3.5 2.b - Volcanic hazards and cascading impacts

Lava flows and pyroclastic density currents

A hazard is a potentially damaging process; disaster losses arise when exposed people and assets are vulnerable to it. Lava and pyroclastic density currents can both move downslope, but they are different physical materials and require different explanations.

A lava flow is moving molten rock. Its direction and extent depend on topography, viscosity, supply rate, cooling and the formation of channels or tubes. Some flows advance slowly enough for evacuation, while channelled, low-viscosity lava on steep slopes can move much faster. There is no universal safe assumption that people can always outrun it. Heat can ignite buildings and vegetation; burial can destroy roads, water pipes and farmland even where people leave beforehand. Solidified lava can make restoration of earlier land uses difficult.

A pyroclastic density current is a hot moving mixture of gas, ash and larger fragments, denser overall than the surrounding air. It can develop when an eruption column collapses, a lava dome fails, or an explosive eruption directs material laterally. Gravity carries the mixture downslope; some dilute turbulent components can overtop relief. Its heat, impact, abrasive particles and suffocating atmosphere can be lethal. It is not simply a fast lava river.

USGS's volcanic-hazard assessment explains the contrasting flow processes and dome-collapse hazards. Valley concentration matters, but a line drawn around one valley is not guaranteed protection from every current. Impact depends on the event's size and properties, terrain, exposure and the ability to act before arrival. A modest eruption close to vulnerable settlements can cause greater losses than a larger remote event.

Gas emissions, tephra and ash

Gas and fragmented rock can affect places well beyond a lava flow, and visible darkness is not a reliable measure of danger. Water vapour is usually the most abundant volcanic gas, but emissions also include carbon dioxide, sulphur dioxide and other compounds. Their effects depend on concentration, duration, wind and topography.

Carbon dioxide can accumulate in low places under suitable stable conditions, displacing breathable air and creating an asphyxiation hazard. It is colourless, so lack of a visible cloud does not establish safety. Sulphur dioxide can irritate airways and contribute to acidic pollution and aerosols downwind. Different gases can also affect vegetation, livestock and infrastructure. The USGS gas-hazard account explains these geographical pathways; gas risk can continue without a major explosive eruption.

Tephra is fragmented material ejected by an eruption. Volcanic ash is its fine fraction, smaller than two millimetres, composed of rock, minerals and volcanic glass rather than soft combustion ash. Large fragments tend to fall nearer the vent, while fine particles can remain airborne and travel far downwind. Wind direction, particle size and plume height produce uneven deposits rather than a uniform circle.

Ash can reduce visibility, abrade machinery and contaminate water supplies. Deposits can smother crops and affect grazing, while roofs face extra load, particularly when wet ash becomes heavier. Collapse depends on both the load and the building's strength and span; a thin deposit does not automatically collapse every roof. USGS's roof-loading guidance makes this exposure–vulnerability connection.

Aviation is vulnerable far from the volcano. Ash can erode components, obscure windscreens and melt within hot engine sections, adhering to parts and disrupting operation. Avoidance, closures and rerouting can interrupt trade and tourism without ash destroying the destination airport. USGS's aviation account uses the 2010 Eyjafjallajökull disruption to illustrate those network effects. Do not assume the later Icelandic fissure eruptions produced the same ash and international-airspace consequences.