Download Plate Tectonics - Scoil Mhuire Geography

Plate Tectonics
Objectives
• Describe and draw the internal workings
of the earth
• Explain the structure of the earth’s crust
• Describe and explain the tectonic cycle
• Explain the theory of plate tectonics
• Understand the formation of landforms
(features) associated with plate margins.
The Earth Keywords
• Crust
• Mantle
• Lithosphere
• Asthenosphere
• Outer core
• Inner core
KEYWORDS STRUCTURE OF THE
EARTH
• Crust – outer part of the earth. Very thin
• Mantle – middle part of the earth semi
molten rock. Moves plates by convection
currents.
• Lithosphere- marks change from crust to
mantle (very thin)
• Asthenosphere – piece of the mantle
nearest the crust.
• Outer core – iron and nickel (liquid)
• Inner core – iron and nickel (solid)
Structure of the Earth
Internal Structure of Earth
• Crust – solid rock, aka lithosphere, 3-10km thick
•
•
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under ocean, 70km thick under continents
Mantle – semi-molten rock (magma), aka
asthenosphere, convection currents cause plate
movements
Outer core – molten rock mainly consisting of
iron
Inner core – up to 5000°C, solid nickel & ironl.
The Earth’ s Crust
• What do you know about the earth’s crust
from Junior Cert??
The Earth’s Crust Keywords
• Oceanic Crust
• Continental Crust
• Plates – When the crust is broken up and
splits into large pieces of rock.
• Plate Tectonics – The study of plates, their
movements and the landforms that are
created.
• Tectonic cycle – when plates collide and
separate.
Plate tectonics keywords
• Wegner
• Pangaea – name given to the earth. One
large area of land.
• Continental Drift – the moving of plates,
land and oceans.
• Sea Floor Spreading – new crust being
created.
• Hess
Theory of Plate Tectonics –
Continental Drift
• Alfred Wegener (1912) – single land mass
called Pangaea, broke to form todays
continents
• Theory backed by similar rocks & fossils
found in Brazil and west Africa
• However, Wegener could not explain how
the continents moved apart
Theory of Plate Tectonics – Sea
Floor Spreading
• Harry Hess (1960s) studied the age of
rocks in the Atlantic Ocean
• The newest rocks were found nearest the
Mid-Atlantic Ridge, while the oldest were
found closer to the US coast
• Theory states that the sea floor is
constantly expanding as new magma
forces its way up between the plates
Tectonic Cycle
• Crustal plates move due to convection currents in the
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•
•
•
mantle
These currents are powered by the Earth’s core, causing
magma to heat, rise, cool and sink in a circular motion
This movement pulls the plates apart
(diverging/separating) or together (converging/colliding)
New crust is created where plates separate
When plates collide, the front of the heavier plate gets
dragged underneath the lighter plate due to gravity –
this is known as SUBDUCTION
Video Clips
• http://www.youtube.com/watch?v=QDqsk
http://www.youtube.com/watch?v=QDqskl
ltCixA&sns=em
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• Continental Drift
http://www.youtube.com/watch?v=GYVS_Yh6dTk&feature=fvwrel
PLATE BOUNDARIES
KEYWORDS
•Destructive Plate Boundary
(Convergent)
•Constructive Plate Boundary
(Divergent)
•Conservative (transverse or
transform) plate boundary
Plate Boundaries
There are three types of plate boundaries.
1. Destructive/convergent (land destroyed)
2. Constructive/divergent (land created)
3. Conservative/transform (land is not
destroyed or created)
1. Destructive Plate Boundaries
Plates move together and collide destroying
the crust. This happens in 3 situations.
1. Between oceanic and continental plates
2. Between two oceanic plates
3. Between two continental plates
Oceanic – Continental Plate Collision
= Subduction zones
• Subduction occurs
• The melted crust turns to magma and moves up through
the continental crust to create some of the most
explosive volcanoes in the world e.g Mount St. Helen in
the USA
• Where the two colliding plates meet a deep trench is
created. (trenches mark all subduction zones).
• The Pacific Ring of Fire is so named because many
active volcanoes are created at subduction zones mark
the edge of the Pacific plate. Eg. Mount Fuji.
Oceanic – oceanic plate collision =
trenches and island arcs
• When two oceanic plates collide one is usually subducted
beneath the other. When one plate goes beneath the other
deep trenches form. These are the deepest part of the
ocean.
• Marked by curved zones of volcanic islands known as island
arcs. These volcanoes form when the subducted plate
descends and melts, creating rising bubbles of magma that
break through the crust above.
• These active volcanoes have built up on the sea floor over
millions of years until they appear above the surface of the
water. E.g Japanese islands.
Continental – Continental
Fold Mountain
• When 2 continental plates collide fold mountains
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•
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are created.
Himalayas and the Alps.
Fold mountians are often composed of sedimentary
rocks such as limstone that formed beneath the
sea.
When two continental plates collide subduction
does not occur . Colliding continental plates tend to
move upwards or sideways, this can sometimes
cause devasting earthquakes e.g Pakistan 2005.
Activity
• Draw a diagram of a destructive plate
boundary
2. Constructive (divergent) plate
boundaries = mid ocean ridges
• These occur where plates are pulled apart by the convection currents
below.
• As the crust splits, a rift valley is formed allowing magma to rise and fill
the gap creating new crust. Shallow earthquakes and volcanoes also
occur e.g. Iceland 2010.
• As magma becomes solid it forces the plates apart even further allowing
more magma to rise.
• Over millions of years the ocean floors have been created this way. (Sea
floor spreading)
• Eg. Mid Atlantic Ridge. The crust is moving about 5cm per year. 25km
per million years.
• E.g. Sea floor spreading in Iceland. Iceland formed where the mid
atlantic ridge appears above the sea. Iceland is getting wider due to the
process of sea floor spreading. It has over 200 active and dormant
volcanoes.
Activity
• Draw the diagram of a constructive plate
boundary
3. Conservative (transform or transverse) =
fault lines
• Where two plates slide past each other.
• These boundaries are marked by fault lines. They are thousands of kilometers
long and up to 8km deep. E.g San Andreas Fault in California. (1,300km
long).
• At this fault the Pacific plate and the North American fault are both moving in
a north west direction.
• The Pacific plate is moving at a faster pace (5cm per year) this is not smooth
and the plate may stick .
• This leads to a build up of pressure between the two plates until they
suddenly go past each other causing an earthquake.
• In the 1906 San Francisco earthquake the Pacific plate moved over 6m in one
minute.
• The Haiti 2010 earthquake occurred on a transform boundary between the
Caribbean and North American plate.
Activity
• Draw the diagram of a conservative
boundary.
Tectonic Activity in the middle of
plates = hotspots
• Volcanoes can be found thousands of miles from
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•
the edges of plates.
Occur where high temps in the mantle produce a
rising fountain (plume) of a molten rock known
as a hotspot.
Hotspots are extremely hot areas deep in the
upper mantle (asthenosphere). The magma rises
up and pushes through the plate above. This
creates a volcanoe. E.g Hawaiian Islands
The Hawaiian Islands
• A chain of volcanic islands in the middle of
the Pacific plate located over 3,200km from
the nearest plate boundary.
• As the pacific plate moves over the hotspot
volcanoes grow, become dormant and
eventually become extinct.
• Hawaiian hotspot is about 500 – 600km
wide and approx 86million years old.
Summary Video
• http://www.youtube.com/watch?v=KCSJN
BMOjJs&sns=em
55 mins long.