Plate Tectonics - Asheboro High School

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Plate Tectonics
Earth Science
Continental Drift
 scientific
theory proposing the
slow, steady movement of
Earth’s continents
Early Observers
 In
the late 1500s, A Dutch
mapmaker noticed that the
continents fit on either side of the
Atlantic Ocean.
 Abraham Ortelius proposed that
North and South America had been
separated from Europe and Africa by
earthquakes and floods.
Early Observers
 Then,
a man named Eduard Suess
hypothesized that the present
southern continents had once been
joined to form a landmass called
Gondwanaland.
Alfred Wegener
 In
1912, created his
hypothesis called
“continental drift.”
 Continental drift =
Alfred Wegener’s
hypothesis which
proposes that the
Earth’s continents
had once been
joined as a single
landmass.
Wegener’s Continental Drift
Hypothesis
 Supercontinent
called “PANGAEA”
began breaking apart about 200
million years ago
– Continents “drifted” to present positions
– Continents “broke” through the ocean
crust
Evidence Used by Wegener
Fit of South
America and Africa
 Matching Fossils
 Matching Rock
Structures
 Ancient Climates

Main Objection to Continental
Drift
Wegener’s theory did not
provide a mechanism for
the movement of
continents
Plate Tectonics
 Theory
proposed in the 1960’s that
replaced the continental drift theory
 Could provide an explanation for the
movement of continents
Plate Tectonics
 Involved
Earth’s Rigid Outer Shell,
the Lithosphere
– Consists of about 20 slabs (plates)
 Plates
are moving slowly
 Largest Plate is the Pacific plate
 Plates are mostly beneath the ocean
Asthenosphere
 The
partially molten, hot, flowing
layer of the mantle
 Found beneath the lithosphere
 Allows for motion of lithosphere
Lithosphere
Asthenosphere
Plate Boundaries
 Where
tectonic plates meet and
interact
 Some boundaries converge,
diverge, or move horizontally
 Associated with:
– Seismic activity (earthquakes)
– Volcanism
– Mountain building
Divergent (Spreading) Boundaries
 Plate
tectonics that MOVE AWAY
from each other
 Most exist along oceanic ridge crests,
in a rift or valley
 Process initiates Seafloor Spreading
– Forms fractures (openings) on the ridge
crests
– Fracture fills with magma—creates new
seafloor
Divergent (Spreading) Boundary
When Divergent Boundaries are on continents,
Rift Valleys form
Rift Valley: long narrow depression
Convergent (Together) Boundary
3
types of Convergent Boundary
– Oceanic-Continental Boundary
– Oceanic-Oceanic Boundary
– Continental-Continental Boundary
Oceanic-Continental Boundary




Forms a subduction
zone with a deepocean trench
Dense ocean plate
subducts beneath
continent
Results in volcanic
mountains
Examples:
– Andes South America
– Sierra Nevadas
California
Oceanic-Oceanic Boundary





1 ocean plate
subducts beneath
another
Creates deep sea
trenches
Often forms volcanoes
on the ocean floor
Island arc forms as
volcanoes emerge
Examples:
– Aleutian Trench &
Islands
– Mariana Trench and
Islands
Continental-Continental Boundary



1 continental plate
pulled into subduction
zone, but not forced
into mantle
Can produce
mountains
Examples:
– Himalayas (India
and Asia)
– Other possibilities
 Alps
 Appalachians
Transform Plate Boundary
2
Plates slide past one another
– No new crust is created
– No crust is destroyed, but deformed or
fractured
– Associated with long faults
 Transform
faults
– Most often in the ocean: 1 land
example San Andreas Fault in
California
Transform Plate Boundaries
Evidence to Support Plate
Tectonics
 1.
Paleomagnetism: study of
Earth’s magnetism
– Ancient Magnetism preserved in rocks
– Paleomagnetic records show
 Earth’s
Magnetic Field Reversals
– Recorded in the Seafloor Rocks
– Confirms seafloor spreading
Evidence to Support Plate
Tectonics
 2.
Earthquake Patterns
– Associated with Plate Boundaries
– Deep-focused Earthquakes along
trenches provide a method for tracking
the plate’s descent
Evidence to Support Plate
Tectonics
 3.
Ocean Rock Age
– Youngest are near ocean ridges
– Oldest at a distance from ocean ridges
– Ocean basins are geologically young
 4.
Ocean Sediment
– Thicker further from o. ridge
Driving Mechanisms of Plate
Tectonics
 No
one model explains all plate
motions
 Earth’s heat is the driving force
 Several models have been proposed
– Convection currents in mantle
– Slab-pull and slab-push model
Slab pull-push
 Slab
pull and slab push
– Descending oceanic crust pulls the plate
 Convergent
boundary
– Elevated ridge system pushes the plate
 Divergent
boundary
Volcanism at convergent
boundaries
Most, 80%
volcanoes occur
at convergent
plate boundaries
 Oceanic-oceanic
volcanic islands
 OceanicContinental
volcanic
mountains
The island of Bora Bora

Mount Shasta in the Sierra
Nevada’s of Oregon
Volcanism at Convergent
boundaries
 Plates
come together at
convergent boundariescreates
a subduction zone plate
descends into the mantle and
melts into magmacomes back
up through the boundaryforms
volcanoes when it reaches the
surface.
Volcanism at divergent boundaries
15% of volcanoes are found at
divergent boundaries
 Magma is forced upward into the
fractures and faults that form as the
plate separates
 Iceland is an example of this
volcanism, its part of the midAtlantic ridge

Hot Spots
10. 5% of volcanoes are hot spots.
11. These volcanoes are found far
away from any plate boundary.
12. Unusually hot regions of the earths
mantle where high temperature
plumes of mantle material rise
toward the surface
Hot Spots
Plume melts rock melt straight
through the mantle and crust 
become volcanoes on earth’s surface
 A Hot Spot!!!
 http://link.brightcove.com/services/li
nk/bcpid1329217914/bctid18774390
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