Theory of Plate Tectonics (Edexcel A Level Geography)

Earth’s structure

• The Earth has three main layers:
  • The crust 
  • The mantle 
  • The core

Crust

• There are two types of crust:
  • Continental - a thicker (45-50km), less dense layer (mostly granite)
  • Oceanic - a thinner (6-10km), denser layer (mostly basalt)
• The crust consists of seven major and several minor tectonic plates
• The Mohorovičić discontinuity - is the boundary between the crust and the mantle is also known as Moho

Mantle

• The mantle is between the crust and core and is the widest layer
  • The upper mantle has two layers:
    • The rigid layer above the asthenosphere, which together with the crust, makes up the lithosphere
    • The asthenosphere is a semi-molten, plastic type layer, which moves under high pressure
  • The lower mantle is hotter and denser than the upper mantle
  • The intense pressure, at depth, keeps the lower mantle solid

Core

• The core is made up of two parts:
  • Inner Core - solid centre, mostly composed of iron
  • Outer core - semi-molten, mostly liquid iron and nickel

Earth's structure

• The tectonic plates move slowly over the asthenosphere

Development of plate tectonic theory

The development of plate tectonic theory

• Scientists agree that the plates move, but there is still debate over the mechanisms that cause the movement

Mantle convection

• In the past, the theory of convection currents was used on its own to explain tectonic plate movement
• The heat from radioactive decay in the core moves upwards into the mantle
• It creates convection currents, which push up into the spreading mid-ocean ridges, forcing them further apart called the ridge push

Convection currents in the mantle

• Other processes are now recognised as being important in plate movement

Seafloor spreading

• Palaeomagnetism provides evidence that the sea floor has gradually moved apart at a mid-ocean ridge
• Lava cools and solidifies with the minerals lining up with the magnetic field
• The direction of the minerals on either side is a mirror image

Seafloor spreading and palaeomagnetism

Subduction and slab pull

• Convection currents in the mantle drag the overlying lithosphere towards each other
• A subduction zone is formed when two plates meet
• The heavier, denser plate subducts under the lighter, less dense plate
• As oceanic crust cools, it becomes denser and thicker, and gravity forces the lithosphere down into the subduction zone 
• As it sinks, it drags or pulls the plate with it
• This is known as slab pull

• Each plate boundary has different processes
• There are four plate boundary types
  • Convergent (destructive)
  • Divergent (constructive)
  • Collision
  • Transform (conservative)

Convergent (destructive) boundary

• At a convergent (destructive) plate boundary, the plates are moving towards each other
• Oceanic plate and continental plates meet:
  • The denser, heavier oceanic plate subducts under the lighter, less dense continental plate
  • This forms deep ocean trenches in the subduction zone
    • Deep sea trenches are long, narrow depressions in the ocean floor with depths of over 6km and up to 11km
    • Trenches are found adjacent to land areas and associated with island arcs
    • The boundary between the Nazca plate and the South American plate is an example
  • Both violent volcanic eruptions and earthquakes occur at this type of plate boundary
    • The narrow area where earthquakes tend to occur in the subduction is known as the Benioff Zone
  • Oceanic and continental convergent plate boundaries are also responsible for fold mountains
    • Fold mountains form the highest of the world’s mountain ranges
    • They are long, relatively narrow belts of mountains
    • The main fold range is made up of a series of smaller ranges

Destructive plate boundary

• When two oceanic plates meet:
  • The heavier of the two oceanic plates subduct, forming deep ocean trenches and island arcs 
  • Island arcs are a series of volcanic islands, formed in an arc shape, e.g. the Caribbean
  • Submarine volcanic eruptions, lead to crust building up and rising above sea level

Constructive

• At the constructive boundary, the plates are moving apart
• The Mid-Atlantic Ridge is an example of a constructive plate boundary
• Both volcanic eruptions and earthquakes can occur at this type of plate boundary

Constructive plate boundary

Collision 

• When two continental plate boundaries meet, both may fold and deform; e.g. the Himalayas are formed by the collision of the Eurasian and Indian plates
• At a collision boundary two plates of similar density move towards each other
• As neither plate can sink into the denser rocks below, they are crushed, crumpled and forced upwards, usually folding in the process
• This creates collision fold mountains such as the Himalayas, which are still being formed upwards, at a rate of 1cm/annum
• As there is no subduction, there is no volcanic activity
• Earthquakes are the main hazard at this type of plate boundary

Collision boundary

Transform or conservative

• Plates move slowly past each other – they do not have to be in different directions but at a different rate of travel in the same direction
• Transform margins are offset at angles, creating zigzag patterns to accommodate movement
• They become stuck and pressure builds, the plates eventually 'snap' past each other
  • These can be called ‘strike-slip’ faults as they strike/stick and then slip/release past each other
• The friction causes earthquakes but not volcanoes
• Land is neither made or destroyed

  • Because no subduction occurs, there is no melting of the crust and so no volcanic activity 

    
    
    

• The processes which occur at the plate boundaries impact on the magnitude of the eruption or earthquake
• The properties of the magma have a crucial role on the magnitude and frequency of eruptions

AWAITING IMAGE

Properties of magma

• At divergent boundaries:
  • Earthquakes tend to be mild and shallow
  • Eruptions tend to be small and effusive
  • The eruptions are usually of basalt lava:
    • Low gas content
    • Low viscosity
    • Higher temperature
• At convergent boundaries:
  • Friction and pressure build up in the Benioff zone (the area within the subduction zone where most friction and pressure build up occurs) causes strong earthquakes
  • Volcanic eruptions tend to be explosive as the magma is forcing its way to the surface
  • These eruptions are often rhyolite lava:
    • High gas content
    • High viscosity
    • Lower temperature
• At transform boundaries:
  • Plates can stick causing a significant build up of pressure and powerful earthquakes

Comparison of Magma Types

Rock type	BASALT	ANDESITE	RHYOLITE
Characteristics	Black to dark grey	Medium to dark grey	Light colour
% on surface	80%	10%	10%
Silica content	45-55%	55-65%	65-75%
Temp	1000-1200°C	800-1000°C	600-900°C
Viscosity	Low	Medium	High
Gas escape	Easy	Medium	Difficult
Eruptive nature	Gentle	Medium	Explosive