What is a plate?:
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Plate Tectonics
Continental Drift
In 1912, Alfred Lothar Wegener (1880-1930), a German meteorologist,
published the hypothesis of continental drift with multiple lines of
evidence to back it up. Wegener proposed that all of the continents were
once joined together as one supercontinent that he called Pangea.
•  Introduction
•  Predecessor of plate tectonics: Continental drift hypothesis
•  What is the theory of plate tectonics?
•  What is a plate?
•  How do we find their edges?
•  How many plates are there?
•  Why is plate tectonics important?
•  Types of relative plate movements (and related stresses)
•  Types of plate boundaries
•  Plate boundaries and earthquake depths
•  Measuring plate motions
•  Why do plates move?
This Dynamic Earth - USGS
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Fossil evidence for once
joined continents
Later studies have allowed the positions of the continents at different
times in Earth history to be determined, and have documented the
breakup of Pangea.
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http://
media.pearsoncmg.co
m/bc/bc_0media_geo/
active_art/
hdew_2e.html?
PlateMoTime
This Dynamic Earth - USGS
Related puzzle activity: http://volcanoes.usgs.gov/about/edu/dynamicplanet/wegener/
This Dynamic Earth - USGS
Before plate tectonics theory
The plates2
Plate Tectonics
Today, we know that it isn’t just the continents that have moved.
Rather, the Earth’s surface is broken into pieces of continental and/or
ocean crust called plates. Plate tectonics deals with the nature of these
plates, what happens at their boundaries, how and why they move, etc.
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Plate tectonics also
explains the locations of
earthquake faults,
volcanoes, and major
mountain ranges.
Photos from: This Dynamic Earth
(USGS) and Understanding Earth
Fig 20.3 - Understanding Earth
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Active continental margins (plate boundary)
vs.
Passive continental margins (no plate boundary)
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Zones of progressively deeper earthquakes revealed the existence of 8
subduction. This led to an understanding of how ocean crust is recycled
at ocean trenches.
The western margin of North America is an active area with earthquakes
(yellow) and volcanic activity (red). In contrast, the eastern margin of North
America is quiet with no active volcanoes and few earthquakes. The eastern
margin is passive. We now know that the active zones coincide with the
boundaries between plates (blue). http://media.pearsoncmg.com/bc/bc_0media_geo/
active_art/hdew_2e.html?PlateBound
Geology 2nd ed. Chernicoff
USGS
Cross-section of the earth1
The discovery of extensive ocean ridges and young ages for the ocean
floor (in the 1940s and 1950s) provided another important clue.
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What is a plate?
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– The brittle outer part of the earth (crust and uppermost
mantle) is called the lithosphere. The lithosphere is broken into plates that move on
the asthenosphere, a part of the mantle which is plastic (able to flow).
Fig 17.27 Understanding Earth
This Dynamic Earth - USGS
Plate boundaries
Types of relative plate movements and boundaries
and related stresses and fault types
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Compression
Extension
Reverse and
thrust faults
Normal faults
Folding
Stretching and
thinning
Shear
Strike-slip faults
Shearing
This Dynamic Earth - USGS
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Oceanic ridge system
The mid-ocean ridge (shown in red) winds its way between the
continents much like the seam on a baseball This Dynamic Earth - USGS
Iceland-map
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Fig 1.15 - Understanding Earth
This Dynamic Earth - USGS
Mid-Atlantic
Ridge at Iceland
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http://media.pearsoncmg.com/bc/bc_0media_geo/ 16
active_art/hdew_2e.html?DivergBound
Continental Rifting
Africa
East African Rift
SOUTH
Gulf of Aden
Red Sea
East African Rift
Continental rifting:
The Afar Triangle
Photo from Space Shuttle:
STS061-079-024 - NASA
This Dynamic Earth - USGS
Jigsaw fit of the continents
Fig 20.3 - Understanding Earth
Next: 20-Plate Divergence.mov
(Red Sea rift)
San Andreas fault and oceanic fracture zones-map
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Geology 2nd ed. - Chernicoff (originally from Bullard,
1969, The Origin of the Continents: Scientific American)
Transform Boundaries
http://media.pearsoncmg.com/bc/
bc_0media_geo/active_art/
hdew_2e.html?TranFault
This Dynamic Earth - USGS
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San Andreas fault
Ocean-ocean subduction
Transform plate boundary
San Andreas fault, California
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Ocean-ocean convergent boundary
This Dynamic Earth - USGS
Fig 1.17 - Understanding Earth
Japan
Ocean-continent subduction
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Ocean-continent convergent boundary
This Dynamic Earth - USGS
Japan, an example of an
oceanic island arc
Photo from Space Shuttle:
STS059-218-044 - NASA
http://www.npr.org/templates/story/story.php?storyId=124361777
(Charles Darwin witnessed 1895 Chilean quake, deduced that the Andes required millions of years to build)
Continental collision
Half dome
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Intrusive igneous rocks formed along an ancient volcanic arc: Part of the
Sierra-Nevada batholith in Yosemite National Park
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Continent-continent convergent boundary
This Dynamic Earth - USGS
Next: 20-Ocean-Continent.mov (Andean subduction zone)
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Earthquake depths and plate tectonic settings
Movement of India
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Formation of the highest
mountains on earth by
continent-continent
collision
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Plate boundaries and earthquake depths
All types of plate boundaries can produce shallow earthquakes.
The deepest earthquakes (and some of the largest earthquakes) occur in
subduction zones.
This Dynamic Earth - USGS
Next: 20-Continent-Continent.mov
(India-Asia collision zone)
PT Summary:
http://media.pearsoncmg.com/bc/bc_0media_geo/
active_art/hdew_2e.html?MotionatPlate12
Fig. 10-16 Chernicoff - Geology
Hot spots
Measuring plate motions using hot spots
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Hotspots and plate movements
http://media.pearsoncmg.com/bc/bc_0media_geo/
active_art/hdew_2e.html?HotSpot12
Geology 2nd ed.
- Chernicoff
Map of part of the Pacific ocean floor showing the 6,000-km-long Hawaiian
Ridge-Emperor Seamounts chain – This volcanic trail of the Hawaiian
“hotspot” could be related to a mantle plume
This Dynamic Earth - USGS
Hot spots and the opening of the south Atlantic Ocean
http://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/hotspots/index.htm
http://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/index.htm
GPS receiver
Ages of the ocean crust
Ages of the ocean crust
Fig 20.11 - Understanding Earth
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How do we know plates move today? From Earthquakes, GPS, VLBI, & SLR
How we know: http://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/index.htm
NASA GPS Time Series plate movements: http://sideshow.jpl.nasa.gov/post/series.html
GPS time series: http://en.wikipedia.org/wiki/File:Global_plate_motion_2008-04-17.jpg
More GPS http://www.dpc.ucar.edu/VoyagerJr/gpsplatebound.html
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A GPS ground receiver at Augustine Volcano (Cook Inlet, Alaska) recording signals
sent by GPS satellites. Artist's conception of a Global Positioning System satellite in
orbit shown in inset. Source: USGS
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Future plate positions
Relative movement directions and velocities
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Major plate boundaries and their rates of movement
Fig 20.12 - Understanding Earth
Projected future positions of the continents - 100 million years from now: If
current plate movements continue unchanged, part of California will become an
island off the coast of Washington, the Mediterranean Sea may close, and eastern
Africa may become an island. Geology 2nd ed. - Chernicoff
Convection
Possible causes of plate motions
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Plate Tectonics:
Why do plates move?
The general scientific consensus is
that plate movements are related
to convection.
Best fit to evidence for
major force causing
plate movements
Convection in the Earth results
from the escape of heat from the
interior and involves the pull of
gravity on rocks of different
densities.
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Time B
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8
7
6
10
9
8
Time A
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Deep
plumes exist,
but are not major
cause of plate
movements
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Fig 20.25 - Understanding Earth
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Subduction via
only rollback
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10
10
5
3
11
Time A
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Partly true, but convection
not as organized as this
Subduction via only
Fig 1.13 - Understanding Earth trenchward motion
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Suggested mechanisms
to drive plate motions
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Essentially, colder, more dense
rocks sink while hotter, less dense
rocks rise.
Doesn’t
match rates
of movement
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Time B
6
7
5
6
4
5
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13
12
11
10
Time C
10
9
9
8
7
Time C
6
5
Initial position
of trench
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Initial position
of trench
left boundary
would necessarily be
Not only do subducting platesspreading
sink,to the
the
can also “roll back”
accomodated by motion of ocean lithosphere
beyond the mid-ocean ridge.
over time. This causes the subduction
zone to pull on both plates.
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9
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Time A
6
5
4
Subduction via
trennchward
motion and
rollback
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Possible modes
of behavior
of oceanic
lithosphere
at convergent
plate boundaries
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- cold, relatively dense lithosphere is pulled downward at subduction
zones and pulls the rest of the plate along behind it
Note that this oceanic lithosphere
need not move relative to the underlying
asthenosphere, in which case any seafloor-
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Initial position
of trench
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Slab pull – probably the most important driver of plate movements
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10
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8
Time B
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6
5
13
12
11
10
Time C
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http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?ConvecTect
Image source: This Dynamic Earth – USGS http://pubs.usgs.gov/publications/text/dynamic.html
LBR SubductionvRollbackNew05 4/2008 rev. 10/2010
Image source: http://www.gly.uga.edu/railsback/DynamicPlateTectonics.html
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Initial
Rollback
Rollback
ridge
mid-ocean
ridge
New sea
floor made
Future plate positions
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Initial
Initial
Compression
of continent
because
of subduction
Extension
of continent
as western
tip
of buoyant
young lithosphere
travels
with west-moving
plate.
mid-ocean
position of
Rollback
Initial
position
of
ridge
ridge
mid-ocean
Initial
position
of
mid-ocean
position of
of trench
ofposition
trench
Initial
position
Initial
Compression of continent because of subduction
of buoyant young lithosphere
of sketches.
Plate boundary locations change over time
thesketches.
sequence
of
Rollback
Rollback
Extension and formation of fore-arc
basin as eastern margin moves
eastward with rollback of trench
Rollback
Rollback
Rollback
Rollback
Subduction
with
rollback pulls on
plates
the throughout
sequence
Rollback
New sea floor
made at
spreading ridge
throughout
is
stationary
Rollback
Rollback
Image source: http://www.gly.uga.edu/railsback/DynamicPlateTectonics.html
Initi
positio
continent as western
margin moves
westward with
rollback of trench
This region
is stationary
ntinent
back-arc
n margin
rd with
ench
his region
ntinent
back-arc
n margin
rd with
ench
Two pieces of lithosphere
generated at same time.
Extension and formation of fore-arc
basin as eastern margin moves
eastward with rollback of trench
LBR
4/2008
DynamicPlateTectonics014
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Projected future positions of the continents - 100 million years from now
assuming current plate movements continue unchanged Geology 2nd ed. - Chernicoff
Extension of continent as western tip
travels with west-moving plate.
Rollback
Possible causes of plate motions
Possible causes of plate motions
LBR
4/2008
DynamicPlateTectonics014
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This Dynamic Earth - U.S. Geological Survey
http://pubs.usgs.gov/publications/text/dynamic.html
Paleomap Project - Continent locations of the past, present and future.
http://www.scotese.com/
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Plate Tectonics
http://www.ucl.ac.uk/EarthSci/people/lidunka/GEOL2014/Geophysics1-%20Plate%20tectonics/PLATE%20TECTONICS.htm
How we know plates move – GPS, VLBI, etc.
http://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/index.htm
NASA Space Geodesy Project – What is VLBI?
Plate Tectonics - U.S.G.S. Cascades Volcano Observatory
http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/framework.html
http://space-geodesy.nasa.gov/techniques/VLBI.html
Digital Tectonic Activity Map (combines topography, plate boundaries, movements, etc.)
http://denali.gsfc.nasa.gov/dtam/
What is Plate Tectonics? – USGS and National Park Service
http://www2.nature.nps.gov/geology/usgsnps/pltec/pltec1.html
Wegener's Puzzling Evidence Exercise (6th Grade Activity)
http://volcanoes.usgs.gov/about/edu/dynamicplanet/wegener/
Plate Tectonics Animations – USGS and National Park Service
http://www2.nature.nps.gov/geology/usgsnps/animate/pltecan.html
CT scan of Earth links deep mantle plumes with volcanic hotspots
http://www.sciencedaily.com/releases/2015/09/150902134939.htm
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