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Plate Tectonics

Compiled by

Susan Weidkamp

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Presentation Objectives:

Describe how continents have moved over the past

250 million years.

Explain evidence supporting the Theory of Plate

Tectonics.

Explain the Physics of Plate Movement.

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The world is like a jigsaw puzzle

Once people looked at a map of the world, they noticed the land fit together like jigsaw puzzles pieces.

Australia

Africa

Antarctic

a

Greenlan

d

South

America

Europ

e

AsiaNorth

America

Central

America

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Moving Continents? No Way!

The land is solid, what forces moved the land

into the current pattern? Ask Alfred Wegener.

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Alfred Wegener

He was the meteorologist

who collected puzzle

evidence in the early

1900’s. In 1912 proposed

the first scientific theory of

continental drift known as

the Theory of Plate

Tectonics.

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Pangaea the Supercontinent

Alfred Wegener proposed that the continents were once gathered together in one large massive form he named Pangaea

Permian 225 million years ago

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Vocabulary

Plate tectonics- Tectonics comes from the Greek root meaning “to build.”

Pangaea (pan JEE uh) Alfred Wegener’s proposed supercontinent composed of all the world’s land masses.

Glossopteris- ancient fern-like plant found in fossil remains in South America, Africa India, Australia, and Antarctica.

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Moving Continents- Ancient Time

Computer modeling

was used to predict

placement of plates

over time

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Dinosaur Days

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Present Day

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Hola Tierra! ¿Como estás? Su cara ha cambiado con el

movimiento de la Tierra y yo no lo reconozco a usted.

Fossils of the same species were found on

several different continents.

Wegener’s Fossil Evidence

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Evidence from the Rocks

Similar layers of rock

were formed in

Antarctica, Australia,

South America, Africa

and India before

Pangaea broke apart.

Glossopteris fossils were

found in the rocks on

each continent.

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Solving the Fossil Puzzle

Wegener proposed that the species dispersed when the continents were connected.

Later as the continents drifted they were carried to their present positions.

Glacier Evidence

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Running the Movie Backwards!

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Glacier Analysis and Theory

Wegener proposed that the

continents were adjacent to

each other during the

glacial event.

Therefore, glaciers spread

over a much smaller area

in the southern hemisphere

and probably did not

influence the climate of the

northern hemisphere.

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Did continents move? Do you see a pattern?

Using the distribution of rock

types, Wegener reconstructed

the distribution of climates

zones at specific times in the

geologic past.

Climates are parallel to

equator, and relative to poles.

What about ancient times?

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Did the poles move? He found that, unlike the present

distribution, in which zones parallel the equator, the past zones occupied very different positions.

This implies that the rotational pole was in very different locations relative to today.

Wegener proposed an alternative interpretation. He believed that the climate zones remained stationary and the continents drifted to different locations.

The drift of the continents caused the apparent movement of the climate zones.

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Still Moving?

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Evidence from space- Still Moving!

A satellite view of the Sinai shows two arms of the Red Seaspreading ridge, exposed on land.

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For Review

What was the first clue that gave people the idea of

plate tectonics?

Who first proposed the theory?

Describe the evidence used to support the

theory of plate tectonics.

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Problems with Wegener's Model of

Continental Drift

Wegener's model was not accepted by all

geologists. Some thought that dispersion by

winds or ocean currents could explain the

distribution of fossil species.

Other geologists thought the poles might wander

and continents remain stationary.

Many geologists thought Wegener's evidence

was insufficient.

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Problems

The greatest shortcoming, at least in the eyes of

American geologists, was the lack of an adequate

mechanism for moving the continents.

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• Wegener proposed that the Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges.

• Geologists at that time understood enough about the strength of rocks to know that this was highly unlikely.

• Wegener's work was largely unaccepted in the northern hemisphere.

• In the southern hemisphere, where geologists were familiar with the rocks that Wegener used to support his hypothesis, continental drift was generally accepted.

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A mechanism to move continents was proposed by Arthur

Holmes, Scottish geologist in 1928.

He believed heat trapped in the Earth caused convection

currents, areas where fluids beneath the Earth's crust rise, flow

laterally, and then fall.

The currents would rise beneath continents, spread laterally,

then plunge beneath the oceans.

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The Physics of Plate Movement

Sea-Floor Spreading

Harry Hammond Hess, professor of

geology at Princeton University

Was very influential in setting the stage for the emerging plate-tectonics theory in the early 1960s.

Believed in the evidence Wegener used to defend his theory of continental drift.

Had very different views about large-scale movements of the Earth.

Sea-Floor Spreading

In 1960 Henry Hess proposed the theory of Sea-Floor Spreading.

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History of Ocean Basins In 1962 Hess published a

paper titled "History of Ocean Basins," which was one of the most important contributions in the development of plate tectonics.

He explained how molten rock (magma) oozes up from the Earth's interior along the mid-oceanic ridges, creating new seafloor that spreads away from the active ridge crest and, eventually, sinks into the deep oceanic trenches.

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If the oceans have existed for at least 4

billion years, as most geologists

believed, why is there so little sediment

deposited on the ocean floor?

Answer: The sediment has been

accumulating for about 300 million years

at most. This interval is approximately

the time needed for the ocean floor to

move from the ridge crest to the trenches,

where oceanic crust descends into the

trench and is destroyed.

Hess’ concept explains puzzles

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A ridge is

created

where the

molten rock

rises.

A trench

forms where

the molten

rock sinks

back into

the mantle.

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How old are the oldest sea floor

fossils?The sea floor is constantly being

recycled. This explains why the

oldest fossils found on the seafloor

are no more than about 180 million

years old.

In contrast, marine fossils in rock

strata on land -- some of which are

found high in the Himalayas, over

8,500 m above sea level -- can be

considerably older.

Magnetic Pole Reversal

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Magnetic

clues to solve

the mystery of

ridge

formation

were left in

the newly

formed rock.

Pole reversal

occurs

periodically.

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Core sampling has determined the pattern is symmetrical.

The Mid-Atlantic Ridge is 6,500 feet (2,000 m) above the

adjacent sea floor, which is at a depth of about 20,000 feet

(6,000 m) below sea level.

Mid-Atlantic Ridge

emerges as the South

American and African

plates pull apart at the

"slow" rate of

approximately 30

millimeters per year.

The axis of the ridge is

marked by a 2 km deep

rift valley, which is

typical of most slow-

spreading ridges.

The map reveals a 12 km jog of the rift valley, a second-order

discontinuity, and also shows a first-order discontinuity called

the Cox transform fault. Colors indicated depths of from 1900

(pink) to 4200 meters (dark blue).

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Theory of Plate Tectonics

The entire lithosphere

of the Earth is divided

into sections called

“plates.”

The plates are moving

slowly.

Plates move at

different rates and in

different directions.

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Plate Boundaries

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Plate Structure

Continental crust is present either in middle or

end of continental plate.

Continental plate has both continental and

oceanic crust. (Land is mixed)

Oceanic plate is entirely oceanic crust. (Oceans

are pure)

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Earthquakes occur along plate boundaries.

Plates can include land and ocean so they are

not the same size as a continent.

Ring of Fire

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The Physics of Plate Movement

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Vocabulary II

Convergent Boundary- Two plates slide toward

each other.

Divergent Boundary- Two plates slide apart

creating a gap where new plate is made.

Transform Boundary- Two plates slip past each

other.

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The Juan de Fuca

ridge off the

Oregon Coast .

What kind of fault

is the San Andreas

Fault?

Convergent

Divergent

Transverse

Plate Boundaries

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Juan de Fuca Plate Subduction

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When a less dense, lighter

(continental) plate overrides

a more dense (oceanic)

plate a subduction zone

forms.

Because the oceanic plate is

bent and driven down, a

deep trench forms at this

collision point.

Trench Formation

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Volcanic Activity

Why do volcanoes form near trenches?

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As the mantle rocks melt they form magma which is

less dense than the surrounding mantle material so it

will rise. Pressure in the magma cracks the overlying

rocks. Then the magma injects into the crack.

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A volcano will form if the magma reaches the

surface. When magma does reach the surface it is

then called lava.

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References

John N. Louie, Phd. Plate Tectonics, the Cause of Earthquakes.

http://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-

tectonics.html May 2, 2001

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/intr

oduction.html

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/par

t5.html

Geology: Plate Tectonics.

http://www.ucmp.berkeley.edu/geology/tectonics.html

http://pubs.usgs.gov/publications/text/HHH.html