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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

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.

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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 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 ?

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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