Download Plate Tectonic Theory

10/11/2009
Plate Tectonic Theory
Lithosphere and Asthenosphere
• Not continental ‘drift’ !!
• Importance:
– The unifying theory for all other theories in the
Earth sciences
– Has resulted in a more detailed understanding
of Earth history
– Has enabled geologists to more precisely
(and cheaply) discover new oil, natural gas,
and other subsurface natural resource
deposits
Important Definitions
• Lithospheric Plate – a large segment of
Earth’s crust and upper parts of mantle;
extends to top of the low velocity zone
• Lithospheric Plate Boundary (Margin)
– Divergent - plates move away from one
another
– Convergent – plates move toward one
another
– Transform – plates move horizontally past
one another
Alfred Lothar Wegener (1880-1930)
Part I
Development of Plate Tectonic
Theory
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Pangaea (Greek, meaning ‘all lands’)
Wegener’s Evidence for Pangaea:
• ‘Fit of the Continents’
– Continental margins fit together like pieces of a
jigsaw puzzle
• Paleontological
– Similar fossils on opposite sides of ocean basins
• Geological
– Similar rock types on opposite sides of ocean basins
• Climatological
– Evidence of glaciation in tropical climates
– Fossils of tropical plant species in polar climates
Positions of the continents approximately 200 million years ago
Fit of the Continents
Paleontological Evidence
Locations of Mesosaurus fossils
Glossopteris
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Geological Evidence
Glossopteris
Geological Evidence
Similarity of rock types and ages in the
Appalachian, British Isles and Caledonian
mountain systems
Scottish Highlands
Reconstruction of the Appalachian mountain
system at the time it formed, approximately
300 million years ago
Highlands, North Carolina
Climatological Evidence
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Glacial Grooves
Why was Wegener’s idea rejected?
Wegener (left) and an Innuit guide on his last expedition in Greenland.
This is one of the last photographs taken of Wegener, taken shortly
before his death.
Discovery of The Mid-Ocean Ridge System
Mid-Ocean Ridge System
Paleomagnetism – the study of Earth’s
magnetic field as recorded in ancient rocks
Magnetic minerals ‘point’ in the direction of
the magnetic north pole.
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Apparent Polar Wandering: Two North Poles?
Use of Magnetic Minerals to Determine Paleolatitude
Apparent Polar Wandering: Single North Pole
Step #2: Determine Paleolatitude
From Magnetic Dip
Step 1:
Determine Magnetic Dip:
a)
90o
(NH)
b) 63o (NH)
c) 37o (NH)
d) 60o (SH)
e) 6o (SH)
Cox and Dallrymple: Magnetic Reversals
from all continents record changes in Earth’s
magnetic field at the same time
a)90o N (North Pole)
b)42o N
c) 18o N
d)38o S
e)3o S
Cox and Dallrymple: The Magnetic Time
Scale
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Harry Hess: The mid-ocean ridge is a site of
sea floor spreading
Vine and Matthews: Magnetic sea floor
stripes record magnetic reversals
Normal Polarity – exhibited by rocks with
magnetic minerals which ‘point’ towards
today’s position of the magnetic north pole
Reversed Polarity – exhibited by rocks with
minerals which ‘point’ towards today’s
position of magnetic south
As the sea floor spreads, repeated ‘stripes’
of normal and reversed polarity ocean floor
basalts are erupted along the MOR
Problem:
What did the theory of sea
floor spreading imply about
the size of the Earth?
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Distribution of shallow-, intermediate-, and
deep-foci earthquakes:
Solution to Problem:
Benioff Zones
Benioff Zones
Part II
The Theory
Types of Lithospheric Plate Boundaries
Types of Lithospheric Plate Boundaries
• Convergent Plate Boundaries
• Transform Plate Boundaries
– Characterized by compressive tectonic stress
– Oceanic – continental convent boundary
– Oceanic-oceanic convergent boundary
– Continental-continental convergent boundary
• Divergent Plate Boundaries
– Characterized by shear tectonic stress
– Continental-continental transform plate
boundary
– Oceanic-oceanic transform plate boundary
– Characterized by extensional tectonic stress
– Oceanic-oceanic divergent boundary
– Continental-continental divergent boundary
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Oceanic-Continental Convergent Plate Boundary
Formation of a subduction zone and continental
margin volcanic arc
• Subduction of higher density (mafic)
oceanic lithosphere beneath lower density
(felsic) continental lithosphere
– Formation of magma in the mantle wedge
– Formation of a continental margin volcanic arc
on the overriding continental lithosphere
• Examples: Cascade Range, Andean Range
Cascade Range
Oceanic-Oceanic Convergent Plate Boundary
Andes Range
Formation of a subduction zone and
volcanic island arc
• Subduction of (mafic) oceanic lithosphere
beneath (mafic) oceanic lithosphere
– Formation of magma in the mantle wedge
– Formation of a volcanic island arc on the
overriding oceanic lithosphere
• Examples: Lesser Antilles, western Aleutians,
Japanese islands
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Lesser Antilles
Japanese Islands
Aleutian Islands
Continental-Continental Convergent Plate
Boundary
• Low density (felsic) continental rocks
pushed upward by compressive tectonic
forces
– Medium to high grade metamorphism
– Earth’s highest mountain ranges
• Examples: Himalayas, Appalachians
Appalachians
Himalayas
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Formation of a mountain range along a
continental-continental convergent plate boundary
The On-Going Collision of India and Asia
Approximately 45 million years ago
Paleolatitudes of India (71 million years to present)
Modern-Day Tectonic Setting of India
Mount Everest, Himalayas
Oceanic Lithosphere-Oceanic Lithosphere
Divergent Plate Boundary
• Results in partial melting of underlying
ultramafic mantle rock, producing a mafic
(basaltic) magma
• Mafic lavas are erupted on ocean floor,
forming new ocean floor basalts
• Lithospheric plates on either side of the
plate boundary mive in opposite directions
• Off-set segments of the ridge axis are
connected by transform faults
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Oceanic Lithosphere-Oceanic Lithosphere
Divergent Plate Boundary
Continental Lithosphere-Continental
Lithosphere Divergent Plate Boundary
Continental Rifting
• Stretching, fracturing and sinking of
continental lithosphere forms a continental
rift (rift valley)
– Example: East African Rift
• Eruption of basaltic lavas and filling with
seawater forms a linear sea
– Examples: Red Sea, Gulf of Aden
• Widening of linear sea forms an ocean
basin
East African Rift
East African Rift Volcanoes
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San Andreas Fault
Transform Boundaries
• Continental lithospheric plates move
laterally past one another
– Transform fault – a fault which connects two
off-set segments of a mid-ocean ridge
• Characterized by frequent earthquake
activity
– Example: San Andreas Fault System
Plate Tectonic Boundaries
Western North America
50 ma
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Present
Cascade Range
Sierra Nevada Range and Coast Range
Basin and Range
Rocky Mountains
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Terrane Accretion – Formation of Coast Range
Terrain vs. Terrane
• Terrain – the natural features of an area of
land; the lay of the land’
• Terrane – A crustal fragment whose
geologic history is distinct from that of
adjoining terranes
– Accreted Terrane - a terrane accreted to
adjoining terranes by tectonic forces
Accreted Terranes in Western N. America
Formation of Rocky Mountains and Basin
and Range Provinces
• Rocky Mountains
– Major intraplate mountain system
– Exhibits evidence of compressive stresses,
typical of a convergent plate margin
• Basin and Range
– Major intraplate mountain system
– Exhibits evidence of extensional stresses,
typical of a divergent plate margin
Rocky Mountains – formed due to ‘shallow’ subduction of
oceanic lithosphere beneath western N. America.
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Rocky Mountains
Basin and Range
Basin and Range – formed due to ‘sinking’ of the
subducted lithosphere, allowing for upwelling of hot mantle
rock beneath the crust.
Rio Grande Rift – part of the Basin and Range
Rio Grande Rift
Rio Grande Rift
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Mantle Plumes – Intraplate Volcanism
Pacific Ocean Hot Spot Tracks
The Driving Force of Plate Tectonics
• Mantle Convection
– Heat transfer from Earth’s core drives convection
currents within the mantle
• Slab-Pull/Slab-Push
– Gravity ‘pushes’ oceanic lithosphere (slabs) away
from mid-ocean ridge axes, and ‘pulls’ slabs into
subduction zones
• Hot Plumes
– Hot rising plumes of mantle rock drive mantle
convection currents
Mantle Convection
Slab-Pull/Slab-Push
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Hot Plumes
Pangaea – 200 million years ago
The Breakup of Pangaea
The Breakup of Pangaea
The Breakup of Pangaea
The Breakup of Pangaea
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Present-day Tectonic Plate Rates and
Directions of Motion
Tectonic Setting of Northern
Sumatra
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