Transcript Document

Plate Tectonics
Mr. Litaker
Chapter 9
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Continents in Motion

Alfred Wegener was a
German meteorologist that
proposed the theory (1912)
that the Earth was not
fixed - it "moved" on
plates. Come to find, in the
1960s, Alfred Wegner´s
theory was proven correct.
This theory is known as
Continental Drift.
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Book written by Wegener

In 1915, Wegener published his
evidence and conclusions in a
now classic book, Die entstehung
der kontinente und ozeane (The
origin of continents and oceans).
Wegener proposed that all
modern continents were once
assembled together in a
supercontinent he named
Pangaea.
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Continental Drift

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Theory that the
continents had once
been part of one or
more landmasses
that had separated
and moved apart.
They are supposed
to have broken up
about 200 million
years ago.
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http://www.ucmp.berkeley.edu/geology/tectonics.html
http://www.ucmp.berkeley.edu/geology/anim1.html
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Pangaea – Means “ All Earth”

Wegener proposed that all continents had once
been joined in a supercontinent he named
Pangaea. Pangaea, he thought, had existed
from the primordial earth until the Mesozoic,
when it began to break up. Wegener at first
considered mantle convection as a possible
driving mechanism, but later rejected that in
favor of Tidal forces as the cause for
Pangaea's breakup and continental drift.
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Enchanted Learning site
http://search.netscape.com/ns/boomframe.jsp?query=the+origi
n+of+continents&page=1&offset=0&result_url=redir%3Fsrc%
3Dwebsearch%26requestId%3Dc5358d4649b76afb%26clicke
dItemRank%3D10%26userQuery%3Dthe%2Borigin%2Bof%2
Bcontinents%26clickedItemURN%3Dhttp%253A%252F%252
Fwww.enchantedlearning.com%252Fsubjects%252Fastronom
y%252Fplanets%252Fearth%252FContinents.shtml%26invoca
tionType%3D%26fromPage%3DNSCPResults%26amp%3BampTest%3D1
&remove_url=http%3A%2F%2Fwww.enchantedlearning.com
%2Fsubjects%2Fastronomy%2Fplanets%2Fearth%2FContinen
ts.shtml
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Panthalassa – Means “All
Seas”
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245 Million years ago: Pangaea
existed when some of the
earliest dinosaurs were roaming
around this big earth. The land
Pangaea was surrounded by a
sea called Panthalassa.
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Pangaea breaks up Forming:

Pangaea started to break up into two
smaller supercontinents, called Laurasia
and Gondwanaland, during the Jurassic
period. By the end of the Cretaceous
period, the continents were separating
into land masses that look like our
modern-day continents.
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3 continents associated
with Laurasia 
4 continents associated
with Gondwana 
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Evidence Supporting
Continental Drift
1.
Continental
Margins –
Giant
Jigsaw
Puzzle.
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These two widely separated continents seem to
resemble two separate pieces of a larger jigsaw
puzzle. North America, too, seems to mirror the
western coastline of Europe.
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Evidence Supporting
Continental Drift

Fossils of
Mesosaurus – A small
2.
reptile that lived 270 million
years ago that was found in
Eastern South America and
western Africa. Mesosaurus
lived near swamps and
rivers and it would have
been impossible for this
reptile to have swam across
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the Atlantic.
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Evidence of Continental Drift
3. Glossopteris Fern
Fossils
Fern fossils have
been found in Africa,
Australia, India, and
Antarctica.
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Evidence of Continental Drift
4.
Glacial Deposits
Evidence of Glacial Deposits was found
on 5 different continents.
Glacial striations on rocks show that glaciers
moved from Africa toward the Atlantic Ocean
and from the Atlantic Ocean onto South
America. Such glaciation is most likely if the
Atlantic Ocean were missing and the
continents joined.
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Evidence of Continental Drift
5.
Geologic Evidence
The age and type of rocks in the coastal
regions of widely separated areas, western
Africa & eastern Brazil, matched closely.
Mountain chains such as the Appalachians
and Scandanavian Mtns. Of Greenland and
Northern Eupope seem to fit closely in age
and structure.
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Wegener’s Theories Rejected
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Most scientist rejected Wegener’s theories --------Why ? ---------It was difficult to conceive of large
continents plowing through the sea floor to
move to new locations. What kind of forces
could be strong enough to move such large
masses of solid rock over such great
distances?
While in Wegener’s lifetime, he never found the
WHY to why the continents moved.
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
Wegener suggested that the continents
simply plowed through the ocean floor, but
Harold Jeffreys, a noted English geophysicist,
argued correctly that it was physically
impossible for a large mass of solid rock to
plow through the ocean floor without
breaking up. Recent evidence from ocean
floor exploration and other studies has
rekindled interest in Wegener's theory, and
lead to the development of the theory of
plate tectonics.
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The WHY is Answered

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The WHY came in 1967, when a group of
scientist set out to map the ocean floor along the
Mid-Atlantic Ridge. This was an undersea
mountain chain with a steep, narrow valley
running down its center. (65,000 km or 40,625
miles long)
Glomar Challanger – Sent out to collect rock
samples at the bottom of the sea for age dating.
Results – Seafloor = 70 to 150 m.y.a. and
Continent = 3 to 4 Billion.
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Total distance penetrated below sea floor 325,548 m
Total interval cored 170,043 m Total core recovered
and stored 97,056 m Overall core recovery 57%
Number of cores recovered 19,119 Number of sites
investigated 624 Deepest penetration beneath ocean
floor 1,741 m Maximum penetration into basaltic
earth crust 1,080 m Deepest water (Leg 60 Site
461A) 7,044 m Total distance traveled 375,632
nautical mi
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Intro to Plate Tectonic
Theory
Plate tectonics is the theory that Earth's outer layer is
made up of plates, which have moved throughout
Earth's history.
The theory explains the how and why behind
mountains, volcanoes, and earthquakes, as well as
how, long ago, similar animals could have lived at the
same time on what are now widely separated
continents.
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Definite Proof

But the purpose of the Glomar Challenger was
scientific exploration. One of the most important
discoveries was made during Leg 3. The crew
drilled 17 holes at 10 different sites along a
oceanic ridge between South America and Africa.
The core samples retrieved provided definitive
proof for continental drift and seafloor renewal at
rift zones. This confirmation of Alfred Wegener's
theory of continental drift strengthened the
proposal of a single, ancient land mass, which is
called Pangaea.
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Navy submarine commander
during World War II.
Harry Hess
Hess proposes sea-floor
spreading
1960
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Suggested that there was actually a break
at the center of the ridge known as a RIFT
ZONE.
Magma from deep within the Earth was
coming to the surface at this Rift Zone and
creating new sea floor.
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Earth’s Layers
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Earth: built by distinct layers:
inner core, outer core, mantle,
crust; the crust is the thinnest
layer
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Lithosphere
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The rigid blocks of the crust and
upper mantle that extend
downwards to about 100 km
deep.
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Asthenosphere
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Below 100 km a plastic like zone of
molten magma extends down to
about 200 km thick called the –
Asthenosphere.
The less dense Lithosphere floats on
the more dense Asthenosphere.
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END
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Types of Boundaries
1.
2.
3.
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Divergent
Convergent
Transform
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D i v e r g e n t Boundary
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A divergent boundary occurs where 2
plates are pulling apart.
The force associated with this is
called – Tension.
Example – Mid-Ocean ridges where
Sea Floor spreading is occuring.
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http://www.uwsp.edu/gEo/faculty/ritter/geog10
1/textbook/earth_materials_structure/ridge_div
ergence_video.html
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Convergent Boundary: 3 Types
A convergent boundary is a boundary
between 2 colliding plates. When 2
plates collide one plate may dive
under the other plate at a
Subduction Zone.
There are 3 types.
1. Ocean – Ocean Convergence
2. Ocean – Continent Convergence
3. Continent – Continent Convergence
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Force associated with
Convergent Boundaries
What type of force is
associated with convergent
boundaries?
Compression
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Ocean – Ocean Convergence
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When two oceanic plates collide creating
deep sea trenches.
Example: Marianas Trench the deepest
part of the Pacific Ocean near the coast of
Japan at at 11033 meters (36201 feet)
deep.
Magma rises to form volcanoes or Island
arcs on the ocean floor parallel to the
trench.
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Ocean to Continent
Convergence
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The Denser Oceanic plate descends into
the less dense Continental Crust and
SUBDUCTS its way down to the
Asthenosphere.
Volcanoes may form
Ex: The Andes in South America were
created when the Pacific plate ran into the
South American plate creating the Andes
Mountain Range.
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New crust is continually being pushed away
from divergent boundaries (where sea-floor
spreading occurs), increasing Earth's surface.
But the Earth isn't getting any bigger. What
happens, then, to keep the Earth the same size?
Subduction Zone
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In locations around the
world, ocean crust subducts,
or slides under, other pieces
of Earth's crust. The
boundary where the two
plates meet is called a
convergent boundary. Deep
trenches appear at these
boundaries, caused by the
oceanic plate bending
downward into the Earth.43
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Continent to Continent Conv.
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Continental rocks have low densities
in which neither is more dense than
the other. This causes them to
buckle up and create Mountains
when they collide.
Very little volcanic activity.
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Earthquakes are CoMmoN.
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Ex: Appalachian’s and Himalayas
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Convergent – Convergent boundary
When two land masses meet neither will slide under the other. Instead,
the two crush together at what is known as a convergent boundary.
They crumple and fold. Some pieces of land are thrust over or under
other pieces. The result is a mountain range.
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The Himalayas, the highest mountains in the world,
were created this way. (In fact, they're still growing.)
So were the European Alps.
Even the Appalachian Mountains formed when two land
masses came together. Although with the Appalachians, the
crushing ended long ago -- all that's left now are the eroded
remnants of a once high mountain range.
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Transform Fault Boundary
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Boundaries where plates are sliding
past one another in opposite
directions or in the same direction
but at different rates.
The Force associated with this
boundary is Shearing.
Ex: San Andreas Fault in California.
YOU GET EARTHQUAKES !!!
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Slippin' and a Slidin'
Transform boundaries neither create nor consume crust.
Rather, two plates move against each other, building up
tension, then releasing the tension in a sudden and often
violent jerk. This sudden jerk creates an earthquake.
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Convection Currents

Convection currents in the mantle appear to be
the driving force that provides the energy
necessary to move the Earth’s plates.
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Convection
is the process by which currents are driven by
temperature differences within fluid bodies.
When fluid is heated, it expands, lowering the
density of the heated material, causing it to rise
through the cooler fluid. As it rises, leaving the
vicinity of the heat source, in this case, hot
magma near the earth's surface, it will cool.
When it becomes more dense (because it is
cooler) than the surrounding fluid, it will begin
to sink. This behavior sets up circulation
currents in the mantle that cause plate
movement.
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Arthur Holmes
believed a fluid mantle possessed convection
currents created by heat trapped beneath the Earth's surface.
Holmes hypothesized that convection currents welled up toward
the surface and then drug continents across the surface.
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Paleomagnetism
based on the principle that magnetic particles will
align themselves with the Earth's magnetic field
is a technique used to cross date ocean cores and to
establish major intervals over wide areas. By
measuring the polarity of the magnetism in samples it
is possible to determine the layer in which the Earth’s
polarity reverses.
Between periods of normal and reversed polarity.
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http://www.pbs.org/wgbh/aso/tryit/tectonics/divergent.html
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Making of a Mountain?
1. How tall is Mt. Everest (feet)?
2. How many people have reached the top?
3. A Syncline is a downward fold in a rock. What is an
Anticline?
4. How many Himalayan peaks are there?
5. What is Orogenesis?
6. When did India hit Asia?
7. How thick is the Lithosphere
8. How many major plates are there?
9. What sea once separated India from Asia?
10. How much are the Himalayas rising each year?
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Answers
1.
2.
29,035 ft
1500 or 1502 people
3.
An upward fold
4.
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5.
Mountain Building
6.
40 mya
7.
100 Km or 62.1 mi.
8.
15 slabs
9.
Tethys Sea
10. 5 mm/yr or .2 in.’s
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