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 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

East African Rift System

– early stages of rifting

– continental rifting

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

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 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