What causes volcanic and earthquake hazards?

Why do volcanic and earthquake hazards vary?

Volcanoes and earthquakes don’t all behave in the same way. Some volcanic eruptions produce slow-moving lava, while others blast ash high into the atmosphere. Some earthquakes barely shake the ground; others trigger deadly landslides and tsunamis.

These contrasts are mainly linked to plate boundary type, magma composition, and the depth and strength of earthquakes.

Conservative (transform) boundaries

Example: San Andreas Fault, California – where the Pacific and North American plates slide past each other.

Volcanic activity

• No volcanoes form because the crust is neither created nor destroyed
• There is no rising magma at the plate boundary.

Earthquake hazards

• Plates grind and lock together, storing tension.
• When the plates suddenly shift, strong earthquakes are released.
• Some quakes reach magnitude 8+, causing major destruction.
• Shallow-focus earthquakes occur close to the surface, making them especially damaging.

Divergent (constructive) boundaries

Example: Mid-Atlantic Ridge between the Eurasian and North American plates, including Iceland.

Volcanic activity

• Plates move apart, allowing magma to rise.
• Basaltic lava escapes easily, producing gentle, runny lava flows.
• Volcanoes are usually shield volcanoes, with wide, gentle slopes.
• Eruptions are frequent but generally not explosive.

Earthquake hazards

• Small, shallow earthquakes are common.
• Magnitudes are often between 5.0 and 6.0 on the Richter scale.
• Although weaker than at other boundaries, shallow quakes can still damage settlements on land.

Convergent (destructive) boundaries

This is where the most powerful hazards often occur, especially when oceanic and continental crust meet.

Example 1: Oceanic–continental collision

Example: West coast of South America — Nazca Plate subducting under the South American Plate (Peru and Chile).

• The oceanic plate sinks into the mantle, forming a deep ocean trench.
• As the plate melts, magma forces its way upwards.
• Produces composite volcanoes with steep sides.
• Lava is andesitic – thick, sticky and gas-rich.
• Eruptions are very explosive, producing ash clouds, pyroclastic flows and lahars.
• Earthquakes can reach magnitude 9.0, and a sudden uplift of the sea floor can trigger tsunamis.

Example 2: Continental–continental collision

Example: Himalayas — where the Indo-Australian Plate collides with the Eurasian Plate.

• Neither plate subducts because both are too buoyant.
• The crust is forced upwards, creating huge fold mountains.
• No volcanoes form because there is no melting of the crust.
• Earthquakes are common and can be extremely destructive.
• Landslides are regularly triggered on steep mountain slopes.