plates

Topics: Plate tectonics, Earth, Lithosphere Pages: 7 (667 words) Published: January 26, 2015
The Structure of the Earth and
Plate Tectonics

Structure of the Earth
Mantle

• The Earth is
made up of 3
main layers:

Outer core
Inner core

– Core
– Mantle
– Crust
Crust

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

How do we know what the
Earth is made of?
• Geophysical surveys: seismic, gravity, magnetics,
electrical, geodesy
– Acquisition: land, air, sea and satellite
– Geological surveys: fieldwork, boreholes, mines

What is Plate Tectonics?



If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.

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.

World Plates

What are tectonic plates made of?
• Plates are
made of rigid
lithosphere.
The lithosphere is
made up of the
crust and the upper
part of the mantle.

What lies beneath the tectonic plates?
• Below the
lithosphere
(which makes
up the tectonic
plates) is the
asthenosphere.

Plate Movement
• “Plates” of lithosphere are moved around by
the underlying hot mantle convection cells

Practical Exercise 1
Supercontinents!

What happens at tectonic
plate boundaries?

Three types of plate boundary
• Divergent

• Convergent

• Transform

Divergent Boundaries

• Spreading ridges
– As plates move apart new material is erupted to
fill the gap

Age of Oceanic Crust

Courtesy of www.ngdc.noaa.gov

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

Himalayas

Continent-Oceanic Crust Collision
• Called SUBDUCTION

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

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

Practical Exercise 2
Where will the UK be in:
1,000 years?
1,000,000 years?
1,000,000,000 years?

Volcanoes and Plate
Tectonics…
…what’s the connection?

Pacific Ring of Fire

Volcanism is
mostly
focused at
plate
margins

Volcanoes are formed by:
- Subduction - Rifting - Hotspots

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

The tectonic plate moves over a fixed hotspot
forming a chain of volcanoes.

The volcanoes get younger from one end to the other.

Earthquakes and Plate
Tectonics…
…what’s the connection?

• As with volcanoes, earthquakes are not
randomly distributed over...
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