EESA06H3 Chapter Notes - Chapter 4: Peridotite, Ultramafic Rock, Himalayas

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12 Aug 2016
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Chapter 4 – The Earth’s Interior
Only rocks geoscientists can study directly in place are Earth’s crust
oEarth’s crust is a think skin of rock, making less than 1% of the earth’s total volume
Mantle rocks brought to Earth’s surface in basalt flows
Diamond-bearing kimberlite pipes and by tectonic attachment of lower parts of the oceanic
lithosphere to the continental crust give geoscientist a glimpse of what underlying mantle might look
like
Evidence of geophysics suggest that the Earth is divided into three major layers
oCrust on the surface
o Rocky mantle beneath the crust
oMetallic core at the centre
The crust and the uppermost mantle can be conveniently divided into the brittle lithosphere and the
plastic asthenosphere
Earth has a radius of about 6 370 km
Geophysics- The application of physical laws and principles to a study of the Earth
o Includes the study of seismic waves and Earth’s magnetic field, gravity and heat
Deep Drilling on Continents
Surface mapping and seismic reflection and refraction suggest continents are largely igneous and
metamorphic rock (granite and gneiss), overlain by a veneer of sedimentary rocks
Sedimentary cover is generally thin, but may thicken to 10km or more in giant sedimentary basins,
where underlying “basement rock” has subsided
Igneous/ metamorphic basement averages 40km thick, making up most of the continental crust (rarely
sampled deeper than 2 or 3km)
oUplift and erosion have exposed some rocks widely thought to have been formed much
deeper in the crust
What Can We Learn from the Study of Seismic Waves?
Seismic waves from a large earthquake may past through the entire Earth
Seismic Reflection – The return of some of the energy of seismic waves of the Earth’s surface after
the waves bounce off a rock boundary
oIf two rock layers of differing densities are separated by a fairly sharp boundary, seismic
waves reflect off the boundary just as light reflects off a mirror
Reflected waves are recorded on a seismogram, showing the amount of time the waves took to travel
down to the boundary, reflect off it and return to the surface
Seismic Refraction – The bending of seismic waves as they pass from one material to another
oSimilar to the way that light waves bend when they pass through the lenses of eyeglasses
oSeismic wave strikes a rock boundary, much of he energy of the wave passes across the
boundary
Wave crosses from one rock layer to another, changing directions
Change of direction/refraction occurs if the velocity of seismic waves is different in
each layer
Canadian Lithoprobe Project
Groundbreaking Canadian scientific effort investigating the composition/structure of the Canadian
Shield and surrounding organic belts since 1984
American continent Shield is formed of distinct geological terranes that were once separate land
masses but brought together by the forces of plate tectonics
Seismic Reflection – Sending waves into the ground and recording them when they bounce back up
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oLithoprobe Project has used a series of large vibroseis trucks to generate sufficiently large
vibrations
oVibroseis trucks (45) termed “dancing elephants” – work together to stamp in unison along a
road bed or road shoulder
Energy waves created pass through Earth & reflect/refract when they encounter a
boundary between rocks with different physical properties
Geophones (cup sized motion sensors) are laid out on the ground to detect the reflected sound waves
Since 1984, Lithoprobe project has collected more than 10 000 km worth of seismic reflection data
from the Canadian Shield, that are now being used to create multidimensional maps of the Earths
crust
What is inside the Earth?
Three main zones of the Earth’s Interior
oCrust: Outer layer of rock, forms a thin skin on Earth’s surface
oMantle: A thick shell of rock that separates the crust above from the core below
oCore: the central part of earth. Probably metallic nad the source of Earth’s magnetic field
Crust
Studies of seismic waves have shown
o1) the crust is thinner beneath the ocean than beneath the continents
o2) Seimsmic waves travel faster in ocean crust than in continental crust (Difference in
velocity; assumption of two types of crust made up of different kinds of rocks)
Seismic P waves travel through the oceanic crust at about 7km/second
oOceanic crust averages 7km in thickness varying 5-8km
Seismic waves travel more slowly through continental crust (6km/ s) same speed as travelling through
granite and gneiss
oContinental crust often called “granite” – usually called felsic by geoscientists
oFelsic- rocks high in feldspare and silicon – for contentinal crust and mafic- rocks high in
magnesium and iron (ferric) for oceanic crust
Continental crust is much thicker than oceanic crust
Seismic waves show crust is thickest under geologically young mountain ranges (andres, Himalayas)
bulging downwards as a mountain root into the mantle
Continental crust is less dense than oceanic crust
Mohorovicic discontinuity- boundary that separates the crust from the mantle beneath it (moho)
Mantle lies closer to the Earth’s surface beneath the ocean than it does beneath continent
Ambitious project: Project Mohole (Early 1990s) was to use equipped ships to drill through the
oceanic crust and obtain samples from the mantle
The mantle
Geoscientists interret it to be made for solid rock based on the way seismic waves pass through
Magma chambers of melted rock may occur as isolated pockets of liquid in both the crust and the
upper mantle, however, most of the mantle seemed to be solid
P waves travel at about 8km/s in the upper mantle, thhe mantle is a different type of rock from either
oceanic crust or continental crust
oHypothesis: consists of ultramafic rock (peridotite)
oUltramafic rock is a dense igneous rock made up chiefly of ferromagneian minerals
Crust and uppermost mantle together form the lithosphere (outer shell of earth that is relatively
strong and brittle)
Seismic waves increase in velocity with depth as increasing pressure alters the properties of the rock
o70-125km = seimic waves travel more slowly than they do in shallower layers, zone is
referred as low-velocity zone
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oZone extending to a depth 200km is called the asthenosphere
Rocks in the asthenosphere may be close to their melting points b/c
o1) it may represent a zone where magma is likely to be generated
o2) rocks here may have relatively little strength and therefore are likely to flow
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