Volcanic hazards are physical phenomena associated with an eruption that threaten life, property, or the environment. These dangers are diverse, ranging from extremely fast, superheated flows to invisible, heavy gases that silently collect in valleys. Understanding these phenomena requires categorizing them by their primary mechanism of danger, such as velocity, temperature, or reliance on water or air for dispersal. A volcano’s overall threat is determined by the specific combination of hazards it produces, which can impact areas both near and far from the vent. The following categories detail the main types of dangers that can occur before, during, or long after a volcanic event.
High-Velocity Hazards
The most immediate and lethal volcanic threats are characterized by extreme speed and high temperature, making avoidance nearly impossible. Pyroclastic flows are among the most dangerous, consisting of a fluidized mixture of superheated gas, ash, and rock fragments. These ground-hugging avalanches can race down volcano slopes at speeds exceeding 100 miles per hour, sometimes reaching 450 miles per hour. Temperatures often range between 390°C and 700°C, instantly incinerating organic matter.
The intense heat and velocity of pyroclastic flows crush, bury, and burn everything in their path. They form either from the collapse of an eruption column or the explosive disintegration of a lava dome. These flows typically follow valleys and depressions, concentrating destructive power in low-lying terrain.
Another fast-moving hazard is the volcanic landslide, often called a debris avalanche. These involve the massive, rapid collapse of a volcano’s flank, triggered by magmatic pressure, earthquakes, or structural weakness. The resulting avalanche is a chaotic mixture of rock and debris that can travel up to 180 miles per hour. The immense volume can bury valleys with hundreds of feet of material.
A debris avalanche can transform into a lahar if it incorporates enough water. The landslide’s force can also trigger an explosive eruption by suddenly removing the confining pressure on the magma chamber. This structural collapse is a persistent danger for nearby communities, even when the volcano is not actively erupting.
Molten Rock Hazards
Molten rock hazards are defined by the movement of lava flows across the ground surface. These streams of incandescent, liquid rock destroy infrastructure and land by burning and burying everything in their path. Lava flows pose a lower danger to human life than other hazards because they typically move slowly enough for evacuation. However, the destruction of homes, roads, and agricultural land in the flow’s path is absolute.
The characteristics of a lava flow are determined by its viscosity, which dictates its speed and surface texture. Less viscous Pahoehoe lava flows easily, creating a smooth, ropey surface as it cools. These flows can travel in insulated underground tubes, advancing long distances from the vent.
In contrast, A’a lava is more viscous and moves as a thicker, bulldozing front. Its surface cools and breaks into sharp, jagged fragments. This rough, blocky texture makes A’a flows difficult to traverse and effective at pushing over structures. Both types represent a long-term threat to property that can persist for months or years.
Water-Driven Hazards
Water-driven volcanic hazards occur when volcanic material interacts significantly with water, creating powerful flows that travel far down river valleys. Lahars, or volcanic mudflows, are destructive examples consisting of a dense slurry of water, ash, rock fragments, and debris. The consistency of a lahar resembles wet concrete, giving it immense power to crush and carry away objects like boulders and bridges.
Lahars can be triggered by the rapid melting of snow and ice during an eruption, or long after an eruption when heavy rainfall mobilizes thick deposits of loose ash and tephra on the slopes. Traveling at speeds of up to tens of meters per second, lahars are dangerous because they follow existing drainage channels and can impact communities many miles away.
Volcanic activity can also generate tsunamis, which are large waves caused by the sudden displacement of a massive volume of water. These events occur when a large volume of debris, such as a volcanic landslide or a pyroclastic flow, rapidly enters a body of water. Underwater explosions or the collapse of a volcano’s caldera can also trigger these waves. Volcanic tsunamis are highly destructive in coastal areas, causing significant loss of life far from the eruption site.
Atmospheric Hazards
Atmospheric hazards involve materials distributed through the air, posing risks over vast distances. Volcanic ash, or tephra, is a widespread hazard composed of pulverized rock, mineral crystals, and shards of volcanic glass, typically measuring less than two millimeters in diameter. This material is hard, abrasive, and electrically conductive, making it a serious threat to infrastructure.
Ashfall accumulates on rooftops, and its weight, especially when wet, can cause structural collapse. The abrasive particles contaminate water supplies, damage electrical power systems, and cause jet engine failure, severely disrupting air travel far from the vent. Inhaling fine ash particles can cause respiratory irritation and pose a long-term health risk.
Volcanic gases present another atmospheric danger, released from dissolved gases in magma during an eruption. The most common dangerous gases include sulfur dioxide (SO2), carbon dioxide (CO2), and hydrogen sulfide (H2S).
Sulfur dioxide reacts with moisture and sunlight to form volcanic smog, known as vog, which causes respiratory irritation and acid rain downwind. Carbon dioxide is colorless, odorless, and denser than air, causing it to pool invisibly in low-lying areas. High concentrations of CO2 displace breathable oxygen, leading to rapid asphyxiation without warning. Hydrogen sulfide is also toxic and can be deadly at high concentrations, though it is often detectable by its rotten egg smell.