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Plate Tectonics
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Continental Drift
A.
B.
Idea that individual continents could shift
position on the globe
Early Scientists
1.
2.
3.
4.
Sir Frances Bacon- 1620 – noticed continents
had similar coastlines
Antonio Snider- 200 years later- did fossil
correlations and similar coastlines
Eduard Suess- 1885- said similar geological
formations in the southern hemisphere could
have been a single land mass called
Gondwanaland
Alfred Wegener- attempted to explain all of the
above and proposed the Theory of Continental
Drift in 1912.
c. Wegener’s Hypothesis and support
1. Fossil evidence
a. Paleontologist found fossil remains of both
plants (seed fern- Glossopteris) and animal
(reptile- Mesosaurs)
b. fossils occurred over limited areas on several
different continents (South America, India,
Aftrica, Australia, and Antarctica)
c. Wegener suggests organisms flourished
when continents were together; continents split
after fossils were entombed in rock.
2. Climate evidence
a. many factors are used to determine an area’s
climate; the most prevalent one is latitude
b. equatorial regions- warmest; polar areascoldest
c. sedimentary rocks preserve evidence of
climatic conditions
d. Wegener’s interpretation was that rocks
indicating cold conditions were depositd when the
continent was near a pole and vice versa for
warm areas.
Changes were the result of continent’s drifting in
and out of different latitudes.
3. Jigsaw Puzzle
a. Land masses looked as though they would fit
together in one large continent (Pangaea)
b. Laurasia- continent in northern hemisphere
which includes Europe, Asia, and North America
c. Gondwanaland- continent in the southern
hemisphere which includes Australia, Antarctica,
India, Africa and South America
d. Ocean surrounding Pangaea was called
Panthalassa
e. Pangaea split 200-225 million years ago
4. This hypothesis did not gain much
support from the scientific community for
many years; however, this theory is
considered quite logical and led to the
theories of sea floor spreading and plate
tectonics
II. Sea Floor Spreading
A. Proposed by Harry Hess and Robert
Dietz in the mid-1960’s
B. Suggests that the sea floor itself is
moving as a result of convection currents
1. moves live a conveyor belt away from
the crest of mid-oceanic ridges
2. moves as a rate of 1-6 cm/yr
C. Driving force is convection currents
(temperature and density)
D. Support for Sea Floor Spreading
1. youngest sea floor should be at the crest of
the ridge with the age of the sea floor getting
progressively older as you move toward the
continents (oldest rock on the ocean floor is
approximately 200 million years old)
2. If mid-ocean ridges were spreading
centers, they should be hot. They are!
3. Ocean floor should be denser as it
moves away from the ridge so the ocean
should be deeper farther from the
spreading center. It is!
4. Must have subduction zones to balance
the spreading.
Evidence of this came from Wadati- Benioff
zones. Zones of concentrated earthquakes
(both subduction zones and spreading
centers)
5. Lack of sediment at ridge.
6. Paleomagnetism (Vine – Matthews
hypothesis)
A. When magma solidifies, the metals
align with the magnetic field (they point
north). When they cool this is frozen in the
rock like a magnetic fossil. Show new
material being pushed away from ridges.
B. Magnetic Anomalies or stripes at the mid-ocean
ridge are mirror images of each side of the ridge.
As magma rises and solidifies it records the
magnetic field (which reverses once every
500,000 years). As the sea floor spreads the new
rock at the ridge is split into two. A pattern of
magnetic anomalies on one side of the ridge is a
mirror on the other side.
C. Rate of mothion can be determined by taking
Distance from ridge/age of the rock
D. Mantle plumes also tell us direction of plate
movement (Hawaiian Islands).
III. Plate Tectonics
A. The surface of the Earth is actually
composed of a series of rigid plates that
are all moving relative to each other.
B. Plate – a large mobile slab of rock that is
part of the earth’s surface
1. includes all of the lithosphere (both
oceanic and continental crust)
2. floats on top of the asthenosphere
C. Lithosphere- is a rigid outer layer approximately
100 km thick. It includes rocks of the earth’s crust
and uppermost portion of the mantle.
a. made up of 6 large plates and approximately
20 small plates
D. Asthenosphere- is a zone approximately 150
km thick and behaves plastically (solid that flows)
because of extreme temperature.
1. This layer acts as a lubricating fluid allowing
the plates to move.
2. Convection currents – the mantle close to
the hot outer core heats, becomes less dense and
rises. As it rises, it spreads and cools. In cooling it
becomes denser and sinks. This carries the
plates. The mantle loses heat at volcanic
eruptions, hot spots, and by seawater. Mantle
sinks at trenches.
E. Subduction – the sliding of sea floor beneath a
continent or island arc
1. If the plate is made up of mostly sea floor it
can be subducted down into mantle forming an
ocean trench
2. If the leading edge of the palte is made of
continental rock, the plate will not subduct. (too
bouyant)
3. The amount of new crust formed by sea
floor spreading is essentially balanced by material
consumed by subducting, folding or faulting of
rocks when two plates collide.
IV. Plate Boundaries
A. Divergent boundaries – plates move
apart <---->
1. Magma wells up from asthenosphere
and new lithosphere is created
2. Ex. Mid Atlantic Ridge
B. Transform boundaries – transform are moving
horizontally past one another
1. Ex. San Andreas Fault
C. Convergent boundaries – plates are moving
toward each other
1. continent and continent results in folding,
faulting, and mountains being formed – ex.
Himalayas
2. ocean and ocean results in island arcs and
trenches
3. Continental and oceanic results in
subduction, deep sea trenches and folded
mountain ranges on coast – ex. Andes Mountains
V. Why do plates move?
A. Convection cells – hot magma rises at
spreading ridges; some escapes to form
new lithosphere, but the rest flows outwrd
and drags the overlying lithosphere with it
B. Subduction zones – the weight of the
dense, cold, down going slab of lithosphere
is the subduction zone pulls the rest of the
trailing plate