Plate Boundaries (Edexcel GCSE Geography B)

Distribution & Characteristics of Plate Boundaries

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

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

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

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

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

Earthquake distribution

• Earthquakes occur at all types of plate boundaries

  • Most occur along the Pacific 'Ring of Fire' (approximately 90%)

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

Causes of Earthquake & Volcanic Hazards

• 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

• 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