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Revision topics – Plate
Tectonics
• Where does it fit in the
specifications?
• GL1 II Energy KI 3
Key terms:
• Lithosphere, Asthenosphere, LVSZ.
• Types of plate boundaries, convection
currents.
• Earth’s magnetism, Curie Point, sea
floor spreading
• Polar wandering curves
LITHOSPHERE
• Rigid part of Earth’s CRUST
• and includes the topmost part of the
MANTLE
• Granite based composition
ASTHENOSPHERE
• Layer of upper mantle, just
underneath the lithosphere
• 5% molten
• Plastic
• Ability to flow and move
LVSZ
• LOW VELOCITY SEISMIC ZONE
• In the Asthenosphere
• Because it is <5% molten, it slows the
seismic waves as they travel through
them
• seismic waves recorded at distances of
100 km to 250 km arrive later than
expected (waves slowed)
• while both the P and S waves travel more
slowly, the S waves are weakened.
• GeoScience
• page 12 and 13 (lithosphere and
asthenosphere)
• Page 30 (strong and weak rocks)
CONVECTION CURRENTS
• Molten asthenosphere can move
• As minerals in the mantle break down
(radioactive processes) generate heat
• Heat rises  upwelling plume
• Eventually the warmth will be lost and
the cold mantle will sink
• MANTLE PLUMES – new term
TYPES OF PLATE
BOUNDARIES
• CONSTRUCTIVE (Diverging )
• DESTRUCTIVE (Converging )
• CONSERVATIVE
CONSTRUCTIVE PLATE
BOUNDARIES
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Plates are diverging

Moving apart
Due to upwelling convection current
Drags plates apart
BASALTIC magma rises to fill the
gap left
Key terms
•
•
•
•
•
•
•
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Upwelling convection current
Basaltic magma
Partial melting of mantle rock
Thin oceanic crust
Mid Ocean Rift (MOR)
Mid Atlantic Ridge (MAR)
Iceland
Shallow seismic activity
DESTRUCTIVE PLATE
BOUNDARIES
•
•
•
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Plates are converging

Moving together
Due to downwelling convection
current
• Pushes plates together
• One plate is pushed under the other
in a SUBDUCTION ZONE
DESTRUCTIVE PLATE
BOUNDARIES 2
• Different types of collisions:
• Ocean crust colliding with ocean crust
(eg. Marianas trench)
• Ocean crust colliding with continental
crust (South America, Andes
mountains)
• Continental crust colliding with
continental crust (Himalayas)
Key terms
• Downwelling convection current
• Partial melting of subducted ocean
crust
• Andesitic volcanoes
• Trench
• Subduction zone, Benioff Zone
• Andes mountains, Himalayas
• Deep seismic activity
Associated metamorphism
• Intense squeezing of two continents
together creates REGIONAL
METAMORPHISM
CONSERVATIVE PLATE
BOUNDARIES
• Plates rub alongside each other
• No crust made or destroyed
• San Andreas Fault line is best
example
• Think of rubbing two matchboxes
together (sandpaper sides together!)
• Friction “sticks” then sudden jerk
movement (earthquake!)
San Andreas Fault scar.
Hot Spot
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Upwelling plume of magma
Bit like a lava lamp!
Best example Hawaii
Hot spot stays still, plate moves over
the hot spot and a chain of volcanic
islands is created
• Basalt
Earth’s magnetism
• Liquid OUTER CORE generates the
magnetic field for the Earth
• Earth’s magnetism is preserved in
rocks
• Iron minerals in the rocks line up and
preserve the direction of the
magnetic field
How do rocks preserve the
magnetic field?
• When rocks are molten, the iron
minerals are free to move around
• As the rock starts to cool, these iron
crystals ALIGN to the magnetic field
of the time
• This is LOCKED in when the rock
solidifies
Curie Point
• The temperature at which the iron
minerals ALIGN in the molten rock
• Different for different rocks!
Earth’s magnetic field
• Is constantly moving
• North Pole moves in a predictable
manner (PRECESSION)
Poles also FLIP
• Due to currents in the liquid outer
core – the N/S can be flipped
• This has happened a lot in geological
time
• It is recorded in rocks as normally
magnetised rocks and reversely
magnetised rocks
Polar wandering
• Rocks which have the magnetic field
preserved can be used to deduce
their LATITUDE of formation
• and also point to where the N Pole
was
Polar wandering
• As the plates move, the direction to
the N Pole changes
• Graphs can be drawn which convert
direction and distance into an easy to
see pattern
• These are called POLAR
WANDERING CURVES
Jigsaw fit
• JLC updated March 2007.