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The Theory of Plate Tectonics
Plate Tectonics is a
theory developed in the
late 1960s, to explain how
the outer layers of the
Earth move and deform.
Why there are
earthquake
prone areas,
why volcanic
eruptions occur
only in specific
areas, and why
there are ocean
basins and
mountains.
EXPANDING EARTH
In the beginning the
Earth’s Crust was evenly
spread over the Mantle ...
… but got stretched and
thinned when the Mantle
expanded forming the
ocean basins.
X X X
X
EXPANDING EARTH
Ocean basins are too wide to
have formed by any reasonable
expansion.
CONTRACTING EARTH
In the beginning the
Earth’s Crust was evenly
spread over the Mantle ...
as the mantle contracted,
the crust wrinkled forming
continents and ocean basins
X
X
X
X
Wrinkles forming continents and
CONTRACTING EARTH
mountains should have formed more
evenly rather than in narrow bands.
GEOSYNCLINES
Weight of sediments
dumped off the edge
of the continents
sinks the crust down
into the Mantle ...
…eventually, it
rebounds up to form
mountain ranges
X X X
GEOSYNCLINES
We see lots of crust
sinking down from the
weight of sediments,
but can’t find any that
are rebounding
Law of Uniformity says that if you
don’t see it happening now, you can’t
assume it happened in the past.
Continental Drift Theory
CONTINENT
CONTINENT
OCEANIC
MANTLE
CRUST
Developed by Alfred Wegner
in the early 1900’s, it said
that the continents are like
rafts sliding over the
oceanic crust.
Continental Drift Theory
Wegener suggested that throughout Earth’s
history the continents have joined and
separated numerous times.
Wegener called the most recent unified
landmass Pangaea. It began to break apart
about 200 million years ago.
Wegener said that
when continents
collided, mountain
ranges formed.
When
continents
separated or
split, ocean
basins formed.
Animations
Bullard Fit of the Continents
Breakup of Pangaea
Wegener's Continental Drift
Continental Drift Theory
Provided answers to some big
mysteries.
Like…
The apparent
puzzle-like fit of the
continents. This
had been noticed
since the early days
of map-making.
And…
The presence of identical fossils on
widely separated continents belonging to
organisms that could not have crossed
the ocean or lived in the kind of climate
currently found in those locations.
And…
The presence of glacial tracks and
deposits in areas that are, today, far
too warm for glaciers. The patterns of
the tracks were also problematic.
And…
Mountain ranges of the same age, elevation, and
rock type on widely separated land masses
But, Continental Drift Theory
had its problems too.
Wegner was a meteorologist, who came up
with his theory from watching ice flows.
This didn’t impress the geologists.
But the biggest problem was that Wegner
couldn’t provide an explanation for how the
continents were able to move that made
sense.
He said the
continents moved
due to the earth’s
rotation…
…and plowed through oceanic crust like an
ice-breaker plowing through ice!
Continental Drip Theory
People said that Wegner
cited no evidence, only
speculations.
This led some people to
offer alternative ideas to
ridicule Continental Drift
For example people noticed that a lot of the continents
looked like drips … suggesting that the continents were
dripping off Antarctica
X X X
X
Continental Drift Theory
Some geologist comments ...
“Anyone who valued his reputation for scientific
sanity would never dare support such a theory.”
"If we are to believe this hypothesis, we must
forget everything we have learned in the last 70
years and start all over again,"
Wegner went back to his ice flows and died
before seeing his ideas gain acceptance.
Video
In the 1940’s Oceanographer, Harry
Hess, discovered a ridge down the middle
of the Atlantic Ocean
Mid-Atlantic Ridge
Temperature
analysis showed
that the center of
the ridge was
very hot, like a
long continuous
volcano
Analysis of cores taken from the ocean bottom also
showed a symmetrical age pattern, younger in the middle,
older at the edges
Geologists knew that the direction of
Magnetic North changed every million
years or so.
They also knew that when magma
cools, the magnetic minerals record
the direction of Magnetic North.
When they
checked the ocean
crust around the
ridges they found
that they recorded
the flip in long
parallel stripes
which
corresponded to
the historical
reversals of
magnetic North.
Animation
Magnetic Reversals
Magnetic Reversals_3 Min
Geologists now had strong evidence that
new crust was being created in the
center of the Atlantic Ocean
In fact, Iceland is a volcanic island that sits
right on top of the Mid-Atlantic Ridge.
Animations
Formation of ocean floor and the
movement of continents
Formation of new ocean and
movement of continents #2
Formation of Rift Valley and New Ocean
Formation of Ocean Crust
If new crust was being created at the center
of the mid-ocean ridge, then somewhere else
the old crust was being destroyed. Otherwise
the earth would be getting larger and larger.
Confirming this was that the age of the ocean crust
around the world showed that none of it was older
than 180 MY, less than a 10th of the age of the
oldest continental crust.
Something was eating up the crust!
Oceanographers began to wonder about the
long deep trenches they had also discovered
in the oceans, particularly the Pacific. Often
these trenches had volcanic mountain chains
along one side and had frequent earthquakes.
Trench
Geologists
discovered that
oceanic crust was
being warped
downward into the
mantle and being
consumed at these
trenches.
As the ocean sediments and crust melted the
lighter elements would well upward forming the
volcanic mountain chains behind the trench.
Animation
Trench and Subduction of Oceanic Plate
Destruction of Oceanic Plate
These trenches formed a ring around the Pacific,
characterized by volcanoes and strong earthquakes
called the “Ring of Fire”
Wegener’s Vindication!
With the discovery that new crust was
being formed at Mid-Ocean Ridges
and old crust was being recycled at
trenches, Wegener’s ideas got a new
lease on life.
Trench
With this new
knowledge scientists
were able to
understand the engine
that was powerful
enough to move
continents and how
the different plate
movements produced
different effects!
Convection and Plate
Movement Animation
Ocean Ridge
Trench
What makes the plates move?
Convection
What makes the plates stop?
X
Convection
shuts off
Or something locks
the plates together
How fast do plates move?
The Arctic Ridge has the slowest rate (less
than 2.5 cm/yr), and the East Pacific Rise
near Easter Island, in the South Pacific
about 3,400 km west of Chile, has the
fastest rate (more than 15 cm/yr).
Geologists eventually discovered that Earth’s crust
was cracked into many pieces, called plates, that are
in constant motion as a result of sea-floor spreading.
Plates are made of
pieces of the
lithosphere (upper
mantle and crust)
Granite:
Main rock of
continents
Less dense
than basalt
Basalt:
Lines the
ocean floor.
Heavy.
Three
types of
plates
Continental
Oceanic
Combination
Different kinds of plate interactions are responsible for
earthquakes, volcanoes, mountains, and other
structures .
Divergent Boundary
Convergent
Boundary
Transform Boundary
Divergent Boundaries and
resulting landforms
Oceanic/Oceanic: Mid Ocean Ridge
Continental : Rift Valley, New Ocean Basin
Rift valley
Convergent Boundaries and
resulting landforms
Continental/Continental : Folded Mountains
Continental/Oceanic : Volcanic Mountains
and Trench
Oceanic/Oceanic : Trench and Volcanic
Islands
Transform Boundary and Resulting Structure
Strike-Slip Fault
Animations
Plate Boundary Interactions
Interactive Plate Boundaries
When Plates Collide
Oceanic Plates Collide
Oceanic Plate and Continental Plates Collide
Continental Plates Collide
The solution to another mystery was suggested by
the moving oceanic crusts. The Pacific has many
long chains of islands that seem to have been made
one at a time in order. The Hawaiian chain is an
example of one of these.
The Hawaiian chain
extends far beyond
the surface island.
Current Hot Spot
The Pacific island
chains record the
motion of the Pacific
plate over time.
Animation
Hot Spot Volcanoes