Transcript Plate Tectonics

Earth Science
A study in change: Plate Tectonics
Plate Tectonics: History
Began as continental drift
 Suggested by Alfred Wegener.
 Continental drift is the theory that all of
the continents used to be one large
landmass, called Pangaea.
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Continental Drift evidence
1.
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4.
Jigsaw Puzzle-shapes of continents fit.
Fossil Evidence-matching fossils on
separate continents. (Mesosaurus)
Geologic Features-mountain ranges, ages
and kinds of rocks match along edges of
continents. (Diamonds)
Ancient climates- glacier markings match
on separate continents, some in areas that
are now tropical.
Continental Drift- a science
failure!
Wegener couldn’t figure out HOW it
worked! He had no driving force.
 No reason for how it worked meant that
people could find other reasons for his
“proof”.
 Science community needed a reason
for making continental drift GO.
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Plate tectonics replaced
continental drift
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The theory of plate tectonics:
– Lithosphere (solid crust and upper mantle)
is broken into plates.
– Tectonic Plate- A large, solid piece of the
earth’s crust that includes the continents
and ocean floor.
– These plates float or ride on the
asthenosphere.
The main difference between plate
tectonics and continental drift =
Plate tectonics has a driving
force = convection currents.
(Demo)
OH, Convection currents!
Density! D= M/V
How does it work?
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Mantle is a liquid.
Recall: Temperature of Earth becomes warmer as
you go deeper into Earth.
So, mantle rock near the core heats up and rises. As
the rock rises, it cools; cool mantle rock then sinks.
(Sound familiar? Like the atmosphere!)
Convection currents within the mantle drive the plate
movement. IT’S ALL ABOUT DENSITY!
– Hot fluids rise, cold fluids sink.
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Think: Grocery store conveyor belt, and we are the
groceries!
So what?
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Fun stuff happens when the plates
move!
Fun stuff like. . .
Earthquakes
 Volcanoes
 Mountains
 Islands

Plate Boundaries - where the
action occurs (3 types)
Divergent- a place where 2 plates
move away from each other.
 Convergent- a place where 2 plates
move toward each other.
 Transform-a place where 2 plates slide
past each other.

Plate Boundaries
Plate Boundaries
Transform Boundary
Transform Boundaries
Areas where two plates slide past each
other.
 Not a smooth movement
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– Slip and stick pattern.
– Cause of earthquakes!
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San Andreas fault in California
Plate Boundaries
Divergent
Boundaries
Divergent Boundaries
Divergent- a place where 2 plates
move away from each other.
 Places where new crust is formed!
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On land = Rift valley
 In water = Mid-ocean ridge
Divergent Boundaries
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Mid-ocean ridges- underwater mountain
ranges. (larger than the ones on land!)
– Have a rift(tear) in the center - allows
magma to flow to the surface.
– Newest crust is in the center. (Igneous
rock)
– New crust forces old crust away from the
ridge.
– Earthquakes and volcanoes common.
Divergent Boundaries
How do we know?
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This week = Sea floor lab:
– Magnetic field tells us!(Paleomagnetism)
– Earth’s magnetic field changes! Reverses!
Every so often. . .
– As new igneous rock forms, magnetic
minerals will line up with Earth’s magnetic
field.
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Bands of rock along rift valleys and midocean ridges alternate N/S directions!
Other ways we know:
“Old” rock is found farther from a rift valley or
mid-ocean ridge than “new” rock.
 Fossils help us know this.
– Fossils from recent eras are close to the
boundary
– Fossils from older eras are far from the
boundary.
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Plate Boundaries
Convergent
Boundaries
Convergent Boundaries
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Convergent- a place where 2 plates move
toward each other.
Places where crust is destroyed and
recycled
Continent + continent = mountain
Continent + ocean = subduction
zone/volcanoes
Ocean + ocean = island arc
Subduction Zones
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Continent + ocean = subduction zone
– Continental crust is less dense and “floats”,
oceanic crust is more dense and“sinks”.
• Due to DENSITY!
– The oceanic crust will get pushed underneath the
continental crust.
• Causes melting
• Associated with volcanoes
• Andes Mountains of South America.
Plate Boundaries - Convergent
Oceanic + Continental
= subduction zone +
mountain (So. America)
Oceanic + Oceanic =
subduction zone +
island arc (Japan, Alaska)
Continental +
Continental =
mountains (Mt. Everest)
Mountains
Continental crust moving toward
continental crust
 Convergent boundary
 No place to go, but UP!
 Mt. Everest (29,035 ft; or 8,850 m)
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Mountains