There are 6 main types of hazards from volcanic eruptions: lava flows, poisonous gases, ashfalls, pyroclastic flows, lahars, and volcanic debris avalanches. Each of these hazards requires different emergency protocols. Most volcanoes are only capable of 3 or 4 of these hazards, or rarely just 1 or 2. Reducing volcanic hazards must address all types of hazards which the local volcano can produce.
Volcano assessment, monitoring, and early eruption detection is essential for mitigating volcanic hazards. The assessment indicates the types of hazards this particular volcano is capable of. Constant onitoring gives a baseline against which changes can be assessed to determine if an eruption is imminent and what actions to take.
Lava flows are the most well-known hazard of a volcanic eruption. However, they are the least dangerous of all volcanic hazards when properly planned for. The paths of likely lava flows from a local volcano can be generally predicted, so that government authorities can zone this land appropriately and destruction of property can be completely avoided. Cell towers and other communication infrastructure should be routed so that they do not intersect areas at risk.
In an emergency, lava flows can be slowed or diverted, but these methods are not always successful. Some ways of slowing and diverting lava flows include cooling the advancing flow with water, or using explosives to create a new channel for the lava flow. These kinds of interventions can backfire if a lava dome is created over a live lava flow.
Most volcanoes eject high levels of hydrogen sulfate and other poisonous gases. Unlike most volcanic hazards, these gases are invisible, and there may be little warning before they reach lethal levels. Even the distinctive “rotten eggs” smell may not help, because human smell sensors shut down at high levels of hydrogen sulfate. Icelandic Heckla-type eruptions often emit high levels of fluorine, which contaminate the land as well as poisoning the air.
Advance warning periods for gas venting during a volcanic eruption are extremely short. An enclosed supply of air will help in an emergency. However, the best way to reduce the hazard from volcanic gases is to evacuate the area. After the eruption is over, land outside the no-go area may need to be decontaminated.
All volcanoes produce large amounts of hot ash, which will spread over a large area. The coverage of the ash cloud is dependent on the prevailing winds at the time of the eruption. Heavier ash, or tephra, settles on everything over hundreds of square miles, and can collapse roofs and bury buildings. Eruptions from underneath glaciers often produce a light, glassy ash which is carried high into the atmosphere and is capable of damaging airplane engines. All ash is caustic and abrasive.
Mitigating risks in ashfall areas will not require evacuation unless the ashfall is extremely heavy or combined with another volcanic hazard. In general, the same precautions should be taken as for an extreme smog alert. Wet cloth over the mouth and nose will prevent light ashfall from entering the trachea and lungs. For heavier ashfall, a respirator may be necessary, along with filters for equipment which depends on ventilation. In case of heavy ashfall or ashfall combined with rain, tephra should be cleared from the roof as it accumulates, and roof integrity should be monitored. Flight paths should be routed around the ash cloud.
Pyroclastic flows and lahars
All stratovolcanoes are capable of producing pyroclastic flows. These consist of an extremely hot mixture of volcanic gases and heavy ash, which can travel at race car speeds or even faster. They are usually limited to the immediate slopes of the volcano.
The only way to reduce the hazard from pyroclastic flows is to evacuate the area as soon as a major eruption is predicted for any stratovolcano. After a pyroclastic flow is spotted, it is already too late.
Lahars and volcanic debris avalanches
Glaciers on an erupting volcano will melt, producing violent mud flows called lahars. These may develop very early in the volcanic eruption, and may even be among the first indicators that an eruption is imminent. Lahars may also result from heavy rainfall during a volcanic eruption. A lahar during or after a volcanic release of hydrogen sulfide may be extremely acidic because of sulfuric acid.
The swarm of small earthquakes which accompany an impending eruption also make the volcano’s slopes extremely unstable. This may result in small and large avalanches of volcanic debris. An avalanche which dumps a large amount of debris into a lake, bay, or other body of water is likely to cause a localized tsunami. If part of the volcano gives way and falls into the ocean, the resulting tsunami could travel around the world.
Evacuation is the best way to reduce the risk from lahars, avalanches, and tsunamis. Lahars often follow generally predictable channels, so appropriate land planning can mostly eliminate this risk. A region which is prone to lahars could also build diversion channels ahead of time. Lahars can also sometimes be redirected into new channels on an emergency basis, although the likelihood of doing this successfully is much lower than the ability to redirect a large river while it is in full flood.
Public awareness is vital to saving lives during a volcanic eruption. Public education before any eruption includes volcano awareness workshops and emergency response planning and exercises. An eruption emergency plan should be prepared which addresses every foreseeable contingency, and should be communicated to the general public well before any eruption.
Clear lines of communication will be essential when a volcanic eruption is imminent. As part of the emergency plan, evacuation routes should have been identified and clearly marked.