Plate Tectonics
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
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
3 types of boundaries

Convergent –
where plates come
together. See
example next slide
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)
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

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
Volcanoes and Earthquakes and
Plate Boundaries
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
Isostasy is balance. A floating object
is balanced in the water, like an
iceberg. If some of the top melts, the
iceberg rises in the water to stay in
balance. If you get into a boat, the
boat sinks to maintain balance. The
same thing happens with mountains.
As plate motions push it higher the
mountain sinks into the mantle to stay
in balance. On the other hand, when
the mountain erodes, it will rise in the
mantle as the top erodes. So, a
mountain may lose 1 meter from
erosion, but regain 0.8 meters from
rising due to isostasy. It maintains
isostatic equilibrium.
•This is a good example of
Newton’s 3rd law. For every
force, there is an equal and
opposing force. In this case
the force of gravity is
opposed by the buoyant
force.
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
Strain

anticline
syncline

A fold above and a fault below

This stress leads to strain
on the crust which bends
it. –
If it is warm, underground,
it can bend. This called
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 faults
–breaks of the earth.
Different faults
Faults move in
different ways,
depending on the
type of stress on
them. Here are 3
types that form.
 1 is a normal fault
 2 is a strike slip
fault
 3 is a reverse fault

1
2
3
Matching

These form at
different plate
boundaries. Can
you figure out
which forms
where? Match
them
A. Convergent
3
B. Divergent
1
C. Transform Fault
2
1
2
3
Can you match the stress and strain?
1
2
3
A is 1st, B is 3rd, C is 2nd