THE THEORY OF PLATE TECTONICS

INTRODUCTION u Tectonics- large scale deformational features of the crust u Plate tectonics – Earth’s outer shell divided into plates – Plates move & change in size thru time u Activity at plate boundaries u Combines: – Continental drift – Sea-floor spreading – Paleomagnetism

INTRODUCTION u Ideas – Continental drift- Alfred Wegener – Sea-floor spreading – Paleomagnetism

Early Case for Continental Drift- Alfred Wegener (F.B. Taylor & H.H. Baker) u Continental coastlines fit together –1620 Sir Francis Bacon: Africa and S. America u Rocks & structures indicated that continents joined – Pangea- supercontinent of the late Paleozoic » Separated into Laurasia & Gondwanaland u Fossil evidence- Glossopteris & Mesosaurus u Late Paleozoic glaciation u Skepticism about Continental Drift – Problem of driving mechanism

Continental Drift u Alfred Wegener 1912 u Pangaea (ALL EARTH) u Evidence: –Continents FIT together like the pieces of a puzzle –Fossils –Rocks & structures –Paleoclimate

Continental Drift u Alfred Wegener 1912 u Pangaea (ALL EARTH) u Evidence: –Continents FIT together like the pieces of a puzzle –Fossils –Rocks and structures –Paleoclimate

Continental Drift u Alfred Wegener 1912 u Pangaea (ALL EARTH) u Evidence: –Continents FIT together like the pieces of a puzzle –Fossils –Rocks and structures –Paleoclimate

INTRODUCTION u Tectonics- large scale deformational features of the crust u Plate tectonics – Earth’s outer shell divided into plates – Plates move & change in size u Activity at plate boundaries u Combined: – Paleomagnetism – Sea-floor spreading

INTRODUCTION u Tectonics- large scale deformational features of the crust u Plate tectonics – Earth’s outer shell divided into plates – Plates move & change in size u Activity at plate boundaries u Combined: – Paleomagnetism – Sea-floor spreading

Paleomagnetism  Iron becomes magnetized below the Curie Point (   C) u Magnetite and hematite aligns on existing magnetic field u Dip indicates old magnetic pole position u Apparent motion of north magnetic pole through time – Split in path – indicates continents split apart

Paleomagnetism u Magnetite aligns on existing magnetic field u Dip indicates old magnetic pole position u Apparent motion of north magnetic pole through time – Split in path – indicates continents split apart

SEA-FLOOR SPREADING u Magnetic anomalies –1950’s detection of 10-50km wide strips symmetrical about ocean ridges –Vine and Matthews: magnetic reversals u Sea-floor moves away from mid-oceanic ridge u Plunges beneath continent or island arc- subduction Plate movement rate of 1 to 20 cm/year, 5 cm/yr average u Driving force – Mantle convection – Ridge Push- Slab Pull forces

SEA-FLOOR SPREADING u Explanations – Mid-oceanic ridge » Hot mantle rock beneath ridge u High heat flow u Basalt eruptions » Rift valley » Shallow-focus earthquakes

SEA-FLOOR SPREADING u 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? u 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 u Plate – Entirely sea floor or – continental and oceanic u Lithosphere – Crust & uppermost mantle – Thickness increases away from ridge u Asthenosphere – Low seismic velocity zone – behaves plastically

Plates and Plate Motion u Plate – Entirely sea floor or – continental and oceanic u Lithosphere – Crust & uppermost mantle – Thickness increases away from ridge u Asthenosphere – Low seismic velocity zone – behaves plastically

History of Continental Positions u Pangea split up 200 m.y. u Continents in motion for at least 2 billion years

How do we know that plates move? u Fracture Zones & Transform Faults – Pattern of earthquakes at ridges and fracture zones – Transform fault u Measuring plate motion directly – Use of satellites

Plates and Plate Motion u Interior of plates relatively inactive- Cratons u 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 u Plate tectonics a unifying theory for geology u Boundaries – Divergent – Convergent – Transform

DIVERGENT BOUNDARIES u During break up of a continent – Rifting, basaltic eruptions (Flood Basalts), uplifting – Extension- normal faults, rift valley (graben) forms – Shallow focus earthquakes u Continental crust separates – Fault blocks along edges – Oceanic crust created – Rock salt may develop in rift

u East African Rift System –early stages of rifting –continental rifting

Red Sea Rift u Red Sea u Gulf of Eilat u Dead Sea u Linear Seas

TRANSFORM BOUNDARIES u Two plates slide past each other u Usually between mid-oceanic ridge segments – Can also connect ridge and trench – Or trench to trench u Origin of offset of ridges

CONVERGENT BOUNDARIES u Plates move toward each other u One plate overrides the other – Subduction zone

CONVERGENT BOUNDARIES u 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 u 10 4 km 2 to 10 8 km 2 u New sea floor added to trailing edge of plate – e.g. North American plate growing at mid- Atlantic ridge u 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 u Thermal Plumes u Explains –Yellowstone volcanism –Hawaiian volcanism –Aseismic ridges

What Causes Plate Motions? u Slab push-pull u Convection in mantle –Deep mantle convection – Two-layer convection

What Causes Plate Motions? u Convection in mantle – Convection a result of plate motion » Ridge push » Slab pull » Trench suction