Intro to Earth Dynamics

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Intro to Earth Dynamics
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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

• Earths 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 Earths surface by tectonic
  processes (xenoliths, kimberlites)

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Earths Core

• Owing to the great pressure inside the Earth the
  Earths 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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Earths Outer Core

• The radius of Earths 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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Earths Outer Core

• The change is density across the CMB is greater
  than that at the Earths 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 Earths magnetic field.
• The outer core is the most homogeneous part of
  the Earth

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Earths 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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Earths 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

• Lithosphere
• Asthenosphere
• 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

• The Earth is cooling over time because it is much
  warmer than the surrounding vacuum of space.
• Heat generally moves from hotter objects or
  regions to cooler objects or regions.
• The movement of heat is generally called heat
  transfer.

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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 Earths 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

• Sometimes conduction is more efficient than
  convection sometimes it is the opposite.
• Which mechanism is more efficient depends on
  material properties.
• The mode of heat transfer that is more efficient
  will dominate.

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Convection

• It turns out that in the Earths mantle
  convection is more efficient than conduction.
• Hence, the Earths mantle is moving just as a pot
  of boiling water does.
• But how can this happen? Isnt the Earths mantle
  a solid ?

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Summary

• Earths 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 Earths
  magnetic field