Earth's Interior A

8/6/2019 Earth's Interior A

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Earths Interior An In-Depth Look into our Home Planet

Mt. Etna, Italy


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Earths Interior

What is one of the first things you notice about adiagram of the Earths interior?


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Distinct Layering

The heaviest materials (metals) appear in the center. The lighter solids (rocks) occupy the middle.

The less dense fluids and gases are found at the top.

Iron-nickel core, rocky mantle and crust, the liquidocean, and the gaseous atmosphere.


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How did we get all of those layers?

Lets say we have a plastic bottle containing thefollowing materials:

Pebbles

Sand

Mud

Water

Air


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What happens when you fill the bottle with all ofthe materials and shake it?

What happens when you stop shaking it?


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Application to the Earth

We can apply the previous experiment to the Earth. The young Earth was molten and composed of a

uniform mixture of iron, nickel, silicates, water, andgases.

As the Earth cooled, similar elements began to grouptogether.

This led to the formation of layers in the Earth.

What is this called?

Differentiation


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Differentiation

What causes layers to form within a planet? The force of gravity is responsible for the Earths

layers.

What can we infer from this piece of information?

Is this true for the Earth?

Iron-nickel core, rocky mantle and crust, the liquid

ocean, and the gaseous atmosphere.


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Seeing Earths Layers

The best way to see inside the Earth would be todig or drill a hole to examine it.

Unfortunately, this is only possible at shallow depths.

The deepest drill hole ever to penetrate the Earth

reached a depth of only 7.5 miles. Thats only 1/500th of the way to the Earths center.

Yet this was an extraordinary feat considering therapid increase in temperature and pressure withdepth.


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Seismic Waves

Fortunately for seismologists, many earthquakes arelarge enough that their seismic waves travel all theway through Earth.

This means they can be detected on the other side of

the Earth. Seismic waves allow us to see into the Earth.

It is like flipping the switch on an X-ray machine.


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However, unlike an X-ray in which the picture isclearly shown, seismic waves are much morecomplex.

Seismic waves usually do not travel along in straightpaths.

Instead, they are reflected, refracted, and diffractedas they pass through the different layers of the Earth.


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Seismic Wave Behaviors

Reflection reflection off of the boundariesbetween layers.

Refraction occurs when passing from one layerinto another.

Much like light bending as it passes from air to water.

Diffraction a seismic wave will diffract aroundobstacles they encounter.


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Reflection

Change in composition cause seismic waves to reflectoff boundaries between layers.

This helped us identify the different layers.

This characteristic of waves is especially important in

the exploration for oil and natural gas.Artificially generated seismic waves are used to find

reservoirs.

Without seismic imaging, a huge number of wells would

have to be randomly drilled to find oil.


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Seismic Wave Behavior

Seismic waves travel at different speeds. Remember P, S, and surface waves?

The speed at which seismic waves travel throughthe Earths surface depends largely on the

properties of the materials encountered. Waves travel faster in stiff (rigid) rock.

This is just one of the pieces of information used to

identify temperature and composition of layers.


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Seismic Wave Behavior

One of the most noticeable behaviors of seismicwaves is that they follow strongly curved paths.

This is because their velocities generally increasewith depth.

Due to pressure increases which squeeze the rock intoa more compact, rigid material.


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Three snapshots showing

the locations of S waveswithin Earths mantlefollowing an earthquake.


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Earths Layers

With the advancement in seismic technology,seismologists have made important discoveriesabout the compositions of Earths layers. Crust

Mantle

Core


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Crust

Earths crust consists of two distinct types: Continental crust

Oceanic crust

Continental and oceanic crusts have very different

compositions, histories, and ages.


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Oceanic Crust

Oceanic crust is compositionally more similar to themantle than continental crust.

Averages about 4.5 miles thick.

Forms at mid-ocean ridges.

P waves travel through oceanic crust at a consistentrate (~5 7 km/hr). This tells us the composition of oceanic crust is quite

uniform.


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Continental Crust

While oceanic crust is fairly uniform, no twocontinental regions have the same structures orcomposition.

Averages 25 miles in thickness.

Can be up to 45 miles thick in mountainous regionssuch as the Andes and Himalayas.

Seismic velocities within continents are quitevariable.

Tells us that the composition also varies greatly.


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The Moho

The Moho is the boundary between the crust andthe mantle.

It was one of the first features of the Earths interiordiscovered using seismic waves.

Named after Andrija Mohorovii, a Croatian, whodiscovered the boundary in 1909.

P wave velocities slightly increase at this boundary.


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Upper Mantle

Extends from the Moho to a depth of 410 miles. The upper mantle itself can be subdivided into 3

shells:

Lithosphere the uppermost mantle and crust

Asthenosphere weak layer beneath lithosphere Transition Zone lower portion of the upper mantle


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Upper Mantle

Rocks brought to the surface by volcanism andother geological processes have provided geologistswith information about the upper mantle.

The velocities at which seismic waves travel through

the upper mantle are similar to those of the rockperidotite.

Mantle peridotite, an ultramafic rock composed ofolivine and pyroxenes, is richer in iron and magnesium

than rocks found in the crust.


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Transition Zone

The transition zone lies at the lowest portions of theupper mantle.

It is called a transition zone because seismic wavesare reflected off of this boundary.

Just as they are at the Moho. Composed of the mineral spinel.

It is calculated that the Transition Zone contains upto 2% of its weight in water.

This means it could potentially hold up to 5X thevolume of Earths oceans.


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Lower Mantle

The lower mantle lies between the transition zoneand the liquid core.

The lower mantle is undoubtedly the Earths largestlayer.

Composes 56% of the volume of our planet. The lower mantle is composed of the mineral

perovskite.

This makes perovskite the single most abundant

material within Earth.


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The D Layer

In the bottom few hundred miles of the mantle, ahighly variable and unusual layer occurs.

Pronounced Dee Double Prime Layer

It is a boundary layer between the rocky mantle

and the liquid iron outer core. The D layer is thought to be a graveyard for

some of the subducted slabs, and the birthplacefor mantle plumes.


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The D Layer

At the very base of the D layer, where the mantleis directly in contact with the hot liquid iron core,there are upside-down mountains that protrudeinto the core.

Also, some of the regions of the D layer may notbe hot enough to be partially molten.


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Discovering the Core-Mantle Boundary

Evidence that Earth has a distinct central core wasuncovered in 1906 by Richard Dixon Oldham.

While studying seismic waves, Oldham observed thatP and S waves were weak or absent at distances

~100 from the epicenter of a large earthquake. In other words, Oldham found evidence for a central

core that produced a shadow zone for seismicwaves.


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As Oldham predicted, Earths core exhibits differentproperties from the mantle above.

This causes considerable refraction of P waves.

Similar to how light is refracted as it passes from air towater.

In addition, because theouter core is liquid iron, itblocks the transmission ofS waves.


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Earths Core

The core accounts for about 1/6 of Earths volume.Yet it accounts for 1/3rd of Earths mass.

Iron is the most dense of the common elements.

When measured by mass, iron is the most abundant

element found within the Earth.


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Inner Core

At the center of the core is a solid sphere of ironwith trace amounts of other elements.

Because the inner core is an actual sphere, andnot a shell like the other layers, drawings make

the inner core appear much larger than it really is. It is actually quite small only 1/142nd the volume

of Earth less than 1%!


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History of the Inner Core

The inner core did not exist early in Earths historywhen our planet was hotter.

As Earth cooled, iron began to crystallize at thecenter to form the solid inner core.

Even today the inner core continues to grow as theplanet cools.


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Inner Core

The inner core is separated from the other solidlayers by the liquid outer core.

This allows the solid inner core to move freely.

Recent studies suggest that the inner core is

actually rotating faster than the crust and themantle.

Lapping them every few hundred years.


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Discovering the Inner/Outer Core Boundary

The boundary between the inner and outer corewas discovered in 1936 by Inge Lehman.

She was unable to determine whether or not theinner core was actually solid from seismic studies.

She simply applied trigonometry to assert that someP-waves were being strongly refracted by suddenvelocity increases.

The ability of a P wave to emerge within a shadowzone confirmed the existence of a solid inner core.


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Earths Temperature


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Earth s Temperature


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Earths Temperature

One way to describe the interior of a planet is toexamine how temperature changes with depth.

Thermodynamics states that themal energy flowsfrom hotter regions toward colder regions.

Earth is about 10,000F at its center and ~32F at itssurface, so heat flows towards the surface.


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Cooling Rate of Earth

Heat does not leave Earths surface at the same ratein all locations.

Heat flow is highest near mid-ocean ridges where hotmagma is consistently rising toward the surface.

Heat flow is the lowest in the deep abyssal plains, whichare areas of old, cold oceanic seafloor.


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How did Earth get so hot?

Like all of the planets in our solar system, Earth hasexperienced 2 thermal stages:

1. Occurred during Earths formation and lasted about50 million years.

During this time, Earth experienced a rapid increase ininternal temperature.

2. We are currently in the 2nd stage cooling. This is a very slow process that has been in action for 4.5

billion years.


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Stage 1: Rapid Increase in Temp

Several factors contributed to the early increase intemperature.

Collisions with countless planetesimals (baby planets) With each collision, kinetic energy was converted into

thermal energy. Short-lived radioactive isotopes decaying to more stable

forms release radiogenic heatas by-products.

Collision of a Mars-sized object that led to the

formation of the Moon.


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If Earths only source of heat was from its earlyformation and the decay of short-lived radioactiveisotopes, our planet would have cooled to a frozencinder long ago.

Then why are we still hot?

The mantle and the crust also contain long-livedradioactive isotopes that keep our planet cooking asif on a slow burner.


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Half-lives of long-lived radioactive isotopes are

billions of years long. Radioactivity plays a vital role as the source of

radiogenic heat keeping mantle convection and platetectonics in motion for billions of years.


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Heat Flow

Heat travels from Earths interior out to space via3 different mechanisms: Radiation

Convection

Conduction Only two of these processes operate within the

Earths interior. Which two?


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Convection Convection is the transfer of heat by moving material

in a fluid-like manner in which hot materials displacethose that are cooler (and vice-versa).

Convection is the primary means of heat transfer

within the Earth.


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Convection Cycles

You are familiar with convection if you have everboiled a pot of water.

The water appears to be rolling rising up in themiddle of the pot, then down the sides.

This pattern is called a convection cycle. Convection cycles occur within the Earths mantle and

outer core, and possiblywithin the inner core as well.


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Convection Convection occurs due to several factors:

Thermal expansion

Gravity induced buoyancy

Fluidity

When water at the bottom of the pot is heated, itexpands and rises (more buoyant), replaces thecooler, denser (less buoyant) water at the top.

Gravity is the driving force for convection.


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Convection in Outer Space?

Do you think it exists? If you tried to boil water in outer space, with no

strong gravity present, you would find that the potof water would not convect.

Besides the fact that the water probably would notstay in the pot.

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Earth's Interior A