Transcript Plate Tectonics

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

Matching
mountain
ranges
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 1950s and 1960s
 World
War II submarines using sonar
found mountains under the oceans –
the mid-ocean 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.
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
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
Confirmation of the Theory of
Plate Tectonics
Evidence supporting the
theory of plate tectonics:
Apparent Polar wandering:
plate movement causes the
apparent position of the
magnetic poles to have shifted.
The paleomagnetic fields in the
rocks would indicate a single
pole until the continents drift
apart.
Confirmation of Plate Tectonics Theory
Paleomagnetism: strips of alternating magnetic polarity at spreading
regions.
The patterns of paleomagnetism support plate tectonic theory.
The molten rocks at the spreading center take on the polarity of
the planet while they are cooling. When Earth’s polarity
reverses, the polarity of newly formed rock changes.
The Plates
Sea-Floor Spreading and Plate
Boundaries
Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
Mid Atlantic Ridge
Mid Atlantic Ridge
Divergent Boundaries

Spreading ridges
– As plates move apart new material is
erupted to fill the gap
Converging Margins: India-Asia
Collision
Transform Faults and Seafloor
Spreading
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
“Plates” of lithosphere are moved around by the underlying hot mantle
convection cells