Earth: From Core to Crust

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Earth From Core to Crust

• Chapter 2

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Reading the Geologic Record

• The difference between geologist and historians
  is that historians think of history over a period
  on thousands of years and geologists over
  billions of years.
• The time frames that geologists talk about are
  referred to as either absolute or relative.
• Relative time refers to an event that happens
  before or after another event
• Absolute time the exact time

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Three Basic Principles to Read the Geologic Record

• Geologic Record uses relative time to date the
  features of the earth.
• James Hutton came up with three principles that
  lead to the creating of the Geologic Record.
• Three principles help Geologists figure out what
  happened and when
• Superposition
• Uniformitarianism
• Fossil Correlation

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1. Superposition

• New sediments are deposited over old ones.
• Therefore the lower the layer of undisturbed rock
  the older it is.

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2. Uniformitarianism

• This principle is based on the idea that natural
  processes occur in the same way today as they did
  in the past
• For example a river would flow the same way and
  cause the same type of erosion and deposition
• This is important because when geologists study
  ancient layer of earth and their appearance they
  can make assumptions based on processes
  occurring on earth today.

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3. Fossil Correlation

• Fossils are organic plant and animal matter that
  has decomposed and been removed and replaced by
  minerals in solution
• This principle is based on the idea that
  different rock deposits that contain the same
  fossil types must be the same age.
• By looking at the fossils in different layers of
  rock geologists can determine when certain animal
  appeared and disappeared.
• Approximate climates can also be determined.

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3. Fossil Correlation
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Layers Beneath our Feet

• As the earth formed the heavier material settled
  in the center and the lighter material on top.
• We cannot drill holes to the center of the earth
  because of the distance and heat so we rely on
  seismology to gain information on the layers.

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Seismology

• A seismograph records the earths vibrations when
  there is an earthquake.
• As the ground shakes different waves are
  generated.
• These waves act differently depending on the
  material that it passes through

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Types of Waves

• There are two types of waves that pass through
  the earth.
• Primary Waves (P waves) squeeze and stretch the
  material they pass through. They pass through any
  type of material and move very quickly.
• Secondary Waves (S waves) vibrate at right
  angles to P waves and only travel through solids

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Types of Waves
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Gutenberg Discontinuity

• An interesting thing happens at about 2,900 Km
  below the surface.
• The P waves slow right down and the S waves stop
  completely
• From readings that Beno Gutenberg took (1914) he
  figured out that this is the depth at which the
  liquid core begins.

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Layers of the Earth
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The Core

• Makes up a third of the total
  mass of the planet
• Made up of two parts
  the inner and the outer
• Inner Core
• Made mostly of iron
• Temperatures range from 4000 C to 6650 C (hotter
  than the sun)
• It is solid because of the immense pressure
• Outer Core
• Not as much pressure so this layer is liquid
• Generates our magnetic field
• Made up mostly of iron
• This layer begins about 5,200 Km below earths
  surface

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The Mantel

• Takes up 80 of earths total volume
• Made up of iron, silicone and magnesium
• Divided into two parts Lower and Upper
• Lower Mantel
• Starts at 2,900 km below surface and ends about
  500 km below
• Moving towards the surface the temperature
  gradually cools.

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The Mantel
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Upper Mantel Asthenosphere

• This layer of the earth is responsible for many
  of the processes that cause the crust to look the
  way it does.
• Hot spots in this layer create convection
  currents that move the earths tectonic plates
• When volcanoes erupt the magma that forms this
  layer of the earth can flow out on to the surface
  as lava.

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Moho Discontinuity

• Discovered by Andrija Mohorovicic
• This is a depth where there is a sharp increase
  in the speed of waves
• This discontinuity form the boundary between the
  asthenosphere and the crust

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Crust

• Very thin covering on the surface of the earth
• Makes up only 0.1 of earths total volume
• Made up of two parts
• sima - beneath the ocean, made up of basaltic
  rock (iron, magnesium and silica)
• sial makes up the continents, contains many
  more minerals (silica, potassium, aluminum to
  name a few), its thicker than the sima

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Sima
Sial