Causes of Earthquakes & Volcanoes (OCR GCSE Geography)

Earthquakes and Plate Boundaries

• Earthquakes can occur anywhere but mostly occur at or near plate boundaries
• Earthquakes happen at all plate boundaries - constructive, destructive, collision and conservative
• 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

Measuring earthquakes

• Seismometers are used to record the size of the seismic waves 
• The magnitude (amount of energy released) by earthquakes is measured on the Moment Magnitude Scale which replaced the Richter scale

The damage caused by earthquakes is measured on the Mercalli Scale

Mercalli Scale Diagram

Mercalli Scale

Earthquake focus

• The depth of the earthquake focus can affect the impact of an earthquake
• A deep focus earthquake starts at a depth greater than 70 km below the surface
• They tend to occur at destructive plate boundaries earthquakes and may be very powerful
• The seismic waves tend to move vertically so a smaller area is affected
• A shallow focus earthquake starts at a depth less than 70 km below the surface
• They can occur at all plate boundaries and tend to be less powerful
• The seismic waves tend to move horizontally so a larger area is affected

Earthquake sequence

• The sequence of an earthquake is the same regardless of the boundary at which it happens:
• As the tectonic plates move, they can get stuck
• Pressure builds as the plates continue to try to move
• Eventually the plates jolt free and the pressure is released as energy
• The point at which the earthquake starts is the focus
• The epicentre is the point directly above the focus on the earth's surface
• The energy passes through the Earth's crust as waves which is the earthquake
• Earthquakes can happen as a result of human activity such as drilling into the crust or mining

Earthquake Features Diagram

Features of an earthquake

Structure of volcanoes

• 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:

Features of a Volcano

Features of a volcano

Types of volcanoes

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

Composite Volcano Features

Features of a composite volcano

• Shield volcanoes
• Gently sloping sides
• Runny/thin lava
• Less explosive - gentle eruptions
• Tend to form on constructive plate boundaries or hot spots
• Frequent eruptions

Sheild Volcano Features

Features of a shield volcano

Volcanoes at a hotspot

• 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

Hotspot Volcano Formation

Volcano formation at a hotspot