Transcript Theory of plate tectonics - 8th Grade Social Studies

PACIFIC NORTHWEST
& IDAHO HISTORY
This Dynamic Earth:
the Story of Plate
Tectonics
(Information accessed and summarized from USGS – U.S. Geological Survey at
http://pubs.usgs.gov/publications/text/dynamic.html)
Fall 2007-Spring 2008
Mrs. Angie Bailey
Theory of plate tectonics
Emerged in the early 1960’s
 Influence on nearly all geologic processes
 While the theory is widely accepted, many
aspects are still being explored
 Geological definitions:
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– Plate is a large, rigid slab of solid rock
– Tectonics from the Greek root “to build”

Theory states the Earth’s outermost layer
is made up of large and small plates that
move against each other
History of Theories

“Catastrophism” – the belief that changes were
sudden and caused by a series of catastrophes.
– Until the 1700’s
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“Uniformitarianism” – based on “Uniformitarian
Principle” proposed by James Hutton in 1785
which believed the present is the key to the past,
meaning the geological forces and processes
acting today are the same as those that have
acted in the geological past
– Mid-19th century to early 1900’s
Wegner and “Continental Drift”
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“Continental Drift” – In 1912, Alfred Wegener published articles
stating the supercontinent Pangaea began to split apart into two
continents
– Laurasia
– Gondwanaland
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They then split further apart into the continents that exist today
In 1596, Abraham Ortelius first noticed the way the Americas
were “torn away from Europe and Africa…by earthquakes and
floods”
Wegener also noticed the remarkable fit of the South American
and African continents
Wegener was intrigued by the fossil species and unusual geologic
structures found in both S. America and Africa
Further evidence existed in the discovery of glacial deposits in
Africa and tropical plant fossils in Antarctica
At the time, Wegener could not explain what force would be
strong enough to move these land masses
New discoveries and evidence lead to further develop Wegner’s
theory into the development of the theory of plate tectonics
Inside the Earth
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Four Main Layers
– Lithosphere
– Crust
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The outermost layer, which is rigid and very thin compared to the other two
Beneath the oceans the crust is generally only 5 km.
Beneath continents it averages about 30 km
Under large mountain ranges, can be as deep as 100 km
The crust is brittle and can break.
– Mantle
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A dense, hot layer of semi-solid rock approximately 2,900 km thick
Contains a great deal of iron, magnesium, and calcium
Temperature and pressure inside the Earth increase with depth making the
mantle hotter and denser than the crust.
– Core
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Is nearly 2x as dense as the mantle because its composition is metallic (iron
and nickel alloy)
Has 2 distinct parts
– A 2,200 km thick liquid outer core
– A 1,250 km thick solid inner core
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As the Earth rotates, the liquid outer core spins, creating the Earth’s
magnetic field
Developing plate tectonic theory:
Four Major Scientific Developments
1.
2.
3.
4.
Demonstration of the ruggedness and
youth of the ocean floor
Confirmation of repeated reversals of the
Earth magnetic field in the geologic past
Emergence of the seafloor-spreading
hypothesis and associated recycling of
oceanic crust
Precise documentation that the world’s
earthquake and volcanic activity is
concentrated along oceanic trenches and
submarine mountain ranges
Ocean Floor Mapping
Most of the geologic processes occurring
on land are linked directly or indirectly to
the dynamics of the ocean floor
 Did you know there are underwater
mountains?
 In fact, the global mid-ocean ridge
mountain chain (4.500 m average)
overshadows all the mountains in the U.S.
except for Mt. McKinley (Denali) in Alaska
(6,194 m)
 It is the most prominent topographic
feature on the surface of our planet.
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Seafloor spreading
 New
magma from deep within the
Earth rises easily through weak
zones and eventually erupts along
the crest of the ridges to create new
oceanic crust.
 Oceanic crust appreciated as a
natural “tape recorder.”
How can new crust be continuously added
along the oceanic ridges without increasing
the size of the Earth?
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Harry H. Hess and Robert S. Dietz suggested new
oceanic crust continuously spread away from the
ridges in a conveyor belt like motion eventually
descending into the oceanic trenches
As old oceanic crust was consumed in the
trenches, new magma rose and erupted along the
spreading ridges to form new crust
In effect, ocean basins are perpetually being
“recycled”, thus Earth does not get bigger with
sea floor spreading
there is little sediment accumulated on the ocean
floor, and oceanic rocks are much younger than
continental rocks
How Seafloor spreading works: 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.
 With seafloor spreading, the continents
did not have to push through the ocean
floor but were carried along as the ocean
floor spread from the ridges (answered
the question that plagued Wegener’s
theory)
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Earthquakes
During the 20th century, scientists realized
earthquakes tend to be concentrated in
certain areas – most notably along the
oceanic trenches and spreading ridges
 The connection between earthquakes and
oceanic trenches is significant because it
helps confirm the seafloor-spreading
hypothesis by pin-pointing the zones
where Hess had predicted oceanic crust is
being generated (along the ridges) and
the zones where oceanic lithosphere sinks
back into the mantle (beneath the
trenches).
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4 Types of Plate Boundaries
1.
2.
3.
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Divergent boundaries – where new
crust is generated as the plates pull
away from each other
Convergent boundaries – where crust
is destroyed as one plate dives under
another
Transform boundaries – where 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
Divergent Boundaries
 Best
known: Mid-Atlantic Ridge
–Submerged mountain range from
the Arctic Ocean to beyond the
southern tip of Africa
 Plates are moving apart and new
crust is created by magma pushing
up from the mantle
The Mid-Atlantic
Ridge, which splits
nearly the entire
Atlantic Ocean north
to south, is probably
the best-known and
most-studied
example of a
divergent-plate
boundary.
(Illustration adapted
from the map This
Dynamic Planet.)
Convergent boundaries
 Destruction
or recycling of the crust
takes place along convergent
boundaries
– Plates are moving toward each other
and sometimes one plate sinks (is
subducted) under another
 Can
occur between oceanic and
continental plates or between two
oceanic plates or two continental
plates
Oceanic-continental convergence
Sustains many of the Earth’s active
volcanoes – eruptive activity is
clearly associated with subduction
 The
most seismically and volcanically
active zone in the world is known as
the “Ring of Fire”
 Notice
“Challenger Deep” on the map
Oceanic-oceanic convergence
Trenches are formed via this process
 The Marianas Trench marks where the
fast-moving Pacific Plate converges
against the slower moving Philippine Plate
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– Challenger Deep, at the southern end of the
Marianas Trench, plunges deeper into the
Earth’s interior (nearly 11,000 m) than Mount
Everest rises above sea level (8,854 m)
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Also result in the formation of volcanoes
Continental-continental
convergence
 The
Himalayan mountain range
dramatically demonstrates one of the
most visible and spectacular
consequences of plate tectonics
 Continental rocks are relatively light,
so the crust buckles and is pushed
upward or sideways
 The Himalayas, towering as high as
8,854 m above sea level, form the
highest continental mountains in the
world
Transform boundaries
 The
zone between two plates sliding
horizontally past one another is
called a transform-fault boundary
or simply a transform boundary
 San Andreas fault zone in California
is an example of one of the few that
occur on land
What drives the plates?
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Until the 1990’s, most scientists emphasized
mantle convection via seafloor spreading - the
slow movement of hot, softened mantle that lies
below the rigid plates
Both the Earth’s surface AND it’s interior are in
motion
The gravity controlled sinking of a cold, denser
oceanic slab into the subduction zone (called
“slab pull”) –dragging the rest of the plate along
with it – is not considered to be the driving force
of plate tectonics
The details of why and how plates move will
continue to challenge scientists into the future.
Left: Conceptual drawing of
assumed convection cells in the
mantle (see text). Below a depth
of about 700 km, the descending
slab begins to soften and flow,
losing its form. Below: Sketch
showing convection cells
commonly seen in boiling water
or soup. This analogy, however,
does not take into account the
huge differences in the size and
the flow rates of these cells.
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Volcanic eruptions
Linked to plate-tectonic processes
 ¾ of all lava erupted on Earth takes place
beneath the ocean
 Subduction-zone volcanoes like Mount St.
Helens are called composite cones and
typically erupt with explosive force
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– Magma is too stiff to allow easy escape of
volcanic gases
– Tremendous internal pressures mount as the
trapped gases expand during ascent
What does it mean to people of the
Pacific Northwest?
Snow-clad Mt. Rainier, a 4,392 m-high volcano built by
plate-tectonic processes, dominates the pastoral scene
around Orting, Washington. This valley is an inviting place
for people to live, work, and play, but it is also highly
vulnerable to destructive mudflows that could be generated
by renewed eruptive activity at Mt. Rainier. Society must
learn to "co-exist" intelligently with active volcanoes.
(Photograph by David E. Wieprecht, USGS.)
Geological Impact - PNW
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Pacific Ocean coastline receded 300 miles
westward
Tectonic plates collided
Erosion caused by numerous alpine glaciers and
massive continental glacier ice sheets
Violent volcanic eruptions
Ongoing mountain building
The physical features of the PNW are much
younger than the rest of the world – some
features may only be a few hundred or a few
thousand years old
Fertile and productive soil
Natural resources provided
 Concentrated
near past or present
plate boundaries
– Fertile soils
– Ore deposits
– Fossil fuels
– Geothermal energy