Transcript plate_tectonics

by Alexandra H., Brooke T., help from Tom
L., Tim D., and Mr. Brown
Edited by Mr. Ledoux
Theory of Plate Tectonics
Earth’s crust is
composed of large
plates
These plates float on
the mantle below
These plates have
always moved (slowly)
This movement shapes
our Earth’s surface
What it Explains
The theory explains the
how and why of…
Mountains
Volcanoes
Earthquakes
Similar prehistoric
animals lived at the
same time on now widely
separated continents
History of Plate Tectonics
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Continental drift was originally proposed by
Alfred Wegener, a German meteorologist, in 1912
His idea was supported by:
The fit of the continents
The distribution of fossils
Similar rock sequences
Ancient climates
The apparent wandering of the Earth's polar
regions
Pangaea
Wegener used his
observations to
hypothesize that all
of the present-day
continents were once
part of a single
supercontinent called
Pangaea.
Species
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Fossils of the same species
were found on several
different continents.
Wegener proposed that the
species dispersed when the
continents were connected
and later carried to their
present positions as the
continents drifted apart.
Rock Sequences
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Rock sequences in
South America,
Africa, India,
Antarctica, and
Australia are very
similar.
Same three layers,
same order, in areas
now separated by
great distances
Why?
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Correct!
He proposed that
the rock layers were
made when all the
continents were part
of Pangaea.
Drifting Magnetic Poles
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Wegener did not
believe it possible that
the magnetic poles
could move far from
the Earth’s poles.
So the apparent drift
over time could be
explained by the
continents moving.
Problems with Wegener’s
Theory
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Wegener’s Theory was
not accepted by most
geologists.
They said that ocean
currents or winds could
scatter the ancient
plants and animals.
They also thought that
maybe it was possible
for the poles to wander.
The Big Problem
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Wegener could not
explain how the
continents could
move.
He couldn’t explain
what forces could
actually cause the
continents to move.
The Big Solution
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Arthur Holmes, (Scottish geologist) made this proposal 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.
Unfortunately, Wegener died in 1930
Theory of Plate Tectonics
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Totally revolutionized
our understanding of
the Earth, past and
present.
Theory ranks with the
theories on evolution,
relativity, the Big
Bang, and Newton’s
Laws.
The Ocean Floor
Landforms on the ocean floor are as
diverse as those on land
These landforms are also formed by
moving plates
Ocean Floor Cont.
The ocean floor, just like land, has
mountains, volcanoes, fault lines, ridges,
valleys, and many other landforms.
The Break Up of Pangaea
Pangaea was
the large landmass
that separated to
form the continents
millions of years
ago
Scientific Basic Facts
CONTINENTAL CRUST
Less Dense (lighter weight): average density = 2.7 gm per cubic
centimeter
Thicker: 0 to 40 km, average 35 km under high mountains, the crust
can be thickened, up to 65 km. Under the craton, the average
thickness is 30 km.
Mineral Composition: granitic (felsic) rocks such as granodiorite and
granite enriched in Potassium (K), Sodium (Na), Aluminum (Al), and
Silica (Si)
"Floats" on top of the Mantle
Oceanic Crust
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OCEANIC CRUST
More dense (heavier): average density = 3.0 gm per cubic
centimeter
Thinner: 0 to 10 km, average 5 km, thinnest at Mid-Ocean Ridges
Mineral Composition: mafic rocks such as basalt and gabbro
enriched in Magnesium (Mg) and Iron (Fe)
Underlies Ocean Basins
Also “floats” on the mantle
Convection Currents Inside the
Earth
The tectonic plates float on the magma that lies
in the core of the earth
This magma is just one big convection current
Earth’s Plates
About Earth’s Plates
The different plates were formed when Pangaea
broke up.
In all there are 14 (depending on your source)
Continental Drift
Continental Drift Continued
Continental drift means that the plates
move across the magma.
It happens very slowly…2 inches a year is
considered very fast!
4 types of plate boundaries:
1.
2.
3.
4.
Divergent boundary -- new crust is created
as the plates pull away from each other
Convergent boundary -- crust is destroyed as
one plate dives under another
Transform boundary -- crust is neither
produced nor destroyed as the plates slide
horizontally past each other
Plate boundary zones -- broad belts in which
boundaries are not well defined and the effects
of plate interaction are unclear
Transform Boundary
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Two plates slip past
each other, moving in
opposite directions
Edges of crust is not
damaged.
Earthquakes often
occur
Example: San
Andreas fault in
California
Divergent Boundaries
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Plates are moving away
from each other
Magma from below is
pushing the plates
apart. This creates
new crust.
Creates mid-ocean
ridges and rift valleys
The rift valley in east
Africa is about 3,000
km long.
Rifting
Convergent Boundaries
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Two plates crash
together
More dense plate
slides under the less
dense plate.
Old crust is recycled
back into the mantle.
Creates:
A trench (at least 1
oceanic plate)
Mountain ranges (2
continental plates)
Subduction
Oceanic Subduction
Continental Subduction
Mountain Building
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Convergent
boundaries can also
cause mountains to
be built up by the
crust folding.
Himalayan Mountains
are formed by India
pushing up into Asia
Borders
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This is where all the action is:
Volcanoes
Earthquakes
Plate Boundaries
Hot Spots
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Plates move over an
area where magma is
close to the surface.
Volcanoes occur at
the same place, but
because the plate is
moving, a chain of
volcanoes can occur.
Hawaiian Islands
were formed this way.
Earth’s Crust
Future Earth
What do you think the world will look like in
a million years???
 It is predicted that the continents will
eventually crash together on the other side
of the world, creating a new Pangaea.
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Future World
Bibliography
http://www.enchantedlearning.com/subject
s/astronomy/planets/earth/Continents.sht
ml
http://www.platetectonics.com/
http://observe.arc.nasa.gov/nasa/earth/tect
onics/GRAPHICS/plate_opening.jpg