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Plate Tectonics Revision Notes Geography A2-1
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Plate Tectonic Notes:Background of the earth:
The core has 2 sections: inner (solid) and outer (liquid).
The mantle is solid however, near the outer edge it can flow/deform
(viscoelasticity. This layer is known as the asthenosphere.
The layer above, composing of the upper mantle and crust in the
lithosphere.
Evidence Supporting the Pangea Theory (1915):
Theory: That the Aarth was once a big singular land mass
Distribution of rock types that have mountain chains could be placed together e.g. Caledonian epoch has
many fold mountain chains positioned on opposite shores.
Fossil Evidence found in similarly aged rocks in South America, Africa, India, and Antarctica, some of which
they wouldn’t be able to survive on.
Tropical, hot desert regions are clear to have had ice sheet covered/carved on top. Location and direction of
flow only makes sense when together.
Harry Hess’ Plate Tectonic Theory Evidence (1960’s):
Theory: That the oceans new crust comes out from the centre
The existence of the great submarine mountain chains and rift valleys in the ocean centre.
At the mid-ocean ridged, the rocks are very young and become more older moving away.
Bands of rock parallel to the ridge have alternating magnetic polarity, reflecting the repeated reversal of the
earth’s magnetic field.
The patterns at submarine volcanoes along the ridges revealed active processes at work. In 1963, a volcano
rose above the surface of the Atlantic near Iceland to form a new island of Surtsey.
Constructive Margin:
Convection currents are when hot spots
deep in the mantle cause magma to rise and
subside within the asthenosphere.
Solid plates are forced upwards causing crust
to stretch and breaks along fault lines.
Zone of weakness marked by tension cracks
with uplifted and slumping blocks gives
mountain ridges and rift valleys.
Magma rising solidifies to create new
oceanic material.
The seafloor spreading may allow a nearby
ocean to spill in and water to flood the rift valley, starting the formation of a new ocean basin. Shallow
earthquakes are also associated.
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, Examples:
The North America and Eurasian Plates are moving away from each other along the Mid Atlantic Ridge
(25mm per year).
When the plates pull apart and magma rises it forms and new land such as Iceland.
Iceland is a summit of the ridge that runs the entire length of the ocean.
The Great Rift Valley of East Africa has already pulled the Arabian plate away from the African plate,
forming the Red Sea.
Mt Kenya and Kilimanjaro are examples of volcanoes built in the rift, sometimes erupting.
Scientists say in 10 years the two plates will be completely separated.
Destructive Margin- Ocean to Continental:
Convection currents are when hot spots deep in the
mantle cause magma to rise and subside within the
asthenosphere.
At destructive margins oceanic and continental plates
move towards each other
The denser oceanic plate subducts
This subduction zone is called the Benioff Zone and is
where earthquakes would have their foci
The subducted oceanic plates lithosphere is melted.
This decompression melt is aided by hydration melting
as cool water pulls the oceanic plate further towards the earth’s core
The melting of the lithosphere creates magma which rises through lines of weakness to form volcanoes and
island arcs
Also at the subduction zone a trench will be formed
Examples:
Eastern Pacific Ocean Basin - Nazca Plate formed at the constructive margin of the East Pacific Rise, moves
Westward to meet the South American plate.
Destructive Margin- Ocean to Ocean:
They are plate boundaries where convection forces (convection currents are when hot spots deep in the
mantle cause magma to rise and subside within the asthenosphere) cause oceanic plates to collide
The denser of the two colliding plates is dragged down into the upper mantle and melts at depths of up to
600 km.
Over a long time period, the growth and reworking of
rock material can produce more substantial masses of
islands.
Where these plates collide, a line or arc of volcanoes
(either submarine or as islands) is often found parallel to
an extremely deep ocean trench, these features are
created due to the subduction of the denser plate
Friction earthquake hotspots are also commonly found
at these plate boundaries due to the releases of energy
and pressure during the subduction period
Example:
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