Mt. Fuji, Japan

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Mt. Fuji, Japan
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The Theory of Plate Tectonics
Steps in Development
1. Benjamin Franklin (late 1700s) Recognized
that crust of Earth was a shell. Surface could
be broken and parts moved about.
2. Alfred Wegener (1912) German
meteorologist-geophysicist Proposed theory of
Continental Drift. Proposed continents float on
a denser underlying interior of the
Earth. CONTINENTS periodically break up and
DRIFT apart.
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The Theory of Plate Tectonics
Wegener believed all continents were joined
together. Supercontinent of Pangaea existed about
200 myBP. Pangaea covered 40 of the Earths
surface.
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The Theory of Plate Tectonics
Majority of Pangaea was in Southern
Hemisphere. Pangaea was surrounded by a single
ocean, the Panthalassic Sea. Pangaea broke up
180 million years ago.
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The Theory of Plate Tectonics
Wegener first published his theory 1912.
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The Theory of Plate Tectonics
Evidence in support of Continental Drift
1. Continental Fit Sir Francis Bacon
(1620) noted that the continents might fit
together. Made observation after seeing some
newly made maps.
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The Theory of Plate Tectonics
2. Habitats of Modern Organisms
Hippopotamus found in Africa and
Madagascar. Marsupials in Australia. Indicate
some migration and evolution took place before
and after drift began.
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The Theory of Plate Tectonics
3. Fossil Record Wegener used the fossil
record. Found fossils of plants and animals
that were found on several continents.
Included animals, Cynognathus, Lystrosaurus,
Mesosaurus, and plants Glossopteris.
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The Theory of Plate Tectonics
4. Similar Rock Types Across Ocean
Basins Mountains of Northern Hemisphere similar
in Greenland, NA, and Europe. Also similar rocks
between South America and Africa.
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The Theory of Plate Tectonics
5. Ancient Climates Glacial striations found in
India, Australia, South America and
Africa. Radiate from a point in southern
Africa. Also coal deposits found in presently
cold regions, such as Norway.
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The Theory of Plate Tectonics
Wegener seemed to have a great idea, and lots of
supporing evidence. However, Plate Tectonics was
not widely accepted until the early 1970s.
Why?
Wegener could not explain how the continents
drift. He could provide no mechanism for the
theory.
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The Theory of Plate Tectonics
Captain Harry Hammond Hess
(later Professor of Geology at Princeton)
Believed in Wegeners hypothesis
Conducted echo-soundings of the oceans during WWII
Mapped the mid-ocean ridges (3000 m high and 2000
m wide)
Located deep-sea trenches (10,000 m deep)
associated with large continental mountain belts
(the Andes) and island arcs (Aleutians, Japan)
Hess presented sea-floor spreading as a mechanism
in 1960s
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Led to the discovery of Oceanic Ridges and
Trenches. Still no explanation.
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Research vessels found alternating bands of rock
on the sea floor on either side of
the ridges. Rocks possessed iron that
indicated magnetic field reversals.
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The Theory of Plate Tectonics
Basic Plate Tectonics Concepts
1. The outer portion of the Earth, the
lithosphere (the upper mantle and crust), is
composed of rigid units called plates. 2.
Plates move slowly. 3. Most of the Earths
large-scale geologic activity, such
as earthquakes and volcanic eruptions, occur at
or near plate boundaries. 4. Interiors of
plates are geologically quiet, with fewer and
usually milder earthquakes and fewer volcanoes
than at plate margins.
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The Theory of Plate Tectonics
Earths Lithospheric Plates
The Earth's surface is broken into 15 crustal
plates
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The Theory of Plate Tectonics
Earths Lithospheric Plates
Continental plates are composite. Include both
continental and oceanic crust. North American
Plate - continental portions are thicker,
lower density lithosphere. - 120 km thick
in continental regions. - oceanic portions
are thinner, higher density lithosphere.
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The Theory of Plate Tectonics
When plates move, everything on them moves with
them. North American Plate moving west, Eurasian
Plate moving east. Rate is about 5-10 cm/yr. Over
long periods of time, movement is large.
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The Theory of Plate Tectonics
So. Has the size of the Earth increased? Why?
Where plates move apart or diverge, at some other
place on the Earth, they must come together
or converge. This is where the Earth gets
exciting.
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The Theory of Plate Tectonics
The tectonic plates are continually moving
Divergent (plates move away from each other)
Convergent (plates collide with each other)
Transform (plates move past each other)
Convection currents in the mantle drive plate
movements
The source of heat is radioactivity deep in the
Earth
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The Theory of Plate Tectonics
Types of Plate Boundaries
convergent
convergent
divergent
divergent
transform
hot spot
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The Theory of Plate Tectonics
Divergent Plate Boundaries
Occur along spreading centers where plates are
moving apart
The Mid-Atlantic Ridge
Spreading at 2.5 cm/yr
You are here
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BRUCE visits the Mid-Atlantic Ridge
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The Theory of Plate Tectonics
Divergent Plate Boundaries
East Pacific Rise
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Alvin!
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ALVIN visits the East Pacific Rise in January,
2002
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East Pacific Rise Sea Floor at 3000 m
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East Pacific Rise Black Smokers
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Divergent Boundaries
The East-African Rift Zone
Oldoinyo Lengai erupting
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