22 Plate TectonicsPlate Tectonics22 Plate TectonicsPlate Tectonics

Chapter 2: Plate Tectonics

Theory of plate tectonicsTheory of plate tectonics

“Plate Tectonics” explains why the plates of the earth are moving.

The main idea (theory) was proposed by Alfred Wegener in 1912.

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

Movement is caused by convection currents in the mantle.

Plates diverge, converge or slide past each other.

Magma Convection currents Magma Convection currents

Causes the movement of plates

Magma heats; rises and moves towards the upper mantle

The magma 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 (Iceland!!)

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

The rocks found further from the plate divide are older so older rocks are found closer towards continents.

Chapter 2: Plate Tectonics

Red represents the newest crust of the earth. The further from red the older the crust. Where is Ireland?

Theory of continental drift

Suggests that the continents are moved 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. ……..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 (pangea divided into Laurasia and Gondwanaland…until these broke further apart into todays continents.)

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:

Divergent (boundaries of construction)

Convergent (boundaries of destruction)

Conservative (passive boundaries)

Chapter 2: Plate Tectonics

Chapter 2: Plate Tectonics

1. Divergent (constructive boundaries)

New crust is formed

Mid-ocean ridges created (Mid-Atlantic Ridge)

Plates separate and move away from each other

Example: American and Eurasian plates

Chapter 2: Plate Tectonics

2. Convergent (boundaries of destruction)

Three types of destructive boundaries:

I.Oceanic-continent: where an ocean and continental plate collide

II.Oceanic-oceanic: where two ocean plates collide

III.Continent-continent: where two continental plates collide

• Heavier, older plate dips • Subducted beneath lighter, younger plate• Deep sea trench may develop

• Heavier oceanic plate subducts • Magma rises to create volcanoes at the

continental plate• Continental plate is buckled and fold

mountains form

• Neither plates sink on collision• Lithosphere subducts • Continental crusts buckle • Formation of fold mountains

Chapter 2: Plate Tectonics

3. Conservative (passive boundaries)

crust 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

e.g. the San Andreas Fault in California

1. Who proposed the Theory of Plate Tectonics?

2. What is the main idea of this theory?3. What causes the plates of the earth

to move?4. What causes magma to move in the

mantle?5. What was Pangea?6. What proof do we have that the

continents of the earth were once all joined together?

7. What are the three types of plate boundary?8. Name examples of each plate boundary?9. What feature is generally found at each boundary?10. Can you name a famous world feature found at each of the main boundary types?