Download The Structure of the Earth and Plate Tectonics

The Structure of the Earth
and Plate Tectonics
Structure of the Earth
• The Earth is
made up of 3
main layers:
– Core
– Mantle
– Crust
Mantle
Outer core
Inner core
Crust
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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 rock
- thin (~7 km)
- dense (sinks under
continental crust)
- young rock
How do we know what the
Earth is made of?
• G
Geophysical
h i l surveys: seismic,
i i gravity,
i
magnetics,
i
electrical, geodesy
– Acquisition: land, air, sea and satellite
– Geological surveys: fieldwork, boreholes, mines
2
What is Plate Tectonics?
When you look at a map of the world, you may notice that some of
the continents could fit together like pieces of a puzzle.
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Plate Tectonics
• The Earth’s crust is divided into 12 major
j
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”
tectonic features.
features
• The word, tectonic, refers to the deformation
(or changes) of the crust as a result of plate
interaction.
World Plates
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What are tectonic plates made of?
• Plates are
made of rigid
lithosphere.
The lithosphere
p
is
made up of the
crust and part of
the upper mantle.
What lies beneath the tectonic
plates?
Below the
lithosphere is
the plasticlike
(neither solid
nor liquid)
asthenosphere.
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Plate Movement
“Plates” of lithosphere
p
are moved around by
y
the underlying hot mantle convection cells.
What happens at tectonic
plate boundaries?
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Three types of plate boundary
1. Divergent
2. Convergent
3. Transform
Divergent Boundaries
• Spreading ridges (ex: Mid-Atlantic Ridge)
– As plates move apart new material (basaltic
magma, or lava) is erupted to fill the gap
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Divergent Boundary
The Mid-Atlantic Ridge
(longest underwater mountain range)
Age of Basaltic Oceanic Crust
Courtesy of www.ngdc.noaa.gov
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Polarity of Basaltic Oceanic Crust
The Earth
Earth’ss
magnetic reversals
provide more
evidence for
seafloor spreading.
The age
g and p
polarity
y
of the basaltic ocean
floor is mirrored on
both sides of the
spreading ridge.
Courtesy of www.ngdc.noaa.gov
Iceland: An example of continental rifting
• Iceland has a divergent
plate
l
boundary
d
running
through its middle
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East African Rift: continental plate rifting
Geologist
G
l i t have
h
already given
names to the
resulting new
plates: Nubian
and Somalian
Plates
Convergent Boundaries
• There are three types of convergent
plate boundaries
1. Continent-continent collision
2. Continent-oceanic crust collision
3 Ocean-ocean
3.
Oc n c n collision
c llisi n
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Continent-Continent Collision
Forms folded mountains like the European Alps, and the
Himalayas
• Usually, no subduction occurs at this type
of convergent plate boundary.
Himalayas: Mt. Everest
“The roof of the world”
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Continent-Oceanic Crust Collision
Ex. Juan de Fuca plate subducting under North American
plate (Mt.
(Mt St.
St Helens).
Helens) Nazca plate subducting under the
South American plate (Andes Mts.).
Subduction
• Oceanic lithosphere
subducts underneath the
continental lithosphere
• Oceanic lithosphere heats
and dehydrates as it
subsides
• The melt rises forming
volcanism
• E.g. The Andes Mountains,
Mt. Saint Helens
12
Ocean-Ocean Plate Collision
The Japanese
Islands were
formed by
subduction of
the Pacific Plate
under the
E
Eurasian
Plate.
l
Mt. Fuji, Japan
Ocean-Ocean Plate Collision
• Wh
When two
t
oceanic
i plates
l t collide,
llid one runs
over the other which causes it to sink into
the mantle forming a subduction zone.
• The subducting plate is bent downward. This
forms the deepest
p
areas in the ocean floor
called trenches.
– Ex. The Mariana Trench is 11 km deep!
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Transform Boundaries
• Where p
plates slide past
p
each other
Above: View of the San Andreas
transform fault, California, USA.
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Volcanoes and Plate
Tectonics…
…what’s the connection?
15
Pacific Ring of Fire
Volcanism is
mostly
focused at
plate
boundaries
Volcanoes are formed by:
- Subduction - Rifting
g - Hotspots
p
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Pacific Ring of Fire
Hotspot
volcanoes
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
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The tectonic plate moves over a fixed hotspot
forming a chain of volcanoes.
The volcanoes get older away from the hotspot.
Earthquakes
q
and Plate
Tectonics…
…what’s the connection?
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• As with volcanoes, earthquakes are not
randomly distributed over the globe
Figure showing
the distribution
of
f earthquakes
th
k
around the globe
• At the boundaries between plates,
friction causes them to stick together.
When built up energy causes them to
break, earthquakes occur.
Where do earthquakes form?
Figure showing the tectonic setting of earthquakes
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Plate Tectonics Summary
• The Earth is made up of 3 main layers
(core, mantle, crust)
• On the surface of the Earth are tectonic
plates that slowly move around the globe
• Plates are made of crust and upper mantle
(lithosphere)
• There are 2 types of plate
• There are 3 types of plate boundaries
• Volcanoes and Earthquakes are closely
linked to the margins of the tectonic plates
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