Plate Boundaries (Edexcel GCSE Geography: B (1GB0))

Plate tectonics

• The Earth's crust is broken into 15 large tectonic plates and a number of smaller ones
• The place where the plates meet is called a plate boundary

Tectonic plates

Types of plate boundary

• Volcanic eruptions and earthquakes most commonly occur at or near plate boundaries
• There are three main types of plate boundaries:
  • Divergent
  • Convergent
  • Conservative 

Divergent plate boundary

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

Divergent plate boundary

Convergent plate boundary

• At a convergent plate boundary the plates are moving together
• The denser, heavier oceanic plate subducts under the lighter, less dense continental plate
• The boundary between the Nazca plate and the South American plate is one example
• Both volcanic eruptions and earthquakes occur at this type of plate boundary

Collision Zone

• When two continental plates collide the rock which makes up the land is folded forming fold mountains
• An example of a collision zone is where the Eurasian and Indo-Australian plates meet forming the Himalayas

Convergent plate boundary

Conservative boundary

• At a conservative boundary the plates move passed each other in opposite directions or in the same direction at different speeds
• An example of this type of boundary would be the San Andreas Fault, California
• Earthquakes are the only hazard at this type of boundary

Conservative plate boundary 

Hot spots 

• At a hot spot the tectonic plate passes over a plume of magma:
  • The magma rises to the surface through cracks in the crust
  • As the tectonic plate moves slowly over the magma plume a line of islands may form e.g. Hawaii

Hot spot

Earthquake distribution

• Earthquakes occur at all types of plate boundaries
  
  • Most occur along the Pacific 'Ring of Fire' (approximately 90%)

Global earthquake distribution

Volcano distribution

• Most volcanoes occur at constructive and destructive plate boundaries
• The majority of active volcanoes (approximately 75%) are located around the rim of the Pacific Ocean called the 'Ring of Fire'
• Hotspots occur away from plate boundaries and are plumes/columns of magma which escape through the Earth's crust

Active volcano distribution

• A volcano is formed when magma erupts onto the Earth's surface as lava through a vent in the Earth's crust
• The magnitude of a volcanic eruption is measured on the Volcanic Explosivity Index (VEI)
  • The scale is open ended but the highest in recorded human history was an 8 (Tambora 1815)
• All volcanoes have features in common, as shown in the diagram below:

• Due to the type of lava erupted the formation of a volcano varies
• Composite (strato-volcanoes) 
  • Steep-sided
  • Sticky (viscous) lava 
  • More explosive eruptions due to higher gas content
  • Formed from alternating layers of ash and lava
  • Tend to form on convergent plate boundaries

• Shield volcanoes
  • Gently sloping sides
  • Runny/thin lava
  • Less explosive due to lower gas content: gentle eruptions
  • Tend to form on divergent plate boundaries or hot spots
  • Frequent eruptions

• Volcanoes may be active, dormant or extinct
  • Active
    • The volcano has recently erupted and is likely to erupt again
  • Dormant
    • Has erupted in the last 2000 years and may possibly erupt again
  • Extinct
    • Shows no evidence of eruption in historic times and there is no evidence of a magma reservoir

Volcanic eruption features

• Eruptions may include a range of features as shown in the table below:

Earthquakes

• An earthquake is the sudden, violent shaking of the ground
• Earthquakes are the result of pressure building when tectonic plates move
• The epicentre is the point on the Earth's surface directly above the focus
• The focus is the point at which the earthquake starts below the Earth's surface: the energy released by the earthquake travels out from the focus
• The magnitude (amount of energy released) by earthquakes is measured on the Moment Magnitude Scale which replaced the Richter scale
• Seismometers are used to measure the magnitude
• The damage caused by earthquakes is measured on the Mercalli Scale

• Earthquakes can occur anywhere but mostly occur at or near plate boundaries
• Earthquakes happen at all plate boundaries: divergent, convergent, collision zones and conservative 
• At a divergent plate boundary, earthquakes tend to be weaker as the plates are moving apart
• At convergent, collision zone and conservative plate boundaries earthquakes tend to be stronger

Primary and secondary hazards

The impacts of earthquakes and volcanic eruptions can be categorised into:

• Primary hazards: these are a direct result of the earthquake or eruption
• Secondary hazards: these occur as a result of the primary effects

Primary and Secondary Hazards

Tsunami

• When an earthquake occurs beneath the sea bed this can lead to a tsunami
  • As the sea bed jolts water is displaced and forced upwards creating a wave
  • As the wave approaches the land they slow and the wavelength becomes compressed
  • This leads to an increase in wave height: they can reach 5-10 meters

Tsunami Formation

• Other causes of tsunami include:
  • Landslides which may be due to earthquakes and volcanic eruptions displacing the water
  • Underwater volcanic eruptions

Effects of earthquakes and volcanic eruptions

• Although the characteristics of earthquakes and volcanic eruptions are very different many of the impacts are similar
• Impacts include:
  • Loss of life and injury: this may be immediate through falling buildings or ash fall. Alternatively, it may be in the days and weeks after the event due to unclean water or disease
  • Collapse or destruction of buildings: this leads to people being homeless for months and even years after the event
  • Transport network: roads, bridges and railways may be damaged or destroyed. This can impact on the speed which aid can get to affected areas
  • Loss of jobs and businesses: this impacts the economy when factories and offices are destroyed or damaged
  • Loss of crops: leads to food shortages and affects farmers income
  • Power and water supplies are damaged: the affects the supply of potable (clean) water
  • Damage to the environment: loss of vegetation and habitats, effects on climate of ash
  • Closure of airports: it is dangerous for jet planes to fly through ash clouds due to possible engine failure, so flights are cancelled