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The Structure of the Earth and Plate
Tectonics
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The Crust
•This is where we live!
•The Earth’s crust is
made of:
Continental Crust
Oceanic Crust
- thick (10-70km)
- buoyant (less dense
than oceanic crust)
- mostly old
- thin (~7 km)
- dense (sinks under
continental crust)
- young
Plate Tectonics
• The Earth’s crust is divided into 12 major plates
which are moved in various directions.
• This plate motion causes them to collide, pull apart,
or scrape against each other.
• Each type of interaction causes a characteristic set
of Earth structures or “tectonic” features.
• The word, tectonic, refers to the deformation of the
crust as a consequence of plate interaction.
Alfred L. Wegener
•Geologist Alfred Wegener
noticed similar rocks & fossil
remains were found on
continents which seemed to
fit together
•He called this “super”
continent called Pangaea
•In 1912 Wegener
published the
first version &
died defending
his theory
PANGAEA - the large landmass that included all
of Earth’s present day continents
Support for Continental Drift
1. SHAPE OF THE CONTINENTS
Continents fit together like puzzle pieces
Support for Continental Drift
2. FOSSIL EVIDENCE
SAME FOSSILS: DIFFERENT CONTINENTS
Mesosaurus was incapable of swimming across a large
ocean.
Support for Continental Drift
3. ROCK EVIDENCE – Mountain ranges
 SAME ROCK:
DIFFERENT RANGE
Existing mountain ranges
separated by vast
oceans contain rocks of
identical mineral content.
 A prime example are the
Appalachian Mountains
in the eastern U.S and
the Caledonian
Mountains in the British
Isles.
Support for Continental Drift
4. GLACIAL SCARS
SAME SCARS:
DIFFERENT CONTINENTS
Support for Continental Drift
5. LOCATION OF COAL DEPOSITS
Coal deposits have been found in temperate and polar
regions; however, coal is formed in tropical regions.
Plate Movement
• “Plates” of lithosphere are moved around by the
underlying hot mantle convection cells
Three types of plate boundary
• Divergent
• Convergent
• Transform
Iceland: An example of continental rifting
• Iceland has a divergent plate
boundary running through its
middle
Convergent Boundaries
•There are three styles of
convergent plate boundaries
•Continent-continent collision
•Continent-oceanic crust
collision
•Ocean-ocean collision
Continent-Continent Collision
• Forms mountains, e.g. European Alps, Himalayas
Continent-Oceanic Crust Collision
•Called SUBDUCTION
Ocean-Ocean Plate Collision
•When two oceanic plates collide, one runs
over the other which causes it to sink into
the mantle forming a subduction zone.
•The subducting plate is bent downward to
form a very deep depression in the ocean
floor called a trench.
•The worlds deepest parts of the ocean are
found along trenches.
•E.g. The Mariana Trench is 11 km deep!
Transform Boundaries
•Where plates slide past each other
Above: View of the San Andreas
transform fault
Pacific Ring of Fire
Volcanism
is mostly
focused at
plate
margins
What are Hotspot Volcanoes?
•Hot mantle plumes breaching the
surface in the middle of a tectonic plate
The Hawaiian island chain are
examples of hotspot volcanoes.
Photo: Tom Pfeiffer / www.volcanodiscovery.com
Where do earthquakes form?
Figure showing the tectonic setting of earthquakes
What is an earthquake?
•They are caused by the release of energy
•They cause the ground to shake
•Reasons:
•Plates Moving
•Faults (cracks in Earth’s surface)
•Caverns collapsing
•Meteor Impacts
•Volcanic Eruptions
The San Andreas is a
•Strike Slip
(Transform) Fault
•One piece of Earth
slides past another
Terms used to describe an earthquake
Focus: area beneath
Earth’s surface where
rock that was under
stress begins to move
or break.
Epicenter: Point on
surface directly above
the focus.
Focus vs. Epicenter
How are earthquakes detected?
•Earthquakes are
detected by
seismographs
•Seismographs
record the
information on a
seismogram
How is the strength of the earthquake determined?
•Intensity
•The effect of the Earthquake on the area
•Measured by the Mercalli Scale (shaking)
•Magnitude
•The actual energy released by the Earthquake
•Measured by the Richter Scale (I-XII)

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