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Mt. Fuji, Japan
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.
The Theory of Plate Tectonics
Wegener believed all continents were joined together.
Supercontinent of Pangaea existed about 200 myBP.
Pangaea covered 40% of the Earth’s surface.
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.
The Theory of Plate Tectonics
Wegener first published his theory 1912.
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.
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.
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.
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.
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.
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 1970’s.
Why?
Wegener could not explain how the continents drift.
He could provide no mechanism for the theory.
The Theory of Plate Tectonics
Captain Harry Hammond Hess
(later Professor of Geology at Princeton)
Believed in Wegener’s 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 1960’s
Led to the discovery of
Oceanic Ridges and
Trenches.
Still no explanation.
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.
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 Earth’s 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.
The Theory of Plate Tectonics
Earth’s Lithospheric Plates
The Earth's surface is broken into ~15 crustal plates
The Theory of Plate Tectonics
Earth’s 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.
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.
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.
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
The Theory of Plate Tectonics
Types of Plate Boundaries
convergent
divergent convergent
transform
hot spot
divergent
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
BRUCE visits the Mid-Atlantic Ridge
The Theory of Plate Tectonics
Divergent Plate Boundaries
East Pacific Rise
“Alvin!”
ALVIN visits the East Pacific Rise in
January, 2002
East Pacific Rise Sea Floor at 3000 m
East Pacific Rise
Black Smokers
Divergent Boundaries
The East-African Rift Zone
Oldoinyo Lengai erupting