EESA06H3 Chapter Notes - Chapter 4: Paleomagnetism, Viscosity, Project Mohole

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26 Jan 2013
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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:
A branch 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
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is in the high-velocity layer (116)
A sharp 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
P Waves 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 UP THE 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
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