Download presentation
Presentation is loading. Please wait.
1
THE THEORY OF PLATE TECTONICS
2
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
4
INTRODUCTION Ideas Continental drift- Alfred Wegener
Sea-floor spreading Paleomagnetism
5
Early Case for Continental Drift- Alfred Wegener (F. B. Taylor & H. H
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
6
Continental Drift Alfred Wegener 1912 Pangaea (ALL EARTH) Evidence:
Continents FIT together like the pieces of a puzzle Fossils Rocks & structures Paleoclimate
7
Continental Drift Alfred Wegener 1912 Pangaea (ALL EARTH) Evidence:
Continents FIT together like the pieces of a puzzle Fossils Rocks and structures Paleoclimate
8
Continental Drift Alfred Wegener 1912 Pangaea (ALL EARTH) Evidence:
Continents FIT together like the pieces of a puzzle Fossils Rocks and structures Paleoclimate
10
Evidence for Continental Drift- Revisited
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
11
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
13
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
14
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
16
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
18
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
19
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
21
SEA-FLOOR SPREADING Explanations Mid-oceanic ridge
Hot mantle rock beneath ridge High heat flow Basalt eruptions Rift valley Shallow-focus earthquakes
22
SEA-FLOOR SPREADING Explanations Oceanic trenches Age of sea floor
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
23
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
27
Plates and Plate Motion
Entirely sea floor or continental and oceanic Lithosphere Crust & uppermost mantle Thickness increases away from ridge Asthenosphere Low seismic velocity zone behaves plastically
29
Plates and Plate Motion
Entirely sea floor or continental and oceanic Lithosphere Crust & uppermost mantle Thickness increases away from ridge Asthenosphere Low seismic velocity zone behaves plastically
31
History of Continental Positions
Pangea split up 200 m.y. Continents in motion for at least 2 billion years
33
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
35
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
36
DIVERGENT BOUNDARIES Continuing divergence Widening sea
Mid-oceanic ridge system 70,000 km in length; 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
37
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
41
East African Rift System
early stages of rifting continental rifting
43
Red Sea Rift Red Sea Gulf of Eilat Dead Sea Linear Seas
45
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
48
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
50
CONVERGENT BOUNDARIES
Plates move toward each other One plate overrides the other Subduction zone
52
CONVERGENT BOUNDARIES
Oceanic-Oceanic Convergence Oceanic trench curved convex to subducting plate Benioff zone (Dipping ; average 450) Magma generated at depth Andesitic volcanism Batholith implacement Island arc forms Angle of subduction determines distance of arc from trench Accretionary wedge Trench migration in time
55
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
57
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
58
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
60
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
61
Intra-Plate Features Thermal Plumes Explains Yellowstone volcanism
Hawaiian volcanism Aseismic ridges
64
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
65
What Causes Plate Motions?
Slab push-pull Convection in mantle Deep mantle convection Two-layer convection
66
What Causes Plate Motions?
Convection in mantle Convection a result of plate motion Ridge push Slab pull Trench suction
Similar presentations
