EESA06H3 Chapter Notes - Chapter 4: Seismic Refraction, Continental Crust, Seismic Wave

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24 Apr 2012
Chapter Four - Page 1 of 8
Chapter Four: The Earth’s Interior
The only rocks that can be studied are those of the Earth’s crust
Mantle rocks brought to the earth’s surface in basalt flow, in diamond bearing kimberlite pipes,
and also by the tectonic attachment of lower parts of the oceanic lithosphere to the continental
Meteorites give clues about the possible composition of the core of the earth
Geophysics evidence suggest that the earth is divided into 3 major layers: crust on the surface,
rocky mantle beneath the crust, and the metallic core at the centre
The crust and uppermost mantle can be divided into the brittle lithosphere and the plastic
Kola peninsula - The deepest scientific well has reached 12km beneath the surface (sedimentary
basins) in Russia. It penetrated ancient Precambrian basement rocks
Earth has a radius of 6.370km
Deep parts of the earth are studied indirectly through geophysics the application of physical
laws and principles to a study of the earth; includes the study of seismic waevs and the earth’s
magnetic field, gravity and heat
Deep Drilling on Continents
Structure and composition of most of the continental crust is unknown
Continents are probably largely igneous and metamorphic rock (such as granite and gneiss,
overlain by a veneer of sedimentary rocks
Second deepest well drilled is the KTB hole in southeastern Germany, which reached 10km
Important way of learning about earth’s interior is via study of seismic reflection
Seismic reflection: the return of some of the energy of seismic waves to the earth’s surface after
the waves bounce off a rock boundary
If two rock layers of differing densities are separated by a fairly sharp boundary, seismic waves
reflect off that boundary just as light reflects off a mirror
These reflected waves are recorded on a seismogram shows the amount of time the waves
took to travel down to the boundary, reflect off it, and return to the surface allows
calculation of depth of the boundary
Canadian lithoprobe project applying seismic reflection techniques to map crustal structures
at the base of the crust
Another method used to locate rock boundaries seismic refraction the bending of seismic
waves as they pass from one material to another. As a seismic waves strikes a rock boundary,
much of the energy of the wave passes across the boundary. As the wave crosses from one rock
layer to another, it changes direction. This change of direction (refraction) only occurs if the
seismic waves velocity is different in each layer (happens if the rock layers differ in
Seismograph station 1 is receiving seismic waves that pass directly through the upper layer A
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Chapter Four - Page 2 of 8
Stations farther from the epicentre, such as station 2, receive seismic waves from 2 pathways:
1. A direct path through layer A
2. A refracted path through layer A to a higher-velocity layer B and back to layer A. Station
2 receives the same wave twice
Seismograph stations close to station 1 receive only the direct wave or possibly two waves, the
direct (upper) wave arriving before the refracted (lower) wave
Stations near station 2 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 B
The distance between this point of transformation and the epicentre of the earthquake is a
function of the depth to the rock boundary between A and B
Sharp rock boundary isnt necessary for the refraction of seismic waves
Canadian Lithoprobe Project
Investigating the composition and structure of the Canadian shield and surrounding organic
Aim is to develop a comprehensive understanding of the geological evolution of north America
The shield is made up of distinct geological terranes that were once separate land masses but
were brought together by the forces of plate tectonics
Will help answer how continental configuration came to be/what tectonics were involved
Will also help evaluate earthquake risk across the shield and find oil/gas reservoirs
Uses seismic reflection
Uses large vibroseis trucks (dancing elephants) work together to stamp in unison
Three main zones of the earth’s interior: crush, mantle and core
Crust: outer layer of rock, which forms a thin skin on earth’s surface
Below the crust lies the mantle a thick shell of rock that separates the crust above from the
core below
The core is the central zone of earth. It is probably metallic and the source of earth’s magnetic
The Crust
Studies of seismic waves have shown:
o The crust is thinner beneath the oceans than beneath the continents
o Seismic waves travel faster in oceanic crust than in continental crust
It’s assumed that the two types of crust are made up of different rocks
Seismic P waves travel through oceanic crust (and basalt and gabbro) at 7km/second
Upper part of the oceanic crust is basalt; lower part is gabbro
Oceanic crust thickness = 7km
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Seismic P waves travel slower through continental crust (and granite and gneiss) 6km/second
Continental crust is also called granitic
Continental crust consists of a crystalline basement composed of granite, other plutonic rocks,
gneisses, and schists, all capped by a layer of sedimentary rocks
Felsic rocks high in feldspar and silicon continental crust
Mafic rocks high in magnesium and iron (ferric) oceanic crust
Continental crust is thicker than oceanic crust 30 to 50 km
Crust is thickest under young mountain ranges andes and Himalayas bulging downward as a
mountain root into the mantle
Continental crust is less dense than oceanic crust
Mohorovicic discontinuity the boundary that separates the crust from the mantle beneath
Mantle lies closer to the earth’s surface beneath the ocean than the continents
Project Mohole use special ships to drill through the oceanic crust and obtain mantle samples
Oceanic Crust basalt underlain by gabbro; 7km
Continental Crust granite, other plutonic rocks, schist, gneiss (with sedimentary rock cover);
The Mantle
Assumed to be mostly made of solid rock
P waves travel at 8km/s in the upper mantle suggests a different rock from oceanic or
continental crust
Best guess about composition of the upper mantle ultramafic rock such as peridotite
o Ultramafic rock is dense igneous rock made up of mostly ferromagnesian minerals such
as olivine and pyroxene
o Some contain garnet; all lack feldspar
The crust and uppermost mantle form the lithosphere
o Outer shell of the earth that is strong and brittle
o Makes up the plate of plate tectonics theory
The lithosphere averages about 70km thick under oceans and 125-250km thick beneath
Seismic waves generally increase in speed with depth as increasing pressure alters the rock
But beginning at 70-125km, seismic waves travel slower than they do at shallow layers low
velocity zone
This zone extends to 200km, and is called the asthenospehere
The rocks here are closer to their melting point (but not hotter) than those above/below it
Melting point controlled by pressure as well as temperature
This zone is important for two reasons:
o It may represent a zone where magma is likely to be generated
o The rocks here may have relatively little strength and therefore are likely to flow
Plates of brittle lithosphere move easier over the asthenosphere120
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