Textbook Notes (363,065)
EESA06H3 (234)
Nick Eyles (205)
Chapter 4

# Chapter 4 Textbook Notes.odt

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School
University of Toronto Scarborough
Department
Environmental Science
Course
EESA06H3
Professor
Nick Eyles
Semester
Winter

Description
Chapter 4 – Earth's Interior ➢ Overview ➢ How gravity measurements can indicate where certain regions of the crust & upper mantle are being held up or down out of their natural position of equilibrium ➢ Magnetic field & reversals ➢ Magnetic anomalies = hidden ore? ➢ Earth's heat ➢ Geoscientists are not able to sample rocks very far below the surface ➢ Rock samples from mines & wells are useful BUT  Mines = 3 km into the Earth  Oil wells = 8 km beneath  Deepest well = 12 km beneath ➢ Directly look at rock  Basalt flows  Intrusion & erosion of diamond-bearing kimberlite pipes ➢ Indirect study of Earth  Earth radius = 6.370 km  So, deep parts of earth are mostly studied inderectly via geophysics  Geophysics:  Abranch of geology  The application of physical laws & principles to the study of the Earth.  Includes study of seismic waves, magnetic field, gravity & heat ➢ What Can We Learn From the Study of Seismic Waves?  Info can be obtained from large earthquakes & nuclear bomb explosion (both generate seismic waves)  2 ways to study Earth's Interior  Seismic Reflection • The return of some of the energy of seismic waves to Earth's surface after waves bounce off rock boundary • Mirror analogy • Reflected waves are recorded on a seismogram = shows amount of time the waves took to travel down to the boundary, reflect off of it, and return to the surface • Calculate depth of boundary based on time of round trip.  Seismic Refraction • The bending of seismic waves as they pass from 1 material to another • Lens analogy • Wave crosses form one rock layer to another and then changes direction = refraction = occurs only is velocity of seismic waves is diff in each layer  Bondaries btwn rock layers are usually distinct enough to be located by seismic refraction techniques  (116) Stations close to epicentre (Station 1) receives seismic waves that pass directly through upper layer  Farther stations (Station 2) receive seismic waves from 2 pathways – receive both the direct and the refracted waves  Even though the refracted wave travels farther, it can arrive at a station first because most of its path is in the high-velocity layer (116)  Asharp rock boundary is not necessary for the refraction of seismic waves  In uniform rock = the waves follow curved path  What is Inside the Earth?  Study of seismic refraction & seismic reflection enabled scientists to plot the 3 mian zones of the Earth's interior • 1) Crust: outer layer of rock (thin skin on Earth's surface) • 2) Mantle: thick shell of rock that separates the crust above from core below • 3) Core: central zone of earth – probably metallice, and the source of Earth's magnetic field.  The Crust  Crust is thinner beneath the oceans than beneath the continents  Seismic waves travel faster in oceanic crusts than in continental crust  Velocity different – therefore two types of crust may be made up of different kinds of rocks  PWaves travel through oceanic crust = 7km/s  Upper part of oceanic crust is basalt and lower part is gabbro  Oceanic crust is 7 km thick  P waves travel more slowly through continental crust = 6 km/s – same as granite  Continental crust known as “granitic” - (pg 117)  So continental crust is called Felsic (feldspar & silicon)  Oceanic crust is called Mafic (magnesium & iron)  Continental crust is much thicker than oceanic crust (30-50 km thick)  Crust is thickest under geologically young mountain ranges –Andes, Himalayas  Even though continental crust is thicker it is actually less dense than oceanic crust  Moho – Mohorovicic Discontinuity: • The boundary that separates the crust from the mantle  Mantle lies closer to the Earth's surface beneath the ocean than it does beneath continents  Project Mohole:Attempt to drill through the mantle.  The Mantle  Made of solid rock  Magma chambers located in crust & upper mantle  But otherwise mantle is pretty solid  Mantle is a diff type of rock than oceanic/continental crust  Most likely ultramafic rock such as peridotite in upper mantle  Ultramafic rock • Dense igneous rock made up of ferromagnesian minerals  Crust & uppermost mantle = lithosphere  Lithosphere = outer shell of Earth that is relatively strong & brittle.  LITHOSPHERE MAKES UPTHE PLATES OF PLATE TECTONICS THEORY  Lithosphere Oceans: 70 km thick, Litho Continents: 125-250 km thick  Usually the deeper they go the more faster waves travel except there's a low velocity zone – where waves travel slower than usual  Low velocity zone = located in asthenosphere  Asthenosphere • Rocks in this zone are closer to their melting point • This zone might be where magma is generated • Very weak rocks that are likely to flow • Acts as a lubricating layer below lithosphere • Exists UNDER OCEANIC CRUST  Debate about mantle composition  The Core  Aregion btwn 103 degrees and 142 degrees which lacks PWaves  P-wave shadow zone  Size and shape of core can be determined by the calculations of P-wave paths  P-waves are missing within the shadow zone because they have been bent (refracted) by the core  S-wave shadow zone • S waves can only travel through solid • Shadow zone is larger than the P-wave shadow zone • Shadow zone indicates that S-wave does not travel through core at all – could be liquid  P-wave refraction suggests... • CO
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