Plate Tectonics
What is plate tectonics?
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Earth’s lithosphere is broken into plates
that move on the asthenosphere.
The movement of these plates is ‘Plate
Tectonics’.
The theory of plate tectonics . . .
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The lithosphere is broken into plates that
move on the asthenosphere.
Volcanoes and earthquakes occur in
specific belts along these plate
boundaries.
The age of rocks on the ocean floor give
us clues to tectonic movement.
Continental Drift theory . . .
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Alfred Wegner, a
German scientist
hypothosized that the
continents have
drifted over time.
Evidence suggesting
this theory is the
shape of coastlines,
fossil distribution, and
distinctive rock
formations.
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Tectonics occurs as the lithosphere of the
crust floats on the plastic asthenosphere at
the surface of the mantle.
How do the plates move?
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There are three types of plate boundaries:
Divergent boundaries -- where new crust is
generated as the plates pull away from each
other.
Convergent boundaries -- where crust is
destroyed as one plate dives under another.
Transform boundaries -- where crust is
neither produced nor destroyed as the plates
slide horizontally past each other.
Divergent Boundaries
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Divergent
boundaries occur
along spreading
centers where
plates are moving
apart and new crust
is created by
magma pushing up
from the mantle.
Convergent Boundaries
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The size of the earth
has not changed
significantly during the
past 600 million years,
and very likely not
since shortly after its
formation 4.6 billion
years ago. The earth's
unchanging size
implies that the crust
must be destroyed at
about the same rate as
it is being created.
Two types of converging
boundaries:
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Subduction Boundary:
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When an ocean plate
subducts [goes under]
either another oceanic
plate or a continental
plate it forms a
subduction boundary.
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Collision Boundary:
Form when the two
converging plates
are both continental.
The collision causes
the crust to be lifted
into mountain
ranges, such as Mt.
Everest in India’s
Himalayan
Mountains.
Transform Boundaries:
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The zone between
two plates sliding
horizontally past one
another is called a
transform boundary.
Most transform faults
are found on the
ocean floor.
Review . . .
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Plates move apart at divergent
boundaries, toward each other at
convergent boundaries, and past each
other at transform boundaries.
Convergent boundaries can be either
collision [between continents] or
subduction [between oceans or between
oceans and continents].
Magnetism helps reveal age of ocean floor:
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Evidence for tectonic
activity is demonstrated by. . .
Magnetic reversals in
rocks on either side
of mid-ocean ridges
High heat flow along
ridges
Youngest crust is
near ridge center
We know plates move, but the question
is; HOW do they move?
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Well, I’m going to tell you . . .
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Three hypotheses describe how plates are
moved:
1. Mantle Convection:
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The transfer of heat
from Earth’s inner
core to its outer core.
These convection
currents of heat may
drag the plate away
from its divergent
boundary.
2. Ridge Push
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Ridge push - Gravity
acting on the
topography of ridges
tries to spread the
ridge outward.
This pushes the rest
of the plate away
from the ridge.
3. Slab Pull
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Slab pull – The edge
of the subducting
plate is cold and
dense; therefore, as it
sinks into the mantle,
it carries the rest of
the plate with it.
So, just to review . . .
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The three hypotheses for plate movement are:
1. Mantle Convection: Convection currents
move the plate.
2. Ridge Push: Gravity acts on the newly
formed rock along ocean ridges forcing them to
slide down [divergent]
3. Slab Pull: As the plate subducts, it pulls the
rest of the plate down with it. [convergent]
Plate Movement and Continental Growth
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Plate movements
have caused Earth’s
continents to change
their positions on the
globe over time.
We started with
Pangea, [from the
Greek, pan = all, gaia
= Earth]
And the continents grow . . .
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New material
continues to be
added to the
continents over
time, constantly
changing both
shape and
position.
Before:
After:
Tectonic activity is constant . . .
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The breakup of
Pangea continues
today as the plates
continue to move
But, what was here first?
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Craton: At the core of
each continent is an
ancient rock called
the craton
The North American
Craton, containing the
oldest rocks on the
continent, date back
about 1 billion years.
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The Canadian Shield
and platform make up
the North American
craton. The shield is
exposed rock, while
the platform is buried
rock.
A little vocab review . . .
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A terrane is a large
block of lithospheric
plate that has been
moved, often a great
distance, and
attached to the edge
of a continent.
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Terranes lead to
continental growth.
When two continental
plates collide, they
can weld to each
other and fuse,
adding to the size of
the original continent.
Just a little more on terranes . . .
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There are three
characteristics used
to identify a terrane:
1. Faults
2. Fossils
3. Magnetic polarity
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Last, but not least, a
terrane. . .
…. is completely
bounded by major
faults.
…has fossils different
than those in
surrounding areas.
…has a different
magnetic record than
that of the surrounding
rock.
So, now we know . . .
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The lithosphere is broken into plates that move
on the asthenosphere.
There are divergent, transform and convergent
boundaries between those plates.
Plate movements are thought to be caused by
mantle convection, ridge push and/or slab pull.
The position and shape of Earth’s continents
has changed over time and is still changing.
And Plate Tectonics is
responsible for . . .
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Volcanoes and the
formation of igneous
rocks
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Earthquakes and
fissures at plate
boundaries
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Dike
Sill
The formation of
metamorphic rocks
at convergent plate
boundaries, igneous
dikes and igneous
sills.