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Plate Tectonics
As people began learning about the structure of the
earth, there appeared to be a geologic mystery….
Fossils, rocks, glaciers, and the bottom of the ocean all
revealed a past that did not reflect the current
conditions on earth. But what did that mean?
Let’s review what scientists found and then look at their
conclusion.
Fossils
• Scientists know that a species appears
in one area on earth, then spreads
out. Some species can only spread
across land.
• Scientists found identical fossil
organisms in S. America and Africa:
they would have to be connected for
this to occur.
Fossils
• Mesosaurus, a freshwater reptile: it should be
more widely spread out if it could swim across
oceans
• Glossopteris, large ferns: the seeds are too heavy
to be blown across oceans
• Lystrosaurus and Cynognathus, land reptiles:
found on 3 continents, which are separated by
oceans, and they cannot swim
Rocks
• On S. America and Africa there are
rock and landscape similarities, such
as matching mountain ranges.
• The rocks in the mountains are of the
same age and type, and fit together
like a puzzle.
Glaciers
• Glaciers are huge ice masses that
move slowly over land. Glaciers
spread outward from the poles.
• Evidence from glaciers 300 million
years ago is found in S.America,
Africa, India and Australia. For this
to happen, these continents would
have had to be closer to the South
Pole than they are now.
Glaciers
• The pattern of rocks can be explained
if the continents were one landmass
when the glacier formed.
• In addition, there is fossil evidence
that the modern day Northern
Hemisphere was tropical swampland.
• These swamps became the coal fields
of the United States, Europe and
Siberia.
So, what did all of this mean?
In 1915, one man, Alfred Wegener, proposed an
idea that explained all of these unusual
findings.
Wegener/Continental Drift
• In 1915, Wegener described
the idea of Continental Drift,
the gradual movement of
continents over time across
the surface of the earth,
which would explain all of
the unusual findings.
• He proposed the idea of
Pangaea, a giant
supercontinent that had once
contained all the world’s
land, that slowly had drifted
apart into today’s continents.
Alfred Wegener/Continental Drift
• Pangea existed 200 million years ago, then
it became fragmented into smaller
continents and started to drift.
• The last landmass to move into current
position was India, 40 million years ago.
Alfred Wegener/Continental Drift
• This idea was great! Except he was missing one
piece of the puzzle… HOW the giant continents
could move around the earth.
• His idea was not accepted during his lifetime
because he did not have a mechanism, a reason,
for the movement of the continents.
• He spent the rest of his life taking trips to
Greenland to test his ideas. In 1930 while
returning to the ship he suffered a heart attack
and died.
• He was buried as he was found, under the snow,
to become part of the ice sheet.
So, was Alfred Wegener correct?
How could scientists explain giant continents
moving around?
The answer to the mechanism of the movement
lay at the bottom of the ocean…
Sea Floor Spreading
• After WWII, oceanographers were able to
use new technology (SONAR) to map the
ocean floor, which also revealed
unexpected findings.
• Deep trenches and steep ridges were found
on the ocean floor, creating a system that
covered the earth.
Sea Floor Spreading
• The ocean floor was covered in Basalt, an igneous
rock, the most abundant rock on the earth.
• How do you explain volcanic rock covering the
bottom of the ocean?
Sea Floor Spreading
• The rocks that were at the center of the
mountain ridges were younger than the
rest of the ocean floor, then became older
as they moved away from the center of the
ridge.
Sea Floor Spreading
• In the 1960’s, Harry Hess proposed
seafloor spreading, the idea that the
oceanic ridge was a crack in the earth’s
crust where the hot mantle pushed
upward. The pieces of crust on either side
of the crack were moving away from each
other and magma pushed up between
them, creating new oceanic rocks.
Sea Floor Spreading
• As rising material spreads laterally,
seafloor is carried conveyor-belt fashion
away from the ridge crust, generating new
oceanic crust.
• To compensate for the other side of the
plate moving, there are deep ocean
trenches where the crust is brought back
down to the mantle.
This was the missing mechanism describing how
land could move!
But why does magma push toward the surface?
Convection!
Convection
• Heat transfer from a lower
temperature to higher
temperature by motion of a
fluid, such as water or
magma.
• Heat from the core moves
toward the surface through the
mantle causing the material of
the crust to circulate.
Convection
• Matter rises in parts and
once cooled, it sinks.
This creates a circle of
heat and matter called
convection.
• This causes rocks to
melt, magma to erupt
and the crust to move.
Convection
• The rising and
sinking material
moves in a circular
motion, which moves
plates because the
plates stick to the
moving mantle due to
friction, which causes
them to move as the
mantle moves.
Convection Currents
• Convection currents are the result of uneven
heating inside the mantle resulting in the
melting of rock; convection, eruption and flow
of magma; and the movement of crustal plates.
• Rock layers are affected by the folding, breaking,
and uplifting of rock layers due to plate motion.
• The movement of crustal plates can cause
earthquakes and volcanic eruptions that can
result in mountain building and trench
formation.
With the mechanism of convection in place,
Wegener’s idea of moving continents was finally
accepted.
Today, we call his idea Plate Tectonics.
Plate Tectonics
• A geological model in which the
Earth's crust is divided into a number
of rigid segments, plates, which
constantly move atop of the mantle.
Rates of Motion
• These average rates of
plate separations can
range widely.
• The Arctic Ridge has the
slowest rate (less than
2.5 cm/yr.
• The East Pacific Rise
near Easter Island, in
the South Pacific, has
the fastest rate (more
than 15 cm/yr).
Boundaries
• If they are moving, what happens when plates
meet?
• All interactions of plates occur along
boundaries, where one ends and another
begins.
• The location of the boundaries was established
by researching locations of earthquakes.
• These boundaries show where the plates are
being subducted, building mountains or
creating new oceanic crust.
Boundaries
• Continental plates: 6 of the 7 major plates, carry
mostly continental crust and some oceanic crust
• Oceanic plates: 1 of the 7 major plates, and
several smaller plates, only made of oceanic
crust
Boundaries
• Convergent: two plates moving together.
• Oceanic-oceanic: one oceanic plate subducts,
sinks under the other plate.
• Oceanic-Continental: oceanic plate, which is
denser, subducts under the continental plate.
• Continental-Continental: neither can move
under the other because they are both thick,
they compress and buckle, forming mountains.
Boundaries
• Divergent: two plates move apart, upwelling of
material from mantle to form new seafloor.
Boundaries
• Transform Fault: two plates grind past each
other without production or destruction of
crust.