4.3 Volcanic hazards and risk assessment

Volcanic Hazards

Volcanic eruptions threaten lives, property, and entire ecosystems. From slow-moving lava flows to superheated pyroclastic avalanches, the hazards vary widely in speed, reach, and destructive power. Understanding each type of hazard, and knowing how scientists assess the risk they pose, is central to protecting communities near active volcanoes.

Primary Hazards of Volcanic Eruptions

Lava flows are streams of molten rock that move downslope from the vent. They destroy buildings, ignite vegetation, and reshape the landscape along their path. Flow speed depends largely on viscosity and composition: basaltic lava (low silica, runny) can travel several kilometers per hour, while rhyolitic lava (high silica, thick) tends to pile up near the vent.

Pyroclastic flows are the deadliest volcanic hazard. These ground-hugging avalanches of superheated ash, pumice, and gas can reach speeds of 700 km/h and temperatures around 1,000°C. The 1902 eruption of Mount Pelée on Martinique killed nearly 30,000 people in minutes, and pyroclastic flows famously buried Pompeii in 79 AD. Because they move so fast, evacuation once a flow begins is nearly impossible.

Lahars are volcanic mudflows made of ash, rock debris, and water. The water can come from melted glacial ice, crater lakes, or heavy rainfall. Lahars funnel down river valleys at high speed and can travel tens of kilometers from the volcano. The 1985 Nevado del Ruiz disaster in Colombia killed over 23,000 people when lahars buried the town of Armero. A critical detail: lahars can occur long after an eruption ends. Around Mount Pinatubo, heavy monsoon rains triggered destructive lahars for years after the 1991 eruption.

Volcanic gases include water vapor, carbon dioxide ($CO_2$), sulfur dioxide ($SO_2$), and hydrogen sulfide. These gases cause respiratory problems, produce acid rain, and can contribute to short-term climate cooling. In low-lying areas, dense gases like $CO_2$ can pool and displace oxygen. The 1986 Lake Nyos disaster in Cameroon released a massive cloud of $CO_2$ that suffocated over 1,700 people in nearby villages. The 1783 Laki eruption in Iceland released enough $SO_2$ to cause crop failures and famine across Europe.

Volcanic Risk Assessment

Risks from Volcanic Hazards

Volcanic hazards create risk across three broad categories:

• Human life: Direct threats include lava flows, pyroclastic flows, and lahars. Volcanic gases cause respiratory illness and, in extreme cases, asphyxiation. Indirect threats matter too: the 2010 Eyjafjallajökull eruption in Iceland didn't kill anyone directly, but it disrupted air travel across Europe for weeks, affecting millions.
• Infrastructure: Eruptions damage or destroy buildings, roads, bridges, and utility networks. During the 1973 Heimaey eruption in Iceland, lava flows threatened to close the island's harbor. Ash fall from Mount Redoubt (1989–1990) disrupted transportation and power systems across parts of Alaska. The 1980 Mount St. Helens eruption caused an estimated $1 billion in damage.
• Environment: Eruptions destroy habitats, alter soil chemistry, and can shift regional climate. The 1991 Mount Pinatubo eruption injected so much $SO_2$ into the stratosphere that global temperatures dropped by about 0.5°C for the following year. Over longer timescales, volcanic ash can actually improve soil fertility, but the immediate effect is destruction of ecosystems.

Importance of Volcanic Risk Assessment

Volcanic risk assessment evaluates both the likelihood of hazards occurring and the consequences if they do. It combines three main activities:

1. Studying the volcano's eruptive history. Past eruptions reveal patterns in eruption style, frequency, and reach. Geologists examine rock layers, ash deposits, and historical records to build a picture of what a volcano has done before and is likely to do again.
2. Monitoring current activity. Scientists track seismic activity, ground deformation, gas emissions, and temperature changes. Increases in any of these can signal that magma is moving toward the surface.
3. Creating hazard and risk maps. Hazard maps show which areas are most likely to be affected by specific hazards (lava flows, lahars, pyroclastic flows). Risk maps layer in population data and infrastructure to show where the consequences would be most severe.

This information feeds directly into practical decisions:

• Evacuation planning: Knowing which zones are highest-risk determines evacuation routes and timing.
• Land-use zoning: Authorities can restrict development in the most dangerous areas.
• Early warning systems: Monitoring data triggers graduated alert levels that tell communities when to prepare and when to evacuate.
• Public education: Awareness campaigns help residents recognize warning signs and know what to do.

Vulnerability Factors for Volcanic Communities

Not every community near a volcano faces the same level of risk. Vulnerability depends on several overlapping factors:

• Proximity to the volcano. Communities closer to the vent face rapid-onset hazards like pyroclastic flows with very little warning time. Villages on the slopes of Merapi in Indonesia, for example, have only minutes to evacuate when flows begin.
• Population density and distribution. Naples, Italy sits within the danger zone of Vesuvius with roughly 3 million people in the metropolitan area. High density means more people exposed. Conversely, isolated communities like Chaitén, Chile may be harder to reach with evacuation resources.
• Socioeconomic factors. Poverty limits a community's ability to prepare, evacuate, and recover. In Goma, Democratic Republic of the Congo, near the active Nyiragongo volcano, many residents lack the resources to relocate even when warnings are issued. The 2018 Fuego eruption in Guatemala disproportionately affected marginalized communities with fewer options.
• Infrastructure and land use. Buildings not designed to withstand ash loading or lahars collapse more easily. Development in river valleys or on steep slopes amplifies the danger. The destruction of Armero, Colombia in 1985 was worsened by the town's location directly in a lahar path.
• Governance and institutional capacity. Effective disaster management requires coordination, funding, and public trust. The Nevado del Ruiz tragedy is partly attributed to delayed and poorly communicated warnings despite scientists identifying the risk. Limited monitoring resources in places like the Philippines (around Mayon volcano) reduce the lead time for warnings. And if communities distrust authorities, as has been documented around Merapi, people may refuse to evacuate even when told to do so.