Transcript Presentation

Forces that Shape the
Earth
State Correlation 4b
Plate Tectonics
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1912 – German scientist Alfred Wegener
Proposed two ideas that are known as
Continental Drift Theory
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Earth’s continents were once joined in a single
large landmass called Pangaea that broke apart
Continents have drifted to their current location
Continental Drift
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It has taken the continents about 225 million years
since the breakup of Pangaea to move to their
present locations.
Evidence for Continental Drift
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Puzzle-like fit of the continents
Similar fossils have been found on different
continents
Arctic areas & glacier deposits in tropical
areas
Similar rock structures are found on different
continents
Seafloor Spreading
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Wegner’s theory was not widely accepted
because no one could explain how or why
continents moved.
Using sound waves scientist discovered
underwater mountain ranges called midocean ridges
In 1960, Harry Hess suggested the theory of
seafloor spreading
Mid-Ocean Ridges
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Form underwater mountain chains where the
ocean crust moves apart allowing magma to
rise forming new ocean crust
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The East Pacific Rise is just one of the many
mid-ocean ridges that wind beneath Earth’s
oceans.
- Sea-Floor Spreading
What Is Sea-Floor Spreading?
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Sea-floor spreading occurs when
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hot, less dense material below Earth’s crust rises
toward the surface of the mid-ocean ridge
flows sideways carrying the carrying the seafloor
away from the ridge in both directions
makes new ocean crust
Works like a conveyer belt
Older rock is found
farther from the ridge
Growing an Ocean
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Because of sea-floor spreading, the distance
between Europe and North America is
increasing by a few centimeters per year.
Theory of Plate Tectonics
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Explains the formation, movement, and
subduction of Earth’s plates.
Claims that Earth’s plates are always in motion
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Plates move
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slowly
at different rates
in different directions
Structure of the Earth
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The Earth is made
of 4 layers
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Inner Core
Outer Core
Mantle
Crust
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Crust
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Mostly solid rocky outer layer of Earth
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includes:
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Earth’s crust & upper mantle are broken into large
plates
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Oceans crust: under the ocean & more dense
Continental crust: surface of earth & less dense
Together the crust and plates make up the lithosphere
Lithosphere is the outermost solid, rocky surface of
the Earth that floats on top of the asthenosphere
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Brittle, rigid, & has fault lines
Lithospheric Plates
Mantle
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Solid layer of hot rock
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Upper mantle melts rocks, forming a substance called
magma which flows like a thick liquid
The asthenosphere is the mechanically weak
deforming region of the upper mantle that is
ductile, plastic, and free-flowing
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Convection currents occur here due to uneven
heating
Convection Currents in the Mantle
• Convection currents can cause plates to move
away from each other or towards each other
• Convection currents:
1. hot less dense rock rises to the lithosphere & spreads
out cooling & shrinking in volume
2. The cooler more dense rock sinks allowing the hot less
dense rock to rise and take its place
3. As convection currents move the plastic rocks sideways
large portions of the crust move called lithospheric
plates
Compare & Contrast
Lithosphere & Asthenosphere
Using a double bubble
Plate Boundaries
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Movement of Earth plates are responsible for
most major geological events and landforms
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Volcanoes, earthquakes, mountain formation
Plate Boundaries are where edges of plates
interact
3 Types of plate boundaries
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Divergent Plate Boundaries
Convergent Plate Boundaries
Transform Plate Boundaries
Plate Boundaries
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Different type of plate movement occurs along
each type of boundary.
Divergent Plate Boundaries
 Forms
when 2 plates move away
from each other
 Two types:
 2 oceanic
 2 continental
Divergent Boundaries:
2 ocean plates
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Form undersea volcanic mountain
ranges called mid-ocean ridges.
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Ex. Mid-Atlantic Ridge
Divergent Boundaries:
2 continental plates
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When two continental plates pull apart a rift
valley forms
Ex. Great Rift Valley in Africa
Convergent Plate Boundaries
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Boundaries that form when 2 plates collide
or come together
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Types
 Oceanic & Continental
 Two Oceanic
 Two Continental
Oceanic & Continental
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The denser oceanic plate is forced below the
less dense continental plate
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The movement of one plate under another forms
a subduction zone
Oceanic plate melts as it subducts into the mantle
forcing hot magma & gas up to the surface of the
continent
Forms a deep-ocean trench & a long chain of
continental volcanic mountains
Ex. : Mt. St. Helens
Two Oceanic Plates
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The less dense ocean plate is forced down
into the mantle forming a deep ocean trench
that sinks & melts
Magma & gas push through the ocean floor
forming a chain volcanoes
If volcanoes become large enough it rises
above sea level & makes a volcanic island
arc
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Ex. Mariana Trench & Mariana Island Arc in the
Western Pacific & Japanese islands
Volcanoes and Plate Boundaries
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Volcanic belts form along the boundaries of
Earth’s plates.
Two Continental Plates
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First, the seafloor between the continents is
forced below one of the continents
Then, when the plates collide the continent
crusts buckle, thicken, & rises up forming
mountain ranges
Compressional forces squeeze plates
together causing Folding & Faulting
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Ex. Himalaya & Appalachian Mountains
Folding
Faulting
Transform Plate Boundaries
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2 plates grind or slide past each other without
creating or destroying the lithosphere
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Form Strike-slip faults
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Earthquakes occur at transform boundaries
San Andreas Fault is an example
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North America & Pacific plates slide past each other in a N – S
direction creating earthquakes
Earthquakes
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Faults are large cracks in the Earth’s crust where
movement occurs
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At Strike-slip faults rocks on either side of the fault are
under pressure & can get locked together
When too much pressure builds up, the rocks suddenly
slide past each other releasing the pressure
The violent shaking of the Earth’s crust is known as an
earthquake
New Madrid Fault