Chapter 2: Plate Tectonics

Theory of plate tectonics

• Proposed by Alfred Wegener

• Earth’s crust consists of plates that move or float on the mantle

• Movement is fuelled by convection currents

• Plates diverge, converge or slide past each other

Chapter 2: Plate Tectonics

Convection currents

• Responsible for the movement of plates

• Magma heats; rises and moves towards the upper mantle

• Flows sidewards, then cools and sinks

• Plate moves with sinking magma

• Circular movement fuels plate movement

Chapter 2: Plate Tectonics

Sea floor spreading

• New rock is formed where plates were split apart

• Ocean floor widens; magma rises from within the mantle and fills the opening

• Magma cools; new ocean floor formed

• New sea floor youngest at the mid-ocean ridge

• Older towards continents

Chapter 2: Plate Tectonics

Theory of continental drift

• Suggests that the continents are transported across the planet by convection currents

• Drifting of the continents is still occurring today

• Continents were once all joined together in one single landmass known as Pangaea. It was effectively a supercontinent

• Approximately 200 million years ago Pangaea began to break apart

• The continents were fuelled by the convection currents and so they began to drift apart

Chapter 2: Plate Tectonics

The process of subduction

• Continents collide

• Oceanic plate is heavier than the continental plate

• Ocean floor pulled down into the mantle

• Subduction occurs

Chapter 2: Plate Tectonics

Plate boundaries

There are three types of plate boundaries:

1.Divergent (boundaries of construction)

2.Convergent (boundaries of destruction)

3.Conservative (passive boundaries)

Chapter 2: Plate Tectonics

1. Divergent (boundaries of construction)

• New rock is formed

• Mid-ocean ridges created

• Plates separate and move away from each other

Chapter 2: Plate Tectonics

2. Convergent (boundaries of destruction)

Three types of destructive boundaries:

I. Oceanic-oceanic: where two ocean plates collide

II. Oceanic-continent: where an ocean and continentalplate collide

III. Continent-continent: where two continental platescollide

Chapter 2: Plate Tectonics

2. Convergent (boundaries of destruction) -

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i. Oceanic-oceanic boundaries

• Heavier, older plate dips

• Subducted beneath lighter, younger plate

• Deep sea trench may develop

• Subducted plate melts as it moves down the mantle

• Volcanic activity

• Island arc active volcanoes, e.g. Japan

• Earthquakes

Chapter 2: Plate Tectonics

2. Convergent (boundaries of destruction) -

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ii. Oceanic-continent boundaries• Heavier oceanic plate subducts

• Ocean trench forms

• Oceanic plate melts

• Magma rises to create volcanoes at the continental plate

• Continental plate is buckled

• Fold mountains belt formed

• Faulting

• Volcanism

• Earthquakes

• e.g. The Andes in South America and the Wicklow Mountains in Ireland

Chapter 2: Plate Tectonics

2. Convergent (boundaries of destruction) -

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iii. Continent-continent boundaries

• Neither plates sink on collision

• Lithosphere subducts

• Continental crusts buckle

• Formation of fold mountains

• Earthquake activity

Chapter 2: Plate Tectonics

3. Conservative (passive boundaries)

• Rock is neither created nor destroyed

• Plates slide past each other

• The line along which the plates slide is known as a fault line

• Known as transform faults

• Also known as plate boundaries

• e.g. the San Andreas Fault in California