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PLATE TECTONICS
Chapter 17
Drifting Continents
Although it is difficult to imagine, the Earth’s
surface is constantly changing.
– South America is moving away from Africa at a rate of
2-3 cm per year.
– What makes up the Hawaiian Islands are migrating
northwest at a rate of 8-9 cm per year!
– In addition to this, Earth’s highest point (Mt. Everest)
continues to slowly rise.
When continents slowly move to their current
positions, we call this continental drift.
Early Observations
Early mapmakers first considered the idea of moving
landmasses because they found many of the continents
had matching coastlines.
• 1500’s - Abraham Ortelius (Dutch) noticed fit of
continents surrounding Atlantic Ocean.
Incorrectly proposed that N America and S America
were separated from Europe and Africa by
earthquakes and floods.
• 1800’s – Eduard Suess (Austrian) proposed southern
continents had once been joined by a single landmass.
• 1900’s – Alfred Wegener (German) hypothesized the
theory of continental drift for the first time in 1912.
Continental Drift
The theory of continental
drift proposed that Earth’s
continents were once
joined as a single
landmass, Pangaea.
• Pangaea is a Greek word
that means “All Earth.”
• Pangaea is proposed to
have begun breaking
apart approximately 200
million years ago.
• Continents then have
slowly moved to their
current positions.
Beginning with a view of Earth with the continents in their present
positions, the continents move back in time to reunite as Pangaea, are
labeled, and locations of fossil evidence that Wegener used to argue in
favor of continental drift are added.
Evidence - Rock Formations
Wegener reasoned that as
Pangaea broke apart, large
geologic structures would
have fractured. He used this
information to hypothesize
that there should be similar
rock bands separated by
oceans.
He found this to be true as
evidenced by:
1. Appalachian Mountains share
similar features with those found
in Greenland and Europe as well
2. Other similar groups of rocks
were found in South America
and Africa, whose coastlines
also match almost perfectly
Evidence - Fossils
Similar fossils of several different animals and plants that once lived on
land had been found on widely separated continents. Support is justified
as land animals could not have possibly swam across large bodies of
water.
Glossopteris (fossil fern) representing continental drift due to finding these
on several continents despite climates. Reasoned that as ferns require
temperate climates, these areas had to have been closer to the equator
at one time.
Based on his strong meteorological background, he indicated that rocks
joining these fossils had to once be joined as he reasoned that the area
separating the fossils was too large to have a single climate.
Evidence - Climate
Evidence of large climate changes were found on some
continents:
1. Coal deposits (requires temperate, swampy areas) were found on
Antarctica dictating a different climate that we know today.
2. Glacial deposits (estimated to be 290 million years old) were found in
Africa, India, Australia and South America suggesting the areas were
once covered by thick ice caps.
Suggestions for Glacial Deposition
Wegener proposed
continents were once
located near the south
pole, before Pangaea
began to fracture,
making suggestions
that:
1.
2.
South Pole had shifted
position
Landmasses had drifted
away from the pole
(preferred as it is doubtful
the Earth changed its axis
of rotation
Flaws to the hypothesis of
Continental Drift
1. Wegener could not satisfactorily explain what was
actually causing the continents to move.
He suggested the rotation of the Earth; however, physicists were
able to show this force was not great enough to move continents.
2. Wegener could not satisfactorily explain how the
continents were moving.
He proposed that landmasses were plowing through the ocean
floor; however, scientists quickly ridiculed this thought and were
able to argue that there would be evidence of fracture on the
ocean floor… and there was not.
Wegener, unfortunately, left
these two questions
unanswered despite the
impressive collection of
evidence he provided to
support his hypothesis.
He did not give up, working to
prove his hypothesis to the
scientific community of his
time. In 1930, he froze to
death on a meteorological
expedition to Greenland –
still firmly believing in the
idea of continental drift.
The Driving Force of
Continental Drift
Shortly after Wegener’s passing, a
scientist named Arthur Holmes
proposed mantle convection as a
driving force of continental drift,
providing a simplistic version of the
idea of convection of the mantle.
This was largely ignored by the scientific
community for roughly 30 years.
Arthur Holmes 1890-1965
Technology Assistance
Until the mid-1900’s, most scientists thought that the ocean floor, unlike the
continents, was essentially flat. Many people had the misconceptions that
oceanic crust was unchanging and was much older than continental crust.
During the 1940’s and 1950’s however, technology advancements proved all of
these widely accepted reasons to be wrong – largely in part to WWII and
submarine warfare.
These tools included the following:
• Sonar – an echo sounding technique used to map topography. Travel times of
waves were used to determine distances to the ocean floor.
• Magnetometer – Device used to detect small changes in magnetic fields,
originally developed to detect magnetic fields generated by submarine hulls.
Additional use of these devices, during more peaceful times,
revealed seafloor bathymetry (study of depth or floors of
water bodies), which led to the discovery of deep-sea
trenches and mid-ocean ridges. These underwater mountain
chains and deep depressions went on to puzzle geologists
for over a decade after their discovery.
Continued Discovery
As tools continued to be utilized
on the ocean floor, scientists
turned their attention to testing
rocks and sediments.
Findings:
•
•
•
Rocks and sediments found near the midocean ridges are younger than those
found near deep-sea trenches
Oceanic crust rocks are much younger
than continental crust rocks
Paleomagnetism (study of magnetic record
of Earth’s rocks) proved that magnetic field
reversal occurs over time, with magnetic
patterns being symmetric around the midocean ridges
Upon new findings of the ocean
floor (to include topographic,
sedimentary, age and
magnetic data) an American
scientist came up with a
theory to explain all prior
observations pertaining to
continental drift.
Harry Hess 1906-1969
Seafloor Spreading Hypothesis
Harry Hess proposed that new
crust was being produced at
mid-ocean ridges (mantle
upwelling) and consumed
(mantle downwelling) at
deep-sea trenches based on
mantle convection.
Each cycle of spreading and
intrusion then resulted in the
formation of another small
section of ocean floor, which
slowly moves away from the
mid-ocean ridge
So, continents are not pushing
through the ocean crust but
are instead moving with the
ocean crust as it pushes
away from the ridge
Seafloor Spreading Animation
Theory of Plate Tectonics
The Earth’s lithosphere is
broken into large slabs of
rock, called plates, which
moves in different
directions at different
speeds. These plates
interact with one another
at plate boundaries.
Each type of boundary has
specific characteristics
and processes associated
with it.
Divergent Boundaries
Characteristics:
• Two tectonic plates move apart,
resulting in thinning of the crust
and creation of new oceanic
crust.
• Most are found on the seafloor,
where they form mid ocean
ridges.
• The formation of new ocean
crust accounts for high heat flow,
resulting in volcanism and
earthquakes.
• Some create rift valley’s, which
occur on land creating long,
narrow depressions.
• Examples: Mid-Ocean Ridge,
Great African Rift Valley
Convergent Boundaries
Characteristics:
•
•
Two plates come together resulting in
thickening of the crust, recycling of
oceanic crust into the mantle, and growth
of continental crust
Three types:
1. Continental / Continental
2. Oceanic / Oceanic
3. Continental / Oceanic
• Result in:
1. Very tall, uplifted mountain ranges
2. Subduction Zones, deep sea trenches,
volcanic island arc
3. Subduction Zones, trenches, and
volcanic island arcs along continental
plate
•
Examples: Himalayas (1), Western
Aleutian Islands (2), and the Western
Coast of South America (3)
Continental / Oceanic Convergence
Transform Boundaries
Characteristics:
• Two plates slip past one another
resulting in no net gain or loss of crust
• Plates can move in same direction,
with one moving at a faster rate
• The Earth’s crust is only deformed,
not created or destroyed
• Results in long faults and/or shallow
earthquakes
• Most are offset sections of ocean
ridges, rarely found on continents
• Examples: San Andreas Fault (CA)
Causes of Plate Motions –
Confirmed!
The transfer of thermal energy by the
movement of heated matter is
called convection.
The heating of matter causes it to
expand and decrease in density.
As it warms, it rises (buoyancy). The
cooler part then sinks (gravity).
This up and down pattern is a
convection current.
In the mantle, convection currents
are set in motion by the transfer of
energy between the Earth’s hot
interior and its cooler exterior.
Ridge Push / Slab Pull
 The upper part of the mantle, the asthenosphere, can flow like a soft plastic,
which ultimately drives plate movement.
 During formation of ocean ridges, mantle forces cause the asthenosphere to rise
and push oceanic plates toward trenches (ridge push)
 At subduction zones, a sinking region of a convection current could then suck an
oceanic plate downward into a subduction zone, wherein the trailing lithosphere is
ultimately pulled into the subduction zone (slab pull)
Plate Tectonics - Unanswered
Questions
Most scientists agree that convection currents in the mantle are
related to the movement of plates. Even today, scientists
seek answers to the following questions in regard to how
these currents originate:
1. Are mantle convections currents permanent features?
2.
3.
4.
5.
Do they shift their positions through geologic time?
How does a convection current start?
What causes the movement to stop?
Does convection only take place in the upper mantle, or do they occur in
the lower mantle as well?
6. Does the convection current cause subduction or vice versa?
7. What causes an upward convection current to form beneath a continent?