Plate Tectonics
Wegener, Continental Drift and
Pangaea
Video shows a fast motion animation over the past 740 million
years
Evidence for Continental Drift
Jigsaw Puzzle fit of
continents
Alfred Wegener
during Greenland
expedition
More evidence
Matching fossils on
continents now
located thousands
of miles apart.
Example =
Mesosaurus, a
freshwater reptile
Many others
More evidence
Matching geologic
structures including:
– Mountain chains
– Ore deposits
– Same rocks of same
age
More evidence
Climate change
evidence
– Glacial deposits at
current equator
– Fossilized palm
trees in Greenland
Map shows why
according to the
placements of
current continents
within Pangaea
Wegener not believed
Why? -
– What could possibly force the continents
to move across the ocean floor in this
way. They would be crushed.
– He was a meteorologist, not a geologist
Developments 50s and 60s
World war 2 submarines found
mountains under the oceans – the
midocean ridges
Sea floor drilling showed rocks
younger than expected and youngest
towards the center of the mid ocean
ridge
Theory of seafloor spreading
suggested by Princeton professor Dr.
Harry Hess
Seafloor spreading
First look at the earth’s layers as shown here. Let’s draw them.
How do we know about them? Animation and Video
Evidence for sea floor spreading
Alvin and other submersibles
found cracks and lava showing
spreading and volcanism at mid
ocean ridges and odd life forms
Matthews and Vine’s survey of
the Indian Ocean sea floor
showed matching stripes of
reversing polarities on either
side of ridge – what caused
these?
– Lava spreading during
alternating magnetic periods
Discussion of
electromagnetism, paleomagnet
ism and earth’s core
The Earth’s magnetic field
It is produced by the
outer core of the earth
which is made of liquid
iron and nickel
This moving magnetic
material produces a
moving magnetic
field, which in turn
produces a moving
electric field. It is a
dynamo!
Earth’s magnetic field
varies over time and it
protects us from VIDEO TRUE or FALSE?
cosmic radiation
Seafloor spreading and
paleomagnetism
Pangaea revisited
By piecing together
this
information, we
can see how the
continents have
moved over the
past 200 million
years, due to
seafloor spreading
Plate tectonics
Sea floor spreading provides the
driving mechanism for movement
However, it is not the continents that
are moving, but the “plates” of
lithosphere “floating” in effect on the
asthenosphere
The lithosphere is made up of about
20 plates which move relative to
each other in several ways
Let’s look at a generalized sketch
The Plates
Sea-Floor Spreading and Plate
Boundaries
3 types of boundaries
Convergent –
where plates come
together. See
example next slide
Converging Margins: India-Asia
Collision
Interesting plate collision
This picture shows a
place in
Newfoundland where
a massive collision
actually forced
mantle rock on top
of the crust, during
the collision that
formed Pangaea and
the Appalachian
mountains. This
looks down the old
plate boundary.
Mantle rocks are toxic
These rocks have very
different compositions
than crustal rocks.
They contain heavy
metals, which do not
support life forms on
the earth’s surface, so
few organisms live
there.
However, in some
places their heavy
metal concentrations
produce rich metal
deposits and are mined
Divergent Boundaries
Found at spreading
centers – either mid
ocean ridges or mid
continental rift zones
Transform fault boundary
This shows the San
Andreas Fault.
It is a transform fault
boundary, where the
plates move sideways
past each other, rather
than away from each
other (at divergent
boundaries), or
towards each other (at
convergent boundaries)
Transform Faults and Seafloor
Spreading
Review of different boundaries
Divergent –mid ocean ridge like Iceland or
continental rift zone like the African Rift
Valley
Convergent
– Ocean/ocean like Japanese Islands
– Continent/ocean like Andes and Cascades
– Continent/continent like the Himalayas
Transform fault like the San Andreas fault
Hot spots are not at plate boundaries, but
give us information about plate motion
Activity
What causes plate tectonics?
Convection in the mantle, as
the plastic asthenosphere
flows, carrying the plates with
it.
This is probably aided by slab
pull at subduction zones and
ridge push at mid ocean ridges
and rising plumes in the
mantle
This diagram shows several
different model hypotheses
Plate tectonics causes
volcanic activity
and earthquakes
It causes rocks to be tilted
Or even to fold or break
What will the future bring?
Link to animation
Volcanoes and Earthquakes and
Plate Boundaries – GIS activity
Structural Geology
Isostasy
Mountains
Stress and strain
Folds
Faults
Formation of mountains
Two forces are
constantly at
work on the
earth.
– Weathering
and erosion
tear structures
down while
– Plate tectonics
builds them up
So we have mountains!
But they will not last forever.
Mountains form in different ways
-Volcanic mountains-
Volcanoes form by subduction and
melting of plates
Volcanic mountains form over hot
spots
Volcanic mountains form at rift zones
Other mountain types
Folded mountains form From converging
continents like the Himalayas
How do these look? Direction?
Fault block mountains form where blocks of
rock drop at faults – mostly near plate
boundaries, but not always
Uplifted mountains – where large sections
of the crust are pushed up, perhaps by
magma, or other forces
Isostasy
Just as a boat sinks or rises with changes in
weight, so does the crust sink or rise with
changes in weight. Plate tectonics builds
mountains and the extra weight causes the crust
to sink. As erosion occurs the weight of the
mountains decreases and the crust rises again.
This process is called isostasy or isostatic change
Isostasy •This is a good example of
Isostasy is balance. A floating object Newton’s 3rd law. For every
is balanced in the water, like an force, there is an equal and
iceberg. If some of the top melts, the opposing force. In this case
iceberg rises in the water to stay in the force of gravity is
balance. If you get into a boat, the opposed by the buoyant
boat sinks to maintain balance. The force.
same thing happens with mountains. •This is also a great
As plate motions push it higher the example of feedback within
mountain sinks into the mantle to stay a system. Is it positive or
in balance. On the other hand, when negative? Figure it out.
the mountain erodes, it will rise in the Remember that maintaining
mantle as the top erodes. So, a equilibrium is __ feedback
mountain may lose 1 meter from because it keeps the system
erosion, but regain 0.8 meters from in balance. So, what is it,
rising due to isostasy. It maintains positive or negative?
isostatic equilibrium.
Right! It’s negative.
Stress
Due mostly to plate movements,
the earth’s crust is under a lot of
stress. There are 3 types, shown
at the right
A occurs where plates pull apart,
divergent boundaries, and is
called tension
B occurs where plates converge,
and is called compression
C occurs where plates move past
each other, at transform fault
boundaries and is called shearing
Stress and strain video
Strain
This stress leads to strain
anticline syncline on the crust which bends
it. –
If it is warm, underground,
it can bend. This called
A fold above and a fault below
ductile deformation.
Features are called folds.
Upturned folds are
anticlines while
downturned folds are
synclines.
Or, the rock may break, if
it is brittle. This causes
Economic value of folds/faults faults –breaks of the earth.
Different faults
Faults move in
different
ways, depending 1
on the type of
stress on them.
Here are 3 types 2
that form.
1 is a normal fault
2 is a strike slip
fault
3
3 is a reverse fault
Matching
These form at
different plate
boundaries. Can 1
you figure out
which forms
where? Match
them 2
A. Convergent 3
B. Divergent 1
C. Transform Fault 2
3
Can you match the stress and strain?
1
2
3
A is 1st, B is 3rd, C is 2nd