Transcript Powerpoint Presentation Physical Geology, 10/e

Lecture Outlines
Physical Geology, 14/e
Plummer, Carlson & Hammersley
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Plate Tectonics: The Unifying
Theory
Physical Geology 14/e, Chapter 19
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Plate Tectonics
Plate tectonics – Earth’s surface is composed of a few large, thick
plates that move slowly and change in size
•combination of continental drift and seafloor spreading hypotheses from the
late 1960s
Plate boundaries – plates move away, toward, or past each other
•intense geologic activity
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Early Case for Continental Drift
Puzzle-piece fit of coastlines of Africa and
South America has long been known
Alfred Wegener – noted South America,
Africa, India, Antarctica, and Australia have
almost identical late Paleozoic rocks and
fossils in early 1900s
• Glossopteris (plant), Lystrosaurus and
Cynognathus (animals) fossils found on all
five continents
• Mesosaurus (reptile) fossils found in Brazil
and South Africa only
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Early Case for Continental Drift
Pangaea – supercontinent proposed by
Wegener
• Laurasia - northern supercontinent
containing North America and Asia
(excluding India)
• Gondwanaland - southern supercontinent
containing South America, Africa, India,
Antarctica, and Australia
Late Paleozoic glaciation patterns –
on southern continents best explained by their
reconstruction into Gondwanaland
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Early Case for Continental Drift
Coal beds – from North America and
Europe support reconstruction into Laurasia
Reconstructed paleoclimate belts –
suggested polar wandering, potential
evidence for continental drift
Continental drift hypothesis initially
rejected
• Wegener could not come up with viable
driving force
• continents should not be able to plow
through sea floor rocks while crumpling
themselves but not the sea floor
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Paleomagnetism & Continental Drift
Revived
Rock magnetism studies – allowed
determination of magnetic pole locations through
time
Paleomagnetism – uses mineral magnetic
properties to determine direction and distance to
the magnetic pole when rocks formed
– steeper dip angles indicate rocks formed closer
to the magnetic poles
– rocks with increasing age point to pole
locations increasingly far from present
magnetic pole positions
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Paleomegnetism & Continental Drift
Revived
Apparent polar wander curves for
different continents suggest real movement
relative to one another
Reconstruction of supercontinents using
paleomagnetic information fits Africa and
South America like puzzle pieces
• improved fit results in rock units (and
glacial ice flow directions) precisely
matching up across continent margins
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Seafloor Spreading
Seafloor spreading – proposed in
1962 by Harry Hess
• seafloor moves away from the
mid-oceanic ridge due to hot
mantle rock rising by convection
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Seafloor Spreading
Increasing distance from ridge – seafloor, and mantle rocks
beneath it, cools and become more dense
Subduction zones – locations where cool and dense rock sink
back into the mantle, giving rise to oceanic trenches
Overall young age for sea floor rocks (everywhere <200 million
years) is explained by this model
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Plates & Plate Motion
Tectonic plates – composed of the relatively
rigid lithosphere
• lithospheric thickness and age of seafloor
increase with distance from mid-oceanic ridge
• float upon ductile asthenosphere
• interact at their boundaries
o divergent boundaries – plates move apart
o convergent boundaries – plates move
together
o transform boundaries – plates slide past
one another
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Marine magnetic Anaomalies
Marine magnetic anomalies –
bands of stronger and weaker than
average magnetic field strength that
parallel mid-oceanic ridges
• symmetric bar-code anomaly pattern
reflects plate motion away from
ridge coupled with magnetic field
reversals
• matches pattern of reversals seen in
continental rocks (Vine and
Matthews)
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Evidence of Plate Motion
Seafloor age increases with distance from mid-oceanic ridge
• rate of plate motion – equals distance from ridge divided by
age of rocks
• symmetric age pattern reflects plate motion away from ridge
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Evidence of Plate Motion
Mid-oceanic ridges – offset along fracture
zones
• transform fault – fracture zone segment
between offset ridge crests
• relative motion along fault is result of seafloor
spreading from adjacent ridges
Plate motion can be measured using satellites,
radar, lasers and global positioning systems
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Divergent Plate Boundaries
Divergent plate boundaries – plates
move away from each other
• can occur in the middle of the ocean or
within a continent
• marked by rifting, basaltic volcanism, and
eventual ridge uplift
• eventually creates a new ocean basin
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Transform Plate Boundaries
Transform plate boundaries– plates slide
horizontally past one another
• marked by transform faults
• transform faults may connect:
o two offset segments of mid-oceanic ridge
o a mid-oceanic ridge and a trench
o two trenches
• transform offsets of mid-oceanic ridges
allow series of straight-line segments to
approximate curved boundaries required
by spheroidal Earth
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Convergent Plate Boundaries
Convergent plate boundaries – plates move toward
one another
– ocean-ocean plate convergence – marked by ocean trench,
Benioff zone, and volcanic island arc
– ocean-continent plate convergence – marked by ocean
trench, Benioff zone, volcanic arc, and mountain belts
– continent-continent plate convergence – marked by
mountain belts and thrust faults
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Movement of Plate Boundaries
Plate boundaries – can move over time
• mid-oceanic ridge crests can migrate
toward or away from subduction zones or
abruptly jump to new positions
• convergent boundaries can migrate if
subduction angle steepens or overlying
plate has a trenchward motion of its own
• transform boundaries can shift as slivers
of plate shear off
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What Causes Plate Motions?
Any proposed mechanism must explain
why:
• mid-oceanic ridges are hot and elevated,
while trenches are cold and deep
• ridge crests have tensional cracks
• the leading edges of some plates are
subducting sea floor, while others are
continents (which cannot subduct)
Mantle convection – may be the cause
or an effect of circulation set up by ridgepush and/or slab-pull
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Mantle Plumes & Hot Spots
Mantle plumes – narrow columns of
hot mantle rock rise through the mantle
• stationary with respect to moving plates
• large mantle plumes may spread out
and tear apart the overlying plate
o flood basalt eruptions
o rifting apart of continental land masses
• new divergent boundaries may form
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Mantle Plumes & Hot Spots
Mantle plume hot spots in the interior of a plate produce
volcanic chains
• orientation of the volcanic chain shows direction of plate
motion over time
• age of volcanic rocks can be used to determine rate of plate
movement
• Hawaiian islands are a good example
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Plate Tectonics & Ore Deposits
Metallic ore deposits –often located near plate boundaries
• commonly associated with igneous activity
• divergent plate boundaries often marked by mineral-rich hot springs
(black smokers) on sea floor
• hydrothermal circulation near island arcs can produce metal-rich
magmatic fluids
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End of Chapter 19
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