the theory of plate tectonics. introduction u tectonics- large scale deformational features of the...
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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
Early Case for Continental Drift-Alfred 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
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 (C)
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
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
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
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
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 Continental-Continental convergence
– Two continents approach each other and collide» Sea floor subducted on one side
» Ocean becomes narrower and narrower
» Continent wedged into subduction zone but not carried down it
» Suture zone
– Crust thickened» Two thrust belts
– Mountain belt in interior of continent
Plate Size
104 km2 to 108 km2
New sea floor added to trailing edge of plate– e.g. North American plate growing at mid-
Atlantic 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
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