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Intro to Earth Dynamics
Lecture-3
1
Topics for Intro to
Earth Dynamics
 The
gross radial structure of the Earth
(chemical and mechanical boundaries)
 Heat
sources and heat transfer
 Convection
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Radial Earth Structure
 The
structure and properties of the Earth
vary most significantly with the radius of
the Earth (equivalently, with depth)
 For
example, a rock sample from St. Louis
is much more similar to a rock taken from
2,500 km to the west (California) than to
one taken from 2,500 km below St. Louis.
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Radial Earth Structure

So the simplest model of the Earth is one in which
all properties vary only with depth

Names for this kind of Earth model include
– radial
– 1D, or one-dimensional
– spherically symmetric
– average
– homogeneous
– layered
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Laterally-Varying Earth Structure
 Obviously,
more complicated (and realistic)
models of the Earth include lateral
variations in structure. These types of
models are called:
– 3D, or three-dimensional
– laterally varying
– heterogeneous
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The Three Major Chemical
Radial Divisions



Crust
Mantle
Core
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The Shallowest Layer
of the Earth: the Crust
 The
crust is the most heterogeneous layer in
the Earth; it is the most poorly described by
a radial model
 The
crust is on average 33 km thick for
continents and 10 km thick beneath oceans;
however it varies from 1-70 km globally.
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The Shallowest Layer
of the Earth: the Crust
 The
boundary between the crust and the
mantle is mostly chemical. The crust and
mantle have different compositions.
 This
boundary is referred to as the “Moho”
 It
was discovered in 1909 by the Croatian
seismologist, Andrya Mohorovicic.
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Middle Earth: The Mantle
 Earth’s
mantle exists from the crust to a
depth of 2891 km (radius of 3480 km)
 It is further subdivided into:
 The
uppermost mantle (crust to 400 km depth)
 The transition zone (400 – 700 km depth)
 The mid-mantle (700 to about 2650 km depth)
 The lowermost mantle, aka D” (2650 – 2891 km
depth)
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Middle Earth: The Mantle
 The
uppermost mantle is composed
dominantly of olivine; lesser components
include pyroxene, enstatite, and garnet
 We
have samples of the upper mantle that
have been shoved to the Earth’s surface by
tectonic processes (xenoliths, kimberlites)
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Earth’s Core
 Owing
to the great pressure inside the Earth
the Earth’s core is actually freezing as the
Earth gradually cools
 The
boundary between the liquid outer core
and the solid inner core occurs at a radius of
about 1220 km.
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Earth’s Outer Core
 The
radius of Earth’s outer core is known to
within 3-4 km (3480 km)
 The
boundary between the mantle and outer
core is sharp
 There
is a dramatic change in chemistry and
phase at the core-mantle boundary (CMB)
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Earth’s Outer Core

The change is density across the CMB is greater
than that at the Earth’s surface!

The outer core is mostly an alloy of iron and
nickel in liquid form.

The viscosity of the outer core is similar to that of
water, it flows kilometers per year and creates the
Earth’s magnetic field.

The outer core is the most homogeneous part of
the Earth
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Earth’s Inner Core
 The
inner core is more purely iron than the
outer core; as the core freezes impurities are
increasingly concentrated in the liquid part.
 As
the core freezes latent heat is released;
this heat causes the outer core to convect
and so generates a magnetic field.
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Earth’s Inner Core
 There
is no major boundary deeper than the
inner core – outer core boundary (ICB)
 Some
speculate that there is a subtle
transition about 200 km below the ICB, but
this is controversial
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Mechanical Layering
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Mechanical
Layers:
1. Lithosphere
2. Asthenosphere
3. Mesosphere
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Mechanical Layers - Lithosphere

The lithosphere is the uppermost 50-100 km of the
Earth.

There is not a strict boundary between the
lithosphere and the asthenosphere as there is
between the crust and mantle

It consists of both crust and mantle

It behaves rigidly, like a solid, over very long time
periods.
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Mechanical Layers Asthenosphere
 The
asthenosphere exists between depths of
100-200 km
 It is the weakest part of the mantle.
 It is a solid over short time scales, but
behaves like a fluid over millions of years
 The asthenosphere decouples the
lithosphere (tectonic plates) from the rest of
the mantle.
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Mechanical Layers: Mesosphere
 The
bulk of the lower mantle is termed the
mesosphere and is stronger than the
asthenosphere
 However, it does behave like a fluid over
long time scales (convects)
 It is not clear if the whole mantle convects
as one layer or two
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Earth Structure Summary
 The
most dramatic variations in Earth
structure occur radially (not laterally)
 The
Earth can be divided radially according
to chemical composition (crust, mantle,
core) or mechanical behavior (lithosphere,
asthenosphere, mesosphere, outer core,
inner core)
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Intro to Earth Dynamics
 So
why are the mantle and core moving
around all the time ?
 The
fundamental answer is heat. The Earth
is trying to cool itself.
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Heat Sources
 Why
–
–
–
–
–
is the Earth hot ?
accretionary heat
radioactive decay
adiabatic compression
tidal influences of moon and Sun (minor)
latent heat from phase transitions (minor)
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Heat Transfer
(1)The
Earth is cooling over time because it is
much warmer than the surrounding vacuum
of space.
(2)Heat generally “moves” from hotter objects
or regions to cooler objects or regions.
(3)The “movement” of heat is generally called
heat transfer.
Lecture-3 24
Heat Transfer
 Heat
transfer generally occurs in one of four
modes:
–
–
–
–
conduction
convection
advection
radiation
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Radiative Heat Transfer
 Materials
can cool themselves by emitting
electromagnetic radiation (light waves)
 If
you stand in the sun and become warmer
you are being radiatively heated by the sun
as it tries to cool itself.
 The
frequency of the radiation emitted by a
body is related to its temperature.
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Conduction
 Conduction
is the most familiar mode of
heat transfer. In this mode heat is
transferred by the diffusion of atomic
vibrations.
 Materials which readily conduct heat are
called conductors. Examples ?
 Materials which are not conductive are
called insulators. Examples ?
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Conduction
 So
in general metals are good conductors
and rocks are poor conductors of heat.
 Thus,
it takes a long time for heat to
conduct through the Earth’s mantle (billions
and billions of years).
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Convection
 Convection
is a mode of heat transfer in
which material (mass) undergoes large scale
motion.
 The
classic example is a pot of boiling
water.
 Advection
is closely related to convection.
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Convection
(1)Sometimes
conduction is more efficient
than convection; sometimes it is the
opposite.
(2)Which mechanism is more efficient depends
on material properties.
(3)The mode of heat transfer that is more
efficient will dominate.
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Convection
 It
turns out that in the Earth’s mantle
convection is more efficient than
conduction.
 Hence, the Earth’s mantle is moving just as
a pot of boiling water does.
 But how can this happen? Isn’t the Earth’s
mantle a solid ?
Lecture-3 31
Summary

Earth’s radial structure can be divided chemically
into three main regions: crust, mantle, core.

The Earth can also be divided mechanically into
the lithosphere, asthenosphere, and mesosphere.

The Earth is furiously trying to cool itself and the
most efficient way for this to happen is by
convection.
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Summary

Both the liquid, metallic core and the solid rocky
mantle are convecting (but separately)

Mantle convection gives rise to plate tectonics
(e.g. continental drift)

Convection in the core gives rise to Earth’s
magnetic field
Lecture-3 33