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Formation and Structure of
the Earth
Standards
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Recognize that radiometric data indicate that
Earth is at least 4 billion years old and that Earth
has changed during that period.
Describe the internal structure of Earth (e.g.,
core, mantle, crust) and the structure of Earth’s
plates.
Describe how waves are used for practical
purposes (e.g., seismic data).
Know that Earth’s systems are driven by internal
(i.e., radioactive decay and gravitational energy)
and external (i.e., the sun) sources of energy.
Formation of the Universe
Our universe began approximately 13.7
Ga (billion years ago) with a cosmic
“explosion” called the Big Bang.
 Before the Big Bang, all matter and energy
were compacted into a single,
inconceivably dense point called a
singularity.
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Formation of the Universe
We don’t know what happened during the
Big Bang or for the first fraction of a
second after it.
 But, during the billions of years that
followed, the universe has expanded and
gas, dust, stars and galaxies have formed.
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The Big Bang
Photo: http://oz.plymouth.edu/~sci_ed/Turski/Courses/Earth_Science/Intro.html
Formation of the Solar System
Earth formed along with the rest of the
solar system (at the same time).
 Our solar system formed from the collapse
of an interstellar cloud (condensation
theory).
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Condensation Theory
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1.
2.
Our model for the formation of the solar
system.
A large cloud of interstellar (between
stars) gas began to collapse under the
influence of its own gravity.
As it contracted, it became denser and
hotter, with most of its matter forming the
sun at its center.
Condensation Theory
3. The rest of the matter formed into a
flattened disk around the sun, hotter in
the inner region where more of the
matter accumulated, than in the less
dense outer regions.
4. Gravitational attraction caused dust and
cooling gas in the disk to collide and
accrete (clump together) as small chunks
called planetismals.
Condensation Theory
5.The planetismals continued to collide and
accrete to form the planets and their
moons.
6. Some planetismals did not clump
together, and now form asteroids and
meteorites.
Condensation Theory of
Solar System Formation
Photo: http://oz.plymouth.edu/~
sci_ed/Turski/Courses/Earth_
Science/Intro.html
Condensation Theory
This all happened 4.6 Ga (billion years
ago).
 But, how do we know when this occurred?
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Dating the Solar System
Meteorites (the original planetismals)
sometimes fall to Earth.
 We can get an age for the meteorites by
radioactive dating (more on this later).
 The meteorites, which are left over pieces
from the formation of the solar system,
have an average age of
4.6 Ga.
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Photo: http://www.nizwa.net/env/meteorites/meteorites.html
Formation of the Earth
Early earth was rocky and uniform in
composition and density throughout.
 Then it began to heat up and melt.
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Photo: http://www.escepticospr.com/
images/early_earth.jpg
3 Sources of Heat
1.
2.
3.
Gravitational collapse – as earth
contracted, it heated up.
Radioactive decay – thermal energy
released by the decay of radioactive
elements.
Surface bombardment by meteorites –
the early solar system contained many
more meteorites than today.
Differentiation Occurs
As Earth heated it became partially
molten.
 30 – 65% of the Earth formed an outer
magma ocean hundreds of kilometers
thick.
 The interior of the Earth became soft.
 The heaviest elements sank to the middle
and the lightest elements floated on top.
 This caused the formation of distinct layers
in the Earth, called differentiation.
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Compositional Layers of the Earth
Layers defined by
what they are made
of
 The innermost layer
of the Earth is the
core (inner & outer)
 The next layer is the
mantle
 The top layer is the
crust
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Core
Made of iron and nickel
 Inner core:
 Solid
 Even though its hot, there is too much
pressure for it to be liquid
 1255 km thick
 Outer core:
 Liquid
 Rotates around the inner core (this gives
us our magnetic field)
 2220 km thick
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Mantle
Region surrounding the core
 Makes up the bulk of the Earth
 Made of peridotite (rock containing iron &
magnesium)
 Is a solid that flows
 2900 km thick
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Crust
Thin outer layer of the Earth
 Solid
 Two types:
1. Continental crust

made of granite
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20 – 70 km thick
2. Oceanic crust
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made of basalt
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thinner than continental crust

7 – 10 km thick
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Mechanical Layers of Earth
Layers defined by how they behave
 Lithosphere
 Asthenosphere
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Lithosphere
Layer that includes the crust and
uppermost mantle.
 Lithospheric Plates - The lithosphere is
broken up into plates that move over the
partially molten mantle (plate tectonics)
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Asthenosphere
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Part of mantle directly below the
lithosphere, on which the plates move.
How do We Know
the Earth’s Structure?
Drilling has only been done to a few
kilometers depth in the crust.
 How then do we know that the Earth is
layered?
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Seismic Waves
Vibrations that travel through the Earth.
 Produced by earthquakes and surface
explosions.
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Seismic Waves
An earthquake generates waves that spread
out in all directions, like light from a light bulb.
 Seismograph stations detect all of the waves
that arrive at that location.
 We can learn about the deep interior of Earth
by:
 Recognizing what kind of waves have arrived
 Knowing exactly when they arrived
 Calculating when and where the earthquake
occurred
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Seismic Waves
Seismic waves refract (bend) and reflect
when they pass from one material into
another material.
 They refract and reflect at the boundaries
in the Earth because the layers are made
of different materials.
 They can also change in acceleration
when passing from one material to
another.
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Seismic Waves
Mohorovičić Discontinuity
 Moho for short
 Boundary between the crust and mantle
 Where seismic waves accelerate as they
pass from the lower density crust into the
higher density mantle
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Types of Seismic Waves
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2.
There are two main types of seismic
waves:
Surface waves
Body waves
Surface Waves
Travel on the Earth’s surface.
 Can’t be used to determine structure.
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Body Waves
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1.
2.
Travel through Earth, so can be used to
determine structure.
Two types of body waves:
Primary or P waves
Secondary or S waves
Primary or P waves
First to arrive at
seismograph station
 Compressional waves
– particle motion is in
same direction as wave
travel
 Can travel through both
solids and liquids
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Secondary or S Waves
Arrive after P waves
 Shear waves – particle
motion is perpendicular
to wave motion.
 Can travel through
solids but not liquids.
 They do not travel
through the outer core,
therefore we know the
outer core is liquid.
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S-Wave Shadow Zone
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Zone in which no S waves are recorded by
seismic stations (because of the outer
liquid core).
Continents
Continental growth: began soon after
differentiation & has continued through
geologic time.
 The less dense materials on Earth’s
surface accrete (clump together) to form
the continents.
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Continents
Continents are made up of large regions of
stable, ancient crystalline rocks (igneous
and metamorphic rocks), called shields.
 The shields are surrounded or buried by
sedimentary rocks called platforms.
 Shields and platforms make up the stable
(no longer undergoing deformation) parts
of the continents, called cratons.
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Earth’s Atmosphere Today
Breathable by humans
 Mixture of gases:
- 78% nitrogen
- 21% oxygen
- Trace amounts of argon, carbon dioxide &
water vapor
 Large amount of oxygen makes Earth’s
atmosphere unique in solar system.
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Origin of Earth’s Atmosphere
Atmosphere of early Earth made up of gases
most common in solar system (hydrogen,
helium, methane, ammonia & water vapor)
 Escaped into space
 Secondary atmosphere was outgassed
(expelled) from planet’s interior by volcanoes
 Surface temperature fell and water vapor
condensed, forming oceans
 Life appeared in oceans and began to produce
oxygen
 Oxygen in present-day atmosphere is direct
consequence of evolution of life on Earth.
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Earth Systems
The Earth system is composed of all the
parts of our planet that work together.
 These are the geosphere, atmosphere,
hydrosphere, biosphere and cryosphere
 It is an open system that exchanges
energy and mass with its surroundings.
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Earth Systems
Earth systems are driven by both external
and internal sources of energy.
 The external source of energy is our sun.
 The sun’s energy drives climate and
weather, and energizes the atmosphere,
hydrosphere, cryosphere and biosphere.
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Earth Systems
The internal sources of energy are
gravitational (trapped heat from Earth’s
formation) and heat from the decay of
radioactive elements.
 Internal energy drives plate tectonics and
the geodynamo (interaction between inner
and outer core) and energizes the
lithosphere, asthenosphere, deep mantle,
and inner & outer core.
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Our Changing Earth
Earth is dynamic - it is always changing
 It continues to change even as we speak
 Change is natural and unstoppable
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