Transcript Plate Tectonics

Plate Tectonics
The Structure of the Earth
A thin crust 10-100km thick
A mantle – has the
properties of a solid
but it can also flow
A core – made of
molten nickel and iron.
Outer part is liquid
and inner part is solid
The average density of the Earth is much higher than
the crust, so the inner core must be very dense
The Crust
Sedimentary
rocks settle in
layers. The
oldest rock is at
the bottom.
Layers of sedimentary rock can be examined to
discover how they were formed. They are often
found folded or fractured:
Tectonic theory
People once thought that the oceans and the continents were formed by
shrinkage from when the Earth cooled down after being formed.
Alfred Wegener proposed something different. Consider Africa and South
America:
These continents look
like they “fit”
together. They also
have similar rock
patterns and fossil
records. These two
pieces of evidence led
me to believe that
there was once a
single land mass. This
is my TECTONIC
THEORY.
Forming mountains
The formation of mountain ranges can be explained by tectonic
theory. Consider the Himalayas at the top of India:
This is where
India is now
This is where
India was millions
of years ago
The intense heat and pressure from this process causes the
rocks to change structure into metamorphic rocks.
The Evidence:
Tectonic theory
1) Some continents look like they used to “fit” together
2) Similar rock patterns and fossil records
The Problems:
Wegener couldn't explain
how continental drift
happened so nobody
believed him
The Answer:
1) Scientists discovered 50 years later that the Earth generates massive
amounts of heat through radioactive decay in the core. This heat
generated convection currents in the mantle causing the crust to move
2) We also now know that the sea floor is spreading outwards from plate
boundaries
Conclusion – scientists now believe Wegener’s Tectonic Theory
The Earth’s crust in motion
Convection currents
• Convection currents beneath the plates
move the crustal plates in different
directions
• The source of heat driving the
convection currents is radioactivity deep
in the Earths mantle
• Convection currents carry heat from the
hot inner mantle to the cooler outer
mantle.
Constructive Margin
Mid-Atlantic Ridge
How is it formed?
• Convection currents cause the plates
to move apart
• As the plates move apart the magma
wells up from the mantle to form new
basaltic oceanic crust (new plate area
is formed)
• The Earth’s surface area increases
due to the formation of new oceanic
crust – sea floor spreading
Mid-Atlantic Ridge
The Mid-Atlantic Ridge passing through Iceland
Destructive Margin
What happens at the
destructive margin?
• The thicker, denser oceanic plate slides
beneath the continental plate (subduction)
• The oceanic plate melts as it reaches the
hot mantle
• Folding of the continental plate forms
mountains
• As the plates slide past each other, friction
stores energy in rocks – energy is
released in earthquakes
Subduction
The Andes
The Andes are found at a
destructive margin along the
western coast of South
America
The Andes
(shown in white)
Collision plate margins
The Collision Zone
• When continental plates collide head on
with other continental plates, the result is
upheaval!
• Similar densities – neither plate is
subducted
• Gradual forward movement of each plate
creates extreme pressures
• Over time, the rock strata become folded,
and rise to create mountain ranges
The Alps and the Himalayas
The Alps
The Himalayas
Conservative Margins
What happens at conservative
margins?
• As the plates slide horizontally past
each other pressure builds up in the
rocks either side of the fault
• Fault surface often rough – friction
creates large strains along the faults
• An earthquake happens when the built
up pressure and energy are released in
a sudden, jerky movement
San Andreas Fault
The San Andreas Fault is the
border between two tectonic
plates — the North American
Plate and Pacific (Nazca) Plate