Causes and Characteristics of Volcanic Hazards

Why volcanic eruptions differ

A volcano forms where magma, molten rock beneath the Earth's surface, rises through openings in the crust and erupts. Once it reaches the surface, it is called lava. Volcanic hazards are potentially damaging processes or materials produced by volcanic activity, such as lava flows, ash fall and pyroclastic flows.

Eruptions do not all behave alike. Some mainly release flowing lava; others violently eject ash, gases and rock fragments. The properties of the magma help explain both the eruption's behaviour and the shape of the volcano it builds.

Magma type, viscosity and escaping gas

Viscosity means resistance to flow. Low-viscosity magma is runny and flows easily; high-viscosity magma is thick and sticky. Think of the contrast between water spreading across a surface and thick syrup piling up more slowly.

Magma composition and temperature influence its viscosity. Basaltic magma has relatively little silica, is typically hotter and has low viscosity. Andesitic magma contains more silica and is generally more viscous than basaltic magma. Rhyolitic magma has still more silica and typically has high viscosity. These names describe magma types, not volcano shapes.

Magma also contains gases. As magma rises, gases expand. Gas can escape relatively easily from runny magma, so pressure is less likely to build up strongly. Sticky magma makes gas escape more difficult. Trapped gas can build pressure until magma and rock are violently fragmented and ejected. This produces a more explosive eruption. Gas content matters too: gas-rich magma has greater potential for explosive activity when the gas cannot escape easily.

Shield volcanoes: spreading lava and gentle slopes

Shield volcanoes tend to form at divergent boundaries, where plates move apart, and at hotspots such as Hawaii. Magma rises through openings in the crust. The magma is typically basaltic, with low viscosity and relatively low gas content.

The lava spreads readily and can travel a considerable distance before cooling and solidifying. Repeated flows build a wide volcano with gently sloping sides, rather than a steep cone. Eruptions are often frequent and effusive: lava flows out relatively gently instead of being blasted into the air.

The main contrast is lower explosivity, not an absence of danger. Flowing lava can cover land and destroy buildings or roads in its path. Runny lava may spread over a wider area than thick lava.

Composite volcanoes: sticky magma and explosive eruptions

Composite volcanoes tend to form at convergent boundaries where subduction occurs: one plate moves beneath another. Magma rises towards the surface above the subduction zone. The Nazca–South American plate boundary is an example of this setting.

These volcanoes commonly erupt andesitic magma, which is more viscous than basaltic magma. Thick lava does not spread as far before solidifying, so it builds up nearer the vent. Explosive eruptions also deposit ash. Successive eruptions produce alternating layers of lava and ash, building a steep-sided composite cone.

Sticky, gas-rich magma makes explosive eruptions more likely because gas escape is difficult and pressure builds. This is why the contrast involves more than shape: the magma's properties influence the hazards as well as the landform.

How explosivity changes the hazards

An explosive eruption can throw ash—very small fragments of volcanic material—high into the atmosphere. Winds can carry it far beyond the volcano, so hazards are not restricted to the immediate slopes. Ash clouds can disrupt aviation because ash can damage aircraft engines.

Explosive activity can also produce volcanic bombs, larger fragments of molten rock ejected from the vent, and pyroclastic flows. These flows of very hot gases, ash and rock fragments race down the volcano, threatening areas below it through their heat and rapid movement. They are different from lava flows: they are not simply streams of molten rock.

Water can create another hazard. Where an eruption melts snow or ice on a volcano, the water can mix with ash to form a fast-moving lahar, or volcanic mudflow.

The central explanation is therefore a connected chain: magma properties affect lava flow and gas escape; these affect volcano shape and explosivity; eruption behaviour affects the hazards produced. Shield and composite volcanoes show useful typical contrasts, although individual eruptions can vary.