own these features have important implications outside the interests and familiarity with the literature. I conclude by fields of seismology and mineral physics, constraining as outlining the picture of mantle structure and composition they do the overall dynamics of the Earth's interior, the which is emerging from these efforts while indicating the deep return flow for plate tectonics, and the geochemical remaining major uncertainties. signatures of source regions of igneous rocks. In the period 1987-1990, great strides have been made in identifying and characterizing mantle discontinuities from a seismological standpoint. Advances in detailed The seismological characterization of mantle discon- observation of reflected and converted phases, analysis of tinuities in both the upper and lower mantle has followed multiple ScS reverberations, stacking of global digital several avenues of inquiry. Investigation has focused data, application of seismometer arrays, and broadband upon establishing the presence or absence of seismic data collection have permitted in~proveddetermination of discontinuities, determining whether they are global or the location, magnitude, and sharpness (depth-extent) of local in extent, measuring the topographic variability of rapid seismic velocity changes in the mantle. Notable their depth of occurrence, and quantifying the magnitude among such advances are the global stacking study of and sharpness of their velocity changes. In addition to Shearer [1990] and the SH-pblarized mantle reverberation numerous local and regional studies, notable advances of studies of Revenaugh and Jordan [1987, 1989, 1990a, the last four years include the use of SH-polarized mantle 1990bl which systematically investigate the fine structure reverberations [Revenaugh and Jordan, 1987, 1989, of seismic velocity variations in the mantle. 1990a, 1990bl and the stacking of five years of global Important progress has also been made in understand- long-period data [Shearer, 19901 to better constrain the ing the physical and chemical properties of mantle fine structure of mantle seismic discontinuities. Work discontinuities by better constraining the effects of pres- continues p. Kennett, pers. comm., 19891 on a new sure, temperature, and compositional variations upon the radial Earth model (IASPR) to fit global body-wave crystal structures and elastic moduli of the mantle's con- travel times more closely than the currently-used radial stituent minerals. Advances in multi-anvil press and model PREM [Dziewonski and Anderson, 19811 while diamond-anvil cell high-pressure technology, calorimetric retaining those features of PREM structure which provide study of mineral thermochemical properties, synchrotron a good fit to free oscillation data. Recently reported radiation analysis of crystal structures, and first-principles depths and magnitudes of mantle seismic discontinuities computation of crystal properties have allowed better are summarized in Table 1. constraints upon both the stability fields and thermoelastic Below the crust-mantle boundary, upper mantle seismic properties of mantle mineral assemblages. Notable velocity structure is dominated by a velocity increase at among these advances are the detailed experimental study about 410 hi depth, followed by an anomalously high of post-spinel phase transformations by Ito and Takahashi velocity gradient throughout the region between this 410 [I9891 and the subsequent thermodynamic analysis by krn discontinuity and the base of the upper mantle near Wood L1989, 19901. 660 krn depth. Recent work also indicates the presence Taken together, these advances allow more meaningful of a smaller velocity increase near 520 km depth, as well correlation of seismological features with mineralogical as suggesting that reported velocity discontinuities near changes. While this permits the placing of stronger con- 220 km depth cannot be globally coherent features. straints upon the composition, structure, and dynamics of Lower mantle velocity structure is dominated by a sharp the interior, there remain significant uncertainties and velocity increase at about 660 km depth, marking the ambiguities. Here I review fundamental advances in both boundary between the upper and lower mantle, and by the seismological and mineralogical understanding of the core-mantle boundary (CMB) at 2890 km depth. The km Copyright 1991 by the American Geophysical Union. lowermost 200-300 of the mantle (the D" layer), just above the CMB, is characterized by anomalous velocity Paper number 91RG00805. structures, consistent with the presence of a velocity 8755-1209/91/91RG00805 515.00 discontinuity with strongly laterally varying properties. 784 BMA MANTLE DISCO;UT?NUTIES TABLE 1. Surnrnary of Reported Mantle Seisnlic Discontinuities Depth (km) Study Reference Characteristics 67 S Revenaugh [I9891 5-8% impedance contrast 86 S Revenaugh and Jordan [I9891 obserued, W Pacific 75-225 S Graves and Helmberger [I9881 LVZ, old Pacific upper 100 S Revenaugh [I9891 LVZ onset, oceanic and tectonic, base not detected 165-215 P LeFevre and Helmhrger [I9891 LVZ, Canadian shield 200 P Bowman and Kennett [1990] 3.3% Vp increase, NW Australia -220 P,S Shearer [1990] not detected -220 S Revenaugh and Jordan 119891 not detected, W Pacific 232 S Revenaugh [I9891 4% impedance contrast, NWC Australia 300 P'IY Wajeman [I9881 Eurasia 400 P Bowman and Kennett [1990] 5.6% Vp increase, NW Australia 405 P LeFevre and Helmberger 119891 5% Vp increase, Canadian shield 405 S Graves and Helmberger [I8891 3.6% Vs increase, old Pacific 410 P,S Shearer [l990] topography 40km 414 S Revenaugh and Jordan [1987, 1989, 1990a,b] refl. coef. 0.04610.010, topog. -12 km 415 P'F' Nakanishi [1988,1989] intermittent 520 P,S Shearer [l990] 3% impedance contrast 520 S Revenaugh and Jordan [1990b] reflection coef. 0.014 630 P Bowman and Kennett [I9901 3.9% Vp increase, NW Australia 655 P'IY Nakanishi [1988,1989] sharp 659 S Graves and Helmberger [I8891 6.8% Vs increase, old Pacific 660 P LeFevre and Helrnberger [I9891 4% Vp increase, Canadian shield 660 P,S Shearer [I9901 topography 40km 660 S Revenaugh and Jordan [1987, 1989, 1990a,b] refl. coef. 0.07210.010, topog. -12 km, slab def. <40 km 670 P'P' Davis et al. [I9891 not detected + 10 km topog. 670 P'P' Wajeman [I9881 7.5% Vp contrast, Eurasia 670 P+S Paulssen [I9881 sharp 660-680 S+P Richards and Wicks [1990] slab def. <50 km, 'Tonga 710 S Revenaugh and Jordan [1989, 1990bI refl. coef. -0.020, tentative 840 P,S Shearer [l990] tentative 900 S Revenaugh and Jordan 11989, 1990b] tentative 2546 P Baumgardt [I9891 2.75% Vp increase 2600 P Davis and Weber [1990] 3% Vp increase, N Siberia 2610 S Young and Lay [1987a] 2.75% Vs increase, Iridian Ocean ---2610 S Revenaugh and Jordan [I9891 not detected, W Pacific Studies are of P-waves (P), S-waves (S), both (P,S), underside reflections (P'P'), and converted phases (P+S, S-+P). LVZ denotes low velocity zone. Magnitudes are given as velocity increases, impedance contrasts, or normal-incidence reflection coefficients. Also noted are long-wavelength topography and apparent deformation by subducting slabs. Wile there is a general lack of evidence for additional evidence for such a feature. This suggests that the significant global discontinuities, minor velocity discon- discontinuity, which appears to be required in certain tinuities tentatively have been suggested at intermediate regional studies, cannot be a globally coherent feature. depths in the lower mantle, and seismic reflectors also Revenaugh and Jordan [1989], using SH-polarized mantle have been observed in subducting slabs. An additional reverberations, find no evidence for this discontinuity conclusion of recent studies is that the 410 and 660 km beneath the western Pacific. In modeling short-period discontinuities exhibit only small variations in depth, waveforms from a hybrid array, however, Bowman and comprising perhaps 20-40 km of relief. Kennett [I9901 require the presence of a 3.3% velocity increase near 200 km depth in their upper mantle P-wave Shallow Mantle Discontinuities velocity profile (NWB-1) for northwest Australia. Furth- ermore, Revenaugh L19891 finds evidence for a 4% Using SH-polarized mantle reverberations, Revenaugh impedance contrast at 232 km depth beneath west-central [I9891 reports a reflector with a 54% impedance con- and northern Australia using SH-polarized mantle rever- trast at 67 km depth in the mantle, which he atmbutes to berations. LeFevre and Helmberger's 119891 upper man- the phase transition from spinel lherzolite to garnet Iher- tle P-wave velocity profile (S25) for the Canadian shield, zolite. In addition, Revenaugh and Jordan [I9891 find determined from waveform modeling
Recommended publications
  • Mantle Transition Zone Structure Beneath Northeast Asia from 2-D
    RESEARCH ARTICLE Mantle Transition Zone Structure Beneath Northeast 10.1029/2018JB016642 Asia From 2‐D Triplicated Waveform Modeling: Key Points: • The 2‐D triplicated waveform Implication for a Segmented Stagnant Slab fi ‐ modeling reveals ne scale velocity Yujing Lai1,2 , Ling Chen1,2,3 , Tao Wang4 , and Zhongwen Zhan5 structure of the Pacific stagnant slab • High V /V ratios imply a hydrous p s 1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, and/or carbonated MTZ beneath 2 3 Northeast Asia Beijing, China, College of Earth Sciences, University of Chinese Academy of Sciences, Beijing, China, CAS Center for • A low‐velocity gap is detected within Excellence in Tibetan Plateau Earth Sciences, Beijing, China, 4Institute of Geophysics and Geodynamics, School of Earth the stagnant slab, probably Sciences and Engineering, Nanjing University, Nanjing, China, 5Seismological Laboratory, California Institute of suggesting a deep origin of the Technology, Pasadena, California, USA Changbaishan intraplate volcanism Supporting Information: Abstract The structure of the mantle transition zone (MTZ) in subduction zones is essential for • Supporting Information S1 understanding subduction dynamics in the deep mantle and its surface responses. We constructed the P (Vp) and SH velocity (Vs) structure images of the MTZ beneath Northeast Asia based on two‐dimensional ‐ Correspondence to: (2 D) triplicated waveform modeling. In the upper MTZ, a normal Vp but 2.5% low Vs layer compared with L. Chen and T. Wang, IASP91 are required by the triplication data. In the lower MTZ, our results show a relatively higher‐velocity [email protected]; layer (+2% V and −0.5% V compared to IASP91) with a thickness of ~140 km and length of ~1,200 km [email protected] p s atop the 660‐km discontinuity.
    [Show full text]
  • STRUCTURE of EARTH S-Wave Shadow P-Wave Shadow P-Wave
    STRUCTURE OF EARTH Earthquake Focus P-wave P-wave shadow shadow S-wave shadow P waves = Primary waves = Pressure waves S waves = Secondary waves = Shear waves (Don't penetrate liquids) CRUST < 50-70 km thick MANTLE = 2900 km thick OUTER CORE (Liquid) = 3200 km thick INNER CORE (Solid) = 1300 km radius. STRUCTURE OF EARTH Low Velocity Crust Zone Whole Mantle Convection Lithosphere Upper Mantle Transition Zone Layered Mantle Convection Lower Mantle S-wave P-wave CRUST : Conrad discontinuity = upper / lower crust boundary Mohorovicic discontinuity = base of Continental Crust (35-50 km continents; 6-8 km oceans) MANTLE: Lithosphere = Rigid Mantle < 100 km depth Asthenosphere = Plastic Mantle > 150 km depth Low Velocity Zone = Partially Melted, 100-150 km depth Upper Mantle < 410 km Transition Zone = 400-600 km --> Velocity increases rapidly Lower Mantle = 600 - 2900 km Outer Core (Liquid) 2900-5100 km Inner Core (Solid) 5100-6400 km Center = 6400 km UPPER MANTLE AND MAGMA GENERATION A. Composition of Earth Density of the Bulk Earth (Uncompressed) = 5.45 gm/cm3 Densities of Common Rocks: Granite = 2.55 gm/cm3 Peridotite, Eclogite = 3.2 to 3.4 gm/cm3 Basalt = 2.85 gm/cm3 Density of the CORE (estimated) = 7.2 gm/cm3 Fe-metal = 8.0 gm/cm3, Ni-metal = 8.5 gm/cm3 EARTH must contain a mix of Rock and Metal . Stony meteorites Remains of broken planets Planetary Interior Rock=Stony Meteorites ÒChondritesÓ = Olivine, Pyroxene, Metal (Fe-Ni) Metal = Fe-Ni Meteorites Core density = 7.2 gm/cm3 -- Too Light for Pure Fe-Ni Light elements = O2 (FeO) or S (FeS) B.
    [Show full text]
  • Deep Earth Structure: Lower Mantle and D"
    This article was originally published in Treatise on Geophysics, Second Edition, published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Lay T Deep Earth Structure: Lower Mantle and D″. In: Gerald Schubert (editor-in-chief) Treatise on Geophysics, 2nd edition, Vol 1. Oxford: Elsevier; 2015. p. 683-723. Author's personal copy 00 1.22 Deep Earth Structure: Lower Mantle and D T Lay, University of California Santa Cruz, Santa Cruz, CA, USA ã 2015 Elsevier B.V. All rights reserved. 1.22.1 Lower Mantle and D00 Basic Structural Attributes 684 1.22.1.1 Elastic Parameters, Density, and Thermal Structure 684 1.22.1.2 Mineralogical Structure 685 1.22.2 One-Dimensional Lower Mantle Structure 686 1.22.2.1 Body-Wave Travel Time and Slowness Constraints 687 1.22.2.2 Surface-Wave/Normal-Mode Constraints 688 1.22.2.3 Attenuation Structure 688 1.22.3 Three-Dimensional
    [Show full text]
  • Continental Flood Basalts Derived from the Hydrous Mantle Transition Zone
    ARTICLE Received 4 Sep 2014 | Accepted 1 Jun 2015 | Published 14 Jul 2015 DOI: 10.1038/ncomms8700 Continental flood basalts derived from the hydrous mantle transition zone Xuan-Ce Wang1, Simon A. Wilde1, Qiu-Li Li2 & Ya-Nan Yang2 It has previously been postulated that the Earth’s hydrous mantle transition zone may play a key role in intraplate magmatism, but no confirmatory evidence has been reported. Here we demonstrate that hydrothermally altered subducted oceanic crust was involved in generating the late Cenozoic Chifeng continental flood basalts of East Asia. This study combines oxygen isotopes with conventional geochemistry to provide evidence for an origin in the hydrous mantle transition zone. These observations lead us to propose an alternative thermochemical model, whereby slab-triggered wet upwelling produces large volumes of melt that may rise from the hydrous mantle transition zone. This model explains the lack of pre-magmatic lithospheric extension or a hotspot track and also the arc-like signatures observed in some large-scale intracontinental magmas. Deep-Earth water cycling, linked to cold subduction, slab stagnation, wet mantle upwelling and assembly/breakup of supercontinents, can potentially account for the chemical diversity of many continental flood basalts. 1 ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia. 2 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O.Box9825, Beijing 100029, China. Correspondence and requests for materials should be addressed to X.-C.W.
    [Show full text]
  • Scientific Research of the Sco Countries: Synergy and Integration 上合组织国家的科学研究:协同和一体化
    SCIENTIFIC RESEARCH OF THE SCO COUNTRIES: SYNERGY AND INTEGRATION 上合组织国家的科学研究:协同和一体化 Materials of the Date: International Conference November 19 Beijing, China 2019 上合组织国家的科学研究:协同和一体化 国际会议 参与者的英文报告 International Conference “Scientific research of the SCO countries: synergy and integration” Part 1: Participants’ reports in English 2019年11月19日。中国北京 November 19, 2019. Beijing, PRC Materials of the International Conference “Scientific research of the SCO countries: synergy and integration”. Part 1 - Reports in English (November 19, 2019. Beijing, PRC) ISBN 978-5-905695-74-2 这些会议文集结合了会议的材料 - 研究论文和科学工作 者的论文报告。 它考察了职业化人格的技术和社会学问题。 一些文章涉及人格职业化研究问题的理论和方法论方法和原 则。 作者对所引用的出版物,事实,数字,引用,统计数据,专 有名称和其他信息的准确性负责 These Conference Proceedings combine materials of the conference – research papers and thesis reports of scientific workers. They examines tecnical and sociological issues of research issues. Some articles deal with theoretical and methodological approaches and principles of research questions of personality professionalization. Authors are responsible for the accuracy of cited publications, facts, figures, quotations, statistics, proper names and other information. ISBN 978-5-905695-74-2 © Scientific publishing house Infinity, 2019 © Group of authors, 2019 CONTENTS ECONOMICS 可再生能源发展的现状与前景 Current state and prospects of renewable energy sources development Linnik Vladimir Yurievich, Linnik Yuri Nikolaevich...........................................12 JURISPRUDENCE 法律领域的未命名合同 Unnamed contracts in the legal field Askarov Nosirjon Ibragimovich...........................................................................19
    [Show full text]
  • Structure of the Earth
    TheThe Earth’sEarth’s StructureStructure fromfrom TravelTravel TimesTimes SphericallySpherically symmetricsymmetric structure:structure: PREMPREM --CCrustalrustal StructuStructurree --UUpperpper MantleMantle structustructurree PhasePhase transitiotransitionnss AnisotropyAnisotropy --LLowerower MantleMantle StructureStructure D”D” --SStructuretructure ofof thethe OuterOuter andand InnerInner CoreCore 3-3-DD StStructureructure ofof thethe MantleMantle fromfrom SeismicSeismic TomoTomoggrraphyaphy --UUpperpper mantlemantle -M-Miidd mmaannttllee -L-Loowweerr MMaannttllee Seismology and the Earth’s Deep Interior The Earth’s Structure SphericallySpherically SymmetricSymmetric StructureStructure ParametersParameters wwhhichich cancan bebe determineddetermined forfor aa referencereferencemodelmodel -P-P--wwaavvee v veeloloccitityy -S-S--wwaavvee v veeloloccitityy -D-Deennssitityy -A-Atttteennuuaattioionn ( (QQ)) --AAnisonisotropictropic parame parametersters -Bulk modulus K -Bulk modulus Kss --rrigidityigidity µ µ −−prepresssuresure - -ggravityravity Seismology and the Earth’s Deep Interior The Earth’s Structure PREM:PREM: velocitiesvelocities andand densitydensity PREMPREM:: PPreliminaryreliminary RReferenceeference EEartharth MMooddelel (Dziewonski(Dziewonski andand Anderson,Anderson, 1981)1981) Seismology and the Earth’s Deep Interior The Earth’s Structure PREM:PREM: AttenuationAttenuation PREMPREM:: PPreliminaryreliminary RReferenceeference EEartharth MMooddelel (Dziewonski(Dziewonski andand Anderson,Anderson, 1981)1981) Seismology and the
    [Show full text]
  • INTERIOR of the EARTH / an El/EMEI^TARY Xdescrrpntion
    N \ N I 1i/ / ' /' \ \ 1/ / / s v N N I ' / ' f , / X GEOLOGICAL SURVEY CIRCULAR 532 / N X \ i INTERIOR OF THE EARTH / AN El/EMEI^TARY xDESCRrPNTION The Interior of the Earth An Elementary Description By Eugene C. Robertson GEOLOGICAL SURVEY CIRCULAR 532 Washington 1966 United States Department of the Interior CECIL D. ANDRUS, Secretary Geological Survey H. William Menard, Director First printing 1966 Second printing 1967 Third printing 1969 Fourth printing 1970 Fifth printing 1972 Sixth printing 1976 Seventh printing 1980 Free on application to Branch of Distribution, U.S. Geological Survey 1200 South Eads Street, Arlington, VA 22202 CONTENTS Page Abstract ......................................................... 1 Introduction ..................................................... 1 Surface observations .............................................. 1 Openings underground in various rocks .......................... 2 Diamond pipes and salt domes .................................. 3 The crust ............................................... f ........ 4 Earthquakes and the earth's crust ............................... 4 Oceanic and continental crust .................................. 5 The mantle ...................................................... 7 The core ......................................................... 8 Earth and moon .................................................. 9 Questions and answers ............................................. 9 Suggested reading ................................................ 10 ILLUSTRATIONS
    [Show full text]
  • The Upper Mantle and Transition Zone
    The Upper Mantle and Transition Zone Daniel J. Frost* DOI: 10.2113/GSELEMENTS.4.3.171 he upper mantle is the source of almost all magmas. It contains major body wave velocity structure, such as PREM (preliminary reference transitions in rheological and thermal behaviour that control the character Earth model) (e.g. Dziewonski and Tof plate tectonics and the style of mantle dynamics. Essential parameters Anderson 1981). in any model to describe these phenomena are the mantle’s compositional The transition zone, between 410 and thermal structure. Most samples of the mantle come from the lithosphere. and 660 km, is an excellent region Although the composition of the underlying asthenospheric mantle can be to perform such a comparison estimated, this is made difficult by the fact that this part of the mantle partially because it is free of the complex thermal and chemical structure melts and differentiates before samples ever reach the surface. The composition imparted on the shallow mantle by and conditions in the mantle at depths significantly below the lithosphere must the lithosphere and melting be interpreted from geophysical observations combined with experimental processes. It contains a number of seismic discontinuities—sharp jumps data on mineral and rock properties. Fortunately, the transition zone, which in seismic velocity, that are gener- extends from approximately 410 to 660 km, has a number of characteristic ally accepted to arise from mineral globally observed seismic properties that should ultimately place essential phase transformations (Agee 1998). These discontinuities have certain constraints on the compositional and thermal state of the mantle. features that correlate directly with characteristics of the mineral trans- KEYWORDS: seismic discontinuity, phase transformation, pyrolite, wadsleyite, ringwoodite formations, such as the proportions of the transforming minerals and the temperature at the discontinu- INTRODUCTION ity.
    [Show full text]
  • Physiography of the Seafloor Hypsometric Curve for Earth’S Solid Surface
    OCN 201 Physiography of the Seafloor Hypsometric Curve for Earth’s solid surface Note histogram Hypsometric curve of Earth shows two modes. Hypsometric curve of Venus shows only one! Why? Ocean Depth vs. Height of the Land Why do we have dry land? • Solid surface of Earth is Hypsometric curve dominated by two levels: – Land with a mean elevation of +840 m = 0.5 mi. (29% of Earth surface area). – Ocean floor with mean depth of -3800 m = 2.4 mi. (71% of Earth surface area). If Earth were smooth, depth of oceans would be 2450 m = 1.5 mi. over the entire globe! Origin of Continents and Oceans • Crust is formed by differentiation from mantle. • A small fraction of mantle melts. • Melt has a different composition from mantle. • Melt rises to form crust, of two types: 1) Oceanic 2) Continental Two Types of Crust on Earth • Oceanic Crust – About 6 km thick – Density is 2.9 g/cm3 – Bulk composition: basalt (Hawaiian islands are made of basalt.) • Continental Crust – About 35 km thick – Density is 2.7 g/cm3 – Bulk composition: andesite Concept of Isostasy: I If I drop a several blocks of wood into a bucket of water, which block will float higher? A. A thick block made of dense wood (koa or oak) B. A thin block made of light wood (balsa or pine) C. A thick block made of light wood (balsa or pine) D. A thin block made of dense wood (koa or oak) Concept of Isostasy: II • Derived from Greek: – Iso equal – Stasia standing • Density and thickness of a body determine how high it will float (and how deep it will sink).
    [Show full text]
  • Effect of Subduction Zones on the Structure of the Small-Scale Currents at Core-Mantle Boundary
    Effect of Subduction Zones on the Structure of the Small-Scale Currents at Core-Mantle Boundary. Sergey Ivanov1, Irina Demina1, Sergey Merkuryev1,2 1St-Petersburg Filial of Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio wave Propagation (SPbF IZMIRAN) 2Saint Petersburg State University, Institute of Earth Sciences Universitetskaya nab., 7-9, St. Petersburg, 199034, Russia [email protected] Abstract. The purpose of this work is to compare kinematics of small-scale current vortices located near the core-mantle boundary with high-speed anomalies of seismic wave velocity in the lowest mantle asso- ciated with the subduction zones. The small-scale vortex paths were early obtained by the authors in the frame of the macro model of the main geomagnetic field sources. Two sources were chosen whose kine- matics are characterized by the complete absence of the western drift and whose paths have a very com- plex shape. Both sources are located in the vicinity of the subduction zones characterized by the extensive coherent regions with increased speed of seismic waves in the lowest mantle. One of them is geograph- ically located near the western coast of Canada and the second one is located in the vicinity of Sumatra. For this study we used the global models of the heterogeneities of seismic wave velocity. It was obtained that the complex trajectories of the vortices is fully consistent with the high-speed anomalies of seismic wave velocity in the lowest mantle. It can be assumed that mixing up with the matter of the lowest man- tle, the substance of the liquid core rises along the lowest mantle channel and promotes its further in- crease.
    [Show full text]
  • Label and Describe the Earth Diagram
    Name_________________________class______ Label and Describe the Earth Diagram Read the definitions then use the information to color code, label and describe IN YOUR OWN WORDS each section of the diagram below. Definitions: crust – (green) the rigid, rocky outer surface of the Earth, composed mostly of basalt and granite. The crust is the thinnest of all layers. It is thicker on continents & thinner under the oceans. inner core – (gray) the solid iron-nickel center of the Earth that is very hot and under great pressure. mantle – (orange) a rocky layer located under the crust - it is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. Convection (heat) currents carry heat from the hot inner mantle to the cooler outer mantle. outer core – (red) the molten iron-nickel layer that surrounds the inner core. ______________________________________________ ______________________________________________ ______________________________________________ _______________________________________ _______________________________________ _______________________________________ ___________________________________ ___________________________________ ___________________________________ ___ __________________________________ __________________________________ __________________________________ __________________________ Name_________________________________cLass________ Label the OUTER LAYERS of the Earth This is a cross section of only the upper layers of the Earth’s surface. Read the definitions below and use the information to locate label and describe IN TWO WORDS the outer layers of the Earth. One has been done for you. continental crust – thick, top Continental Crust – (green) the thick parts of the Earth's crust, not located under the ocean; makes up the comtinents. Oceanic Crust – (brown) thinner more dense parts of the Earth's crust located under the oceans. Ocean – (blue) large bodies of water sitting atop oceanic crust. Lithosphere– (outline in black) made of BOTH the crust plus the rigid upper part of the upper mantle.
    [Show full text]
  • Geochemistry
    GEOCHEMISTRY “Structure & Composition of the Earth” (M.Sc. Sem IV) Shekhar Assistant Professor Department of Geology Patna Science College Patna University E-mail: [email protected] Mob: +91-7004784271 Structure of the Earth • Earth structure and it’s composition is the essential component of Geochemistry. • Seismology is the main tool for the determination of the Earth’s interior. • Interpretation of the property is based on the behaviour of two body waves travelling within the interior. Structure of the Earth Variation in seismic body-wave paths, which in turn represents the variation in properties of the earth’s interior. Structure of the Earth Based on seismic data, Earth is broadly divided into- i. The Crust ii. The Mantle iii. The Core *Figure courtesy Geologycafe.com Density, Pressure & Temperature variation with Depth *From Bullen, An introduction to the theory of seismology. Courtesy of Cambridge Cambridge University Press *Figure courtesy Brian Mason: Principle of Geochemistry The crust • The crust is the outermost layer of the earth. • It consist 0.5-1.0 per cent of the earth’s volume and less than 1 per cent of Earth’s mass. • The average density is about “2.7 g/cm3” (average density of the earth is 5.51 g/cm³). • The crust is differentiated into- i) Oceanic crust ii) Continental crust The Oceanic crust • Covers approx. ~ 70% of the Earth’s surface area. • Average thickness ~ 6km (~4km at MOR : ~10km at volcanic plateau) • Mostly mafic in nature. • Relatively younger in age. The Oceanic crust Seismic study shows layered structural arrangement. - Seawater - Sediments - Basaltic layer - Gabbroic layer *Figure courtesy W.M.White: Geochemistry The Continental crust • Heterogeneous in nature.
    [Show full text]