Transcript Plate Tectonics

Warm-up
1. Draw and label the layers of Earth.
2. Describe the core.
3. Pick one type of plate movement to
describe.
Use complete sentences any time you
see “describe” or “explain”
Topic 8:
Plate Tectonics
Evolution of the Earth
How do we know anything about
the Earth?
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Interior structure
Volcanoes
Earthquakes
Tectonic plates
Tectonic motion
Reconstruction of the Earth’s history
Interior
Interior
Plate Tectonics
• Plate tectonics helps to
explain
– earthquakes
– volcanic eruptions
– formation of
mountains
– location of
continents
– location of ocean
basins
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Tectonic interactions affect
 atmospheric and
oceanic circulation and
climate
 geographic distribution,
 evolution and extinction
of organisms
 distribution and
formation of resources
Alfred Wegener and the
Continental Drift Hypothesis
• German
meteorologist
• Credited with
hypothesis of
continental drift1912 in a scientific
presentation –
published a book
in 1915.
Alfred Wegener and the
Continental Drift Hypothesis
• He proposed that all landmasses were
originally united into a supercontinent he
named Pangaea from the Greek meaning “all
land” (pan = all; gaea = Earth)
• He presented a series of maps showing the
breakup of Pangaea
• He amassed a tremendous amount of
geologic, paleontologic, and climatologic
evidence
Wegener’s Evidence
• Shorelines of continents fit together
– matching marine, nonmarine, and glacial
rock sequences from Pennsylvanian to
Jurassic age for all five Gondwana
continents including Antarctica
• Mountain ranges and glacial deposits
match up when continents are united into
a single landmass
Jigsaw-Puzzle Fit of Continents
• Continental Fit
Pangea
The break up
of Pangea
Jigsaw-Puzzle Fit of Continents
• Matching mountain
ranges
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Matching glacial
evidence
Matching Fossils
The Perceived Problem with
Continental Drift
• Most geologists did not accept the idea of
moving continents
– There was no suitable mechanism to explain
how continents could move over Earth’s
surface
• Interest in continental drift only revived
when new evidence from studies of Earth’s
magnetic field and oceanographic research
showed that the ocean basins were
geologically young features
Checkpoint
1. Give two sources of evidence for
continental movement.
2. What was the name of the
supercontinent?
Fig. 3-4, p. 39
Further Evidence
• Radiometric dating of sediments and rocks
• Found the maximum age of the ocean floor
and its sediments were less than 200 million
years
• Centers of continents are much older
– Parts are more than 3.9 billion years old
Seafloor Age
Evidence is based
on seafloor ages
which get younger
as we approach
sea floor ridges
So why is the oceanic crust so young?
Seafloor Spreading
• Professor Harry Hess (1960, Princeton) suggested that the new
seafloor forms at the Mid-Atlantic Ridge and spreads outward
from this line of origin.
• As one boundary of a plate is being subducted and destroyed,
the other boundary is having new material added to it - Plates
move away!
• Magma rises to the crustal surface and forms
mid ocean ridges.
• As the lava cools it forms new seafloor and
features: rift valleys; seamounts; abyssal hills
peaks)
• As new material reaches the surface, the
plates are pushed apart
(volcanic
What type of plate boundary (movement) do
you think is responsible for seafloor spreading?
Mid-ocean Ridges
Plate Tectonics
• Ideas of continental drift and seafloor spreading
were integrated into the concept of plate
tectonics
– Tekton = builder
• Earth’s outer layer consist of about a dozen plates
floating on the
asthenosphere (hot
plastic layer of upper mantel)
Based on Composition
• Crust – solid, relatively low density silicate rock
• Mantle – Semi fluid, denser, mafic (iron and
magnesium bearing) rocks
• Core – Liquid then solid iron and nickel with traces of
heavier elements
Composition vs. Motion
We can look at the interior of the Earth based
on the composition of the rocks or based on the movement
Earth’s Moving Plates
The oceanic crust shows signs of regular
movement.

This movement is associated with convection
currents in the mantle.
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The rising up, spreading out and sinking of
gasses, liquids or molten material create
convection currents.
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Convection Currents
• When heated from
below, the fluid
asthenosphere expands,
becomes less dense, and
rises.
• It turns aside when it
reaches the lithosphere
(above asthenosphere),
and drags the plates until
it cools and turns under
again to make a cycle
Asthenosphere
Cracks in the Earth’s Crust
• The crust, when it is solid, acts as a heat
insulator for the hot interior of the Earth.
• The molten material, magma, below the crust
builds up tremendous heat and pressure.
• The magma creates convection currents and
rises to the surface.
• These currents can crack the crust!
Cracks
• The lava cools to form new rock for the
crust.
• There is a world wide system of “cracks”
in the crust (both oceanic and
continental).
• These cracks separate the crust into
plates (huge sections of the Earth that
moves relative to each other).
Tectonic Plates
“Through the great expanse of geological time, this slow
movement of the plates remakes the surface of the Earth,
expands and splits continents, and forms and destroys ocean
basins.” – Oceanography, pg. 63
Review
• SONAR
– Active
– Passive
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SOFAR
Sound
Shadow Zone
Speed of Sound and
SOFAR Channel
• Echolocation
• Seafloor Spreading
• Layers of Earth
• Core
• Mantel
• Crust
• Alfred Wegener
• Continental Drift
• Pangaea
• Convection Currents
• Subduction
• Convergence
• Divergence
• Transform Boundary
• Plates
Plate Boundaries
• Subduction – when one plate plunges
beneath another
• Continental Collision (Convergent) – plates
move toward one another, shoved together
• Sea Floor Spreading (Divergent) – plates
move away from each other
• Transform fault – plate moves sideways from
each other, slide past each other
Subduction
• The downward movement of an oceanic
plate (more dense) into the mantle
• As the crust enters the mantle, pressure
breaks the crustal rock.
• Heat from friction melts it.
• It forms a pool of magma.
• This magma is called andesite lava, which
is a mixture of basalt from the oceanic crust
and granite from the continental crust.
• Might reemerge through a volcano
Results of Subduction
• Places where subduction occurs are also
sites of deep earthquakes caused by rocks
slipping over other rocks deep in the mantle.
Results of Subduction Cont...
• The lava will travel along channels in the crust,
causing great explosive eruptions and form composite
volcanoes – steep sided, cone shaped mountains with
alternating layers of lava and rock fragments
• Ocean trenches form at the regions where one plate
moves downward beneath another. These trenches
are deep, narrow and long.
Continental Collision
Occurs if 2 plates carrying continents collide and
the subduction is interrupted.
• Because the continental
crust is made of low density
material, it does not sink.
• The crust moves upward, folds and buckles and
breaks.
• Mountain ranges were formed by continental
collision.
Convergent Plates
Divergence and Seafloor
Spreading
• Two plates are moving
apart
• Magma rises to the crustal
surface and forms mid
ocean ridges.
• As the lava cools it forms
new seafloor and features:
rift valleys; seamounts;
abyssal hills (volcanic
peaks)
• As new material reaches
the surface, the
plates are pushed apart
Divergent Plates
Ring of Fire
• Plates are diverging and oceanic plates
are moving under the continental plates
(subduction)
Transform Fault
• Perpendicular breaks or fractive zones
occur when sections of plates slip pass
one another
• This causes shallow earth quakes
Transform Plates
Transform Plates
San Andreas
Fault
Why do the Plates Move?
• No single idea explains everything but we can
identify several forces that contribute to the
movement of the plates.
– Slab pull
• The sinking of the cooled dense oceanic plates pulls on the
rest of the plate
– Ridge rises
• The material deposited on the top of the ridge slides downs
from the rise pushing on the plate
– Convection
• Movement within the mantle could be part of the driving force
behind the motion of the plates.
Review
1. Name and describe the feature of the
ocean floor shown at A.
2. Describe the process shown occurring at
B, and explain what results from this
Review
3. What process is shown occurring at C,
and why does it occur?
4. The process by which the ocean floor
sinks into the mantle is called?
Plate Boundaries
The Big Picture
Tectonic Plates
Today plate boundaries are determined by examining
the location of volcanoes and earthquakes.
Volcanoes result from the friction (heat) of the plates’
motion.
Earthquakes occur where plates rub against one another.
Tectonic Plates
Volcanoes
Volcanoes
• Volcanoes are the result of hot spots within the
crust or mantle of the earth.
• The hot, liquid rock will break through weak
spots in the surface and form volcanoes or flood
basalts.
• Many volcanoes do not release lava, instead
they spit ash and small bits of lava called lapilli.
• Some eruptions are quiet with very fluid (low
viscosity) lava flows while others are explosive
Volcanoes
Quiet
lava
flows
Volcanoes
Mt. St. Helen before the explosive eruption
Volcanoes
Time lapse of the eruption
Volcanoes
Mt. St. Helen after the eruption
Volcanoes
Volcano Locations
Underwater Volcanoes
• Some are active and blast steam and
rock-debris high above the surface of the
sea.
• Many others lie at such great depths that
the tremendous weight of the water above
them results in high, confining pressure
and prevents the formation and explosive
release of steam and gases.
FROM: www.geog.ucsb.edu
Hot Spots and Mantle Plumes
• Hot spots are locations where stationary
columns of magma originating deep within
the mantle, called mantle plumes slowly rise
to the surface
• Mantle plumes remain stationary although
some evidence suggests they may move
• When plates move over them, hot spots
leave trails of extinct, progressively older
volcanoes
Hot Spots and Mantle Plumes
Example: Emperor Seamount-Hawaiian
Island chain
Age
increases
with plate
movement
Earthquakes
• Earthquakes are a result of motion within the
earth.
• Caused by shifting masses of rock miles below
the surface
• This only occurs where the earth is solid and
therefore can only occur within about 100 miles
of the surface
• Earthquakes provide the best evidence
regarding the interior structure of the Earth.
Tectonic Plates
P-Waves and S-Waves
P-waves
• Compression Waves
• Move through solids,
liquids, and gases
• Move quickest through
denser material
S-waves
• Travel through solids only
• Speed and direction
change as density changes
Surface Waves
• Result when P- and S-waves reach the
surface and travel outward along the
surface from an earthquake’s epicenter
(where the earthquake starts).
– These cause damage to buildings, etc.
Earthquakes
Earthquakes
Shadow Zone
• Seismic waves change speed and direction as they travel
through materials of different density.
•When P-waves enter the boundary between the mantle and
core, the difference in densities causes them to bend sharply.
•When S-waves reach the boundary between the mantle and the
core, they bounce back because they cannot travel through the
dense fluid in the outer core.
•The result is a shadow zone.
Tectonic Plates
Earthquakes
Location of worldwide earthquakes
Earthquakes
Tidal waves or Tsunamis result when a large section of the sea
floor suddenly moves and therefore displaces a massive amount
of water.
Where are we going?
We appear to be headed for another
super continent as North America,
South America, Asia and Australia converge in the
ever shrinking Pacific Ocean
Pangaea
Pangea