THE THEORY OF
PLATE TECTONICS
INTRODUCTION
 Tectonics-
large scale deformational
features of the crust
 Plate
tectonics
– Earth’s outer shell divided into plates
– Plates move & change in size thru time
 Activity at plate boundaries
 Combines:
– Continental drift
– Sea-floor spreading
– Paleomagnetism
INTRODUCTION
 Ideas
–
–
–
Continental drift- Alfred Wegener
Sea-floor spreading
Paleomagnetism
Early Case for Continental DriftAlfred Wegener (F.B. Taylor & H.H. Baker)
 Continental
coastlines fit together
– 1620 Sir Francis Bacon: Africa and S. America
 Rocks
–
& structures indicated that continents joined
Pangea- supercontinent of the late Paleozoic
»
Separated into Laurasia & Gondwanaland
 Fossil
evidence- Glossopteris & Mesosaurus
 Late Paleozoic glaciation
 Skepticism
–
about Continental Drift
Problem of driving mechanism
Continental Drift



Alfred Wegener 1912
Pangaea (ALL EARTH)
Evidence:
– Continents FIT
together like the pieces
of a puzzle
– Fossils
– Rocks & structures
– Paleoclimate
Continental Drift



Alfred Wegener 1912
Pangaea (ALL EARTH)
Evidence:
– Continents FIT
together like the pieces
of a puzzle
– Fossils
– Rocks and structures
– Paleoclimate
Continental Drift



Alfred Wegener 1912
Pangaea (ALL EARTH)
Evidence:
– Continents FIT
together like the pieces
of a puzzle
– Fossils
– Rocks and structures
– Paleoclimate
Evidence for Continental DriftRevisited
 Fitting
continents at continental slope rather
than shoreline- 1000m depth
 Refined matches of rocks between
continents
 Isotopic ages support matches
 Glacial evidence
 Matches between Africa and South America
are particularly convincing
INTRODUCTION
 Tectonics-
large scale deformational
features of the crust
 Plate
tectonics
– Earth’s outer shell divided into plates
– Plates move & change in size
 Activity at plate boundaries
 Combined:
– Paleomagnetism
– Sea-floor spreading
INTRODUCTION
 Tectonics-
large scale deformational
features of the crust
 Plate
tectonics
– Earth’s outer shell divided into plates
– Plates move & change in size
 Activity at plate boundaries
 Combined:
– Paleomagnetism
– Sea-floor spreading
Paleomagnetism
 Iron
becomes magnetized below the Curie Point
(600oC)
 Magnetite and hematite aligns on existing
magnetic field
 Dip indicates old magnetic pole position
 Apparent motion of north magnetic pole through
time
– Split in path
– indicates continents split apart
Paleomagnetism
 Magnetite
aligns on existing magnetic field
 Dip indicates old magnetic pole position
 Apparent motion of north magnetic pole through
time
– Split in path
– indicates continents split apart
INTRODUCTION
 Tectonics-
large scale deformational
features of the crust
 Plate
tectonics
– Earth’s outer shell divided into plates
– Plates move & change in size
 Activity at plate boundaries
 Combined:
– Paleomagnetism
– Sea-floor spreading
SEA-FLOOR SPREADING
 Magnetic
anomalies
– 1950’s detection of 10-50km wide strips symmetrical
about ocean ridges
– Vine and Matthews: magnetic reversals
 Sea-floor
moves away from mid-oceanic ridge
 Plunges beneath continent or island arc- subduction
(earthquake define zone, Benioff zone)
 Plate
movement rate of 1 to 20 cm/year, 5 cm/yr average
 Driving force
–
–
Mantle convection
Ridge Push- Slab Pull forces
SEA-FLOOR SPREADING
 Explanations
–
Mid-oceanic ridge
»
Hot mantle rock beneath ridge
High heat flow
 Basalt eruptions

»
»
Rift valley
Shallow-focus earthquakes
SEA-FLOOR SPREADING
 Explanations
–
Oceanic trenches
»
»
»
»
–
Low heat flow
Negative gravity anomalies
Benioff zone earthquakes
Andesitic volcanism
Age of sea floor
»
»
»
Young age of sea floor rocks (oldest 160 my)
Implies youngest should be at ridges, oldest at
trenches
Explains pattern of pelagic sediment
How do we know that plates
move?
 Marine
–
magnetic anomalies
Vine-Matthews Hypothesis
»
»
»
»
–
–
Anomalies
Reversals
Normal and reverse polarity
Positive and negative anomalies
Measuring the rate of sea floor spreading
Predicting sea floor age
Plates and Plate Motion
 Plate
–
–
Entirely sea floor or
continental and oceanic
 Lithosphere
–
–
Crust & uppermost mantle
Thickness increases away from ridge
 Asthenosphere
–
–
Low seismic velocity zone
behaves plastically
Plates and Plate Motion
 Plate
–
–
Entirely sea floor or
continental and oceanic
 Lithosphere
–
–
Crust & uppermost mantle
Thickness increases away from ridge
 Asthenosphere
–
–
Low seismic velocity zone
behaves plastically
History of Continental Positions
 Pangea
split up 200 m.y.
 Continents in motion for at least 2 billion
years
How do we know that plates
move?
 Fracture
Zones & Transform Faults
– Pattern of earthquakes at ridges and fracture
zones
– Transform fault
 Measuring plate motion directly
– Use of satellites
Plates and Plate Motion
 Interior
of plates relatively inactive- Cratons
 Activity along boundaries
– Trenches (zone of subduction), melanges (complex of
shear rock), accretionary prism (sedimentary and volcanic
wedges separated by high angle faults)
– e.g., earthquakes, volcanoes, young mountain belts
 Plate
tectonics a unifying theory for geology
 Boundaries
– Divergent
– Convergent
– Transform
DIVERGENT BOUNDARIES
 Continuing
–
–
divergence
Widening sea
Mid-oceanic ridge system
»
–
70,000 km in length; 30-35 km wide; 1-2 km deep
New crust formed at mid-oceanic ridge
»
Ophiolite Sequence (FROM THE TOP DOWN)
 Pillow
basalt
 sheeted dikes
 Layered Gabbro
 Peridotites: layered ultramafic rocks
DIVERGENT BOUNDARIES
 During
break up of a continent
– Rifting, basaltic eruptions (Flood Basalts),
uplifting
– Extension- normal faults, rift valley (graben)
forms
– Shallow focus earthquakes
 Continental crust separates
– Fault blocks along edges
– Oceanic crust created
– Rock salt may develop in rift

East African Rift System
– early stages of rifting
– continental rifting
Red Sea Rift




Red Sea
Gulf of Eilat
Dead Sea
Linear Seas
DIVERGENT BOUNDARIES
 Continuing
–
–
–
divergence
Widening sea
Mid-oceanic ridge
New crust formed at mid-oceanic ridge
»
Ophiolite Sequence (FROM THE TOP DOWN)
 Pillow
basalt
 sheeted dikes
 Layered Gabbro
 Peridotites: layered ultramafic rocks
–
–
Marine sediment covers continental edges
Passive continental margin
TRANSFORM BOUNDARIES
 Two
plates slide past each other
 Usually between mid-oceanic ridge segments
– Can also connect ridge and trench
– Or trench to trench
 Origin of offset of ridges
CONVERGENT BOUNDARIES
 Plates
move toward each other
 One plate overrides the other
–
Subduction zone
CONVERGENT BOUNDARIES
 Oceanic-Oceanic
–
Convergence
Oceanic trench
»
–
–
Benioff zone (Dipping 200-900; average 450)
Magma generated at depth
»
»
–
–
Andesitic volcanism
Batholith implacement
Island arc forms
»
–
curved convex to subducting plate
Angle of subduction determines distance of arc from trench
Accretionary wedge
Trench migration in time
CONVERGENT BOUNDARIES
 Oceanic-Continental
–
Convergence
Active continental margin
»
»
–
–
–
–
Subduction of oceanic lithosphere beneath
continental lithosphere
Accretionary wedge & forearc basin
Magmatic arc- volcanoes & plutons
Crustal thickening and mountain belts
Regional metamorphism
Thrust faulting & folding on continental side
»
Backarc basin
CONVERGENT BOUNDARIES
 Continental-Continental
–
convergence
Two continents approach each other and
collide
»
»
»
»
–
Crust thickened
»
–
Sea floor subducted on one side
Ocean becomes narrower and narrower
Continent wedged into subduction zone but not
carried down it
Suture zone
Two thrust belts
Mountain belt in interior of continent
Motion of Plate boundaries
 Boundaries
move as well as plates
 Ridge crests may jump to a new position
 Convergent boundaries can migrate or jump
 Transform boundaries can change position
– San Andreas fault may shift
Plate Size
 104
km2 to 108 km2
 New sea floor added to trailing edge of plate
– e.g. North American plate growing at midAtlantic ridge
 Oceanic plate might get smaller as continetal
plate overrides it
– e.g. Eastward moving Nazca plate subducted
beneath westward moving South American
plate
Intra-Plate Features
 Thermal
Plumes
 Explains
– Yellowstone volcanism
– Hawaiian volcanism
– Aseismic ridges
Attractiveness of Plate Tectonics
Theory
 Many
–
of earth features explained. Summary:
Distribution of volcanoes
»
»
–
–
–
Basaltic at diverging boundaries
Andesitic at converging boundaries
Earthquake distribution
Young mountain belts
Sea floor
»
»
»
Mid-oceanic ridge
Oceanic trenches
Fracture zones
What Causes Plate Motions?
Slab
push-pull
Convection in mantle
–Deep mantle convection
– Two-layer convection
What Causes Plate Motions?
 Convection
–
in mantle
Convection a result of plate motion
»
»
»
Ridge push
Slab pull
Trench suction