Earthquakes & Volcanoes (SQA National 5 Geography): Revision Note

Distribution of Earthquakes and Volcanoes

• Most earthquakes and volcanoes occur at plate boundaries

Earthquake distribution

• Earthquakes occur at all types of plate boundaries

• Most (approximately 90%) occur around the rim of the Pacific Ocean

• This zone is called the ‘Ring of Fire’

Volcano distribution

• Most volcanoes occur at divergent (constructive) and convergent (destructive) plate boundaries

• Like earthquakes, the majority of active volcanoes (approximately 75%) are located around the Pacific ‘Ring of Fire’

• Volcanoes also occur at hotspots away from plate boundaries and are plumes/columns of magma which escape through the Earth’s crust

Plate Tectonics and Plate Boundaries

The Structure of the Earth

• The Earth is composed of various layers

  • Inner core: About 1400km in diameter, a solid and dense layer composed of iron and nickel with temperatures of about 5000 - 6000°C

  • Outer core: About 2100km thick, a semi-molten metal layer with temperatures between about 4000 and 6000 °C

  • Mantle: About 2900km thick, a semi-molten layer which is less dense than the outer core. This is split into two parts 

  • Crust: the thickness varies and there are two types of crust

    • Continental crust is thick (25–90km), old and not as dense as oceanic crust

    • Oceanic crust is thinner (5–10km), younger and denser than continental crust

Plate tectonics

• The Earth’s crust is broken into many tectonic plates

• These plates move on top of the semi-molten mantle below

• The movement of the plates is in part due to the convection currents within the mantle

• The places where the plates meet are called plate boundaries

Types of plate boundary

• There are four main types of plate boundaries:

  • constructive

  • destructive

  • conservative

  • collision 

Constructive plate boundary

• At a constructive plate boundary, the plates are moving apart

  • As they move apart, magma rises to fill the gap

  • This leads to the formation of volcanoes and eruptions

• 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

Destructive plate boundary

• At a destructive plate boundary, the plates are moving together

  • The denser, heavier oceanic plate subducts under the lighter, less dense continental plate

  • This leads to friction and heat, which melt the crust and form magma

  • Magma rises to the surface through cracks in the crust

  • It erupts on the surface as lava and forms a volcano

• The boundary between the Nazca plate and the South American plate is an example

• Both volcanic eruptions and earthquakes occur at this type of plate boundary

Conservative boundary

• At a conservative boundary, the plates move past each other in opposite directions or in the same direction at different speeds

• The San Andreas Fault is an example of a conservative boundary

• Earthquakes are the only hazard at this type of boundary

Collision boundary

• At a collision boundary, two continental plates of similar density move towards each other

  • Neither is dense enough to subduct, so the land is pushed upwards

  • This process forms fold mountains such as the Himalayas

• Earthquakes are the only hazard at this type of plate boundary

Causes of Earthquakes

• Earthquakes can occur anywhere, but mostly occur at or near plate boundaries

• Earthquakes happen at all plate boundaries: constructive, destructive, conservative and collision

  • At constructive plate boundaries, earthquakes tend to be weaker

  • At destructive, conservative and collision plate boundaries, earthquakes tend to be stronger

Features of an earthquake

• An earthquake is the sudden, violent shaking of the ground

• Earthquakes are the result of pressure building when tectonic plates move

• The focus is the point where the earthquake starts below the Earth’s surface

• The epicentre is the point on the Earth’s surface directly above the focus

• The magnitude (amount of energy released) by earthquakes is measured on the Moment Magnitude Scale (MMS), which replaced the Richter scale

Earthquake sequence

• The sequence of an earthquake is the same regardless of the type of plate boundary:

  • As the tectonic plates move, they can become stuck

  • Pressure builds as the plates continue to try to move

  • Eventually, the plates jolt free and the pressure is released as energy

  • The energy passes through the Earth’s crust as seismic waves, which are the earthquake

Causes of Volcanoes

• Volcanoes occur at constructive and destructive plate boundaries 

• Volcanoes do not occur at conservative or collision boundaries

  • At constructive plate boundaries, eruptions tend to be more gentle

  • At destructive plate boundaries, eruptions tend to be more explosive 

Features of a volcano

• A volcano forms when magma erupts onto the Earth’s surface as lava through a vent in the Earth’s crust

  • When magma erupts onto the surface, it is called lava

• The magnitude of a volcanic eruption is measured on the Volcanic Explosivity Index (VEI)