Safety on volcanoes: volcanic risk zones around volcano
The volcanic risk is roughly a product of the time spent in a given area and the combined likelyhood of hazards during that time in that particular area, and is reduced by possible factors such as degree of experience, preparedness, and availability of suitable protection or escape possibilities.
The current state of a volcano is extremely variable with time and it has no meaning to define fixed risk zones around volcanoes (e.g. "from 300 to 1000m from the crater") unless one relates such risk zones to a particular period in time and a particular state of the volcano. It follows that the extent of risk zones are variables and can change even during your visit. Volcanoes are always capable of surprises. The following is a distinction of 5 risk zones in qualitative terms:
1) Extreme Risk Zone ("No Go", "Death zone")
There is an area around an active vent where the likelyhood to be killed is extremely high, so high in fact that almost no one who cares about his life ever chooses to enter it at all and outside from suicide attempts there is realistically no good reason to do so.
Unfortunately, there is NO way to give any number (such as "from 0 to tens of meters") for the actual extent of this zone,- since it varies too much with different volcanoes in different eruption situations. The distinction between the Extreme and the High risk zone is also fluid and it can happen, while you are in the High Risk Zone already, typically very close to activity, that you enter into the Extreme Risk zone, either by accident or because of a sudden increase in activity.
Examples:
- Climbing down into one of Stromboli's craters, i.e. approaching the actual vents to a range of few meters to a few tens of meters would likely result in your death even if you plan to stay there for just a few minutes, because eruptions typically do occur there every few minutes.
- In contrast, entering one of Etna's craters is sometimes possible without putting yourself into excessive danger, while at other times it would mean entering the death zone.
- When Mt. St. Helens erupted in 1980, the paths of the debris avalanche and the directed blast defined an elongated death zone streching for more than 25 km away from the volcano!
2) High Risk Zone
This is the area where chances to be injured or killed are considerable (above 1:1000 per hour stay in this zone).
There is no default distance (e.g. "100 m to 500 m") for this zone. It depends totally on the given volcano and its current state. This zone, like the previous one, is only present at volcanoes that are erupting (or show strong signs of an impending eruption). It is usually the area frequently affected by the eruptive phenomena that volcano exhibits.
For instance, High Risk Zones are the areas where volcanic projectiles fall regularly. If you see fresh impacts of such projectiles on the ground where you are, you are in this zone. With small to medium eruptions, this zone extends often up to 1 km from the vents. With bigger eruptions producing heavy ash fall and pyroclastic flows, the High Risk Zone may well extend for several kilometers away from the volcano, especially if you are near valleys where such pyroclastic flows are channeled.
If you choose to enter this zone, you should limit time in this region to the shortest possible and have a strong reason to be there at all!
Examples (not recommended!): you want to take spectacular close-up shots of blocks and volcanic bombs ejected by a moderate explosive eruption. Then you might choose to enter this zone deliberately, for a short time, accepting the risk. To approach the crater of a volcano in strombolian eruption such as Semeru puts you into this high risk zone. In fact, people have been killed at this spot, including volcanologists. The crater rims of most erupting volcanoes are well within this zone.
3) Medium Risk Zone
In this zone, the risk of injury or death from a volcanic eruption is comparable to other activities where an increased risk is commonly accepted (driving a car, skiing etc.). This is the area that is usually most interesting for volcano watchers and volcanologist, since it permits you to be reasonably close while limiting the risk to a degree most people are comfortable with.
Typically, it consists of the area around an active vent where hazards are present only occasionally. For instance, the area where single bombs do fall but very rarely and where the chance of dodging them are high. Explosions of bigger-than average size, with repeat rates of a few per year threaten this zone. Although it is impossible to give precise ranges for this zone, its limits are typically about 500-1000m away frmo the craters.
Examples:
- Stromboli's summit where many tourists climb is at the limits of this zone and fatal accidents, although very few, have occurred here.
- The summit craters of Etna when there is no activity and no detected warning signs of increased concern are always at least in this zone. In 1971, a sudden, unpredicted explosion from one of its summit craters killed 9 tourists and injured several others. None of the mountain guides was injured; their survival indicates also that their experience and proper reactions (dodging from blombs vs. panicking and running away) helped in reducing the risk.
Take note:
The medium risk zone should not be considered safe ground, although people often stay there for several hours without noticing any danger. You should not stay unnecessary long here and you should not engage in activites that are not related to your direct goals (e.g. observation or study). In particular, you should not camp or sleep in this area).
Volcano guides will usually limit their approach to a volcano to this area. In this area, your safety can be greatly increased by experience and knowledge of the volcano and how to interpret its activity, as well as preparedness of proper reactions.
4) Low Risk Zone
In this zone, the volcanic risk in this zone is extremely small and is usually overestimated.
Observing volcanic activity is still possible and rewarding. Typically you are more than a 1 km away from volcanic vents (with effusive or small to moderate explosive activity), but more than 5-10 km away from volcanoes producing moderate to strong explosive activity and you are outside valleys where pyroclastic flows could be channeled.
Only exceptional eruptions still affect this zone, but they do so frequently enough that the risk for immobile goods (houses) is considered by most as being too large to build here. Unfortunately, it happens around many volcanoes, especially in underdevelopped countries, that land pressure (or people's forgetting about the volcanic risk) is so high that this zone is often densely populated. This is the zone where evacuations often occur and where an eruption's toll on property can be immense.
Examples of this zone include the villages around Merapi volcano in Indonesia, but also the densely habitated area around Vesuvius volcano in Italy.
Note:
The volcanic risks for people in this zone is so small that other often much underestimated risks become in fact much more important, in particular risks connected to weather and general mountain situations: most accidents in this zone are caused by: lost orientation, lightning, accidents on rocks and cliffs, dehydration, hydrothermia, sunburn etc.
5) Safe Zone (habitation zone)
In this area, usually many kilometers from the craters, only exceptional eruptions cause damage; such eruptions typically occurr at intervals of many decades or centuries or even longer. The volcanic risk to life is neglectable here and the risk for property is small enough that these areas are often inhabited. The volcanic risk, however, is still large enough that critical structures of high value (e.g. nuclear reactors) cannot be accepted in this zone. Examples are the villages around Etna's slopes, the city of Seattle, the town of Hilo or the village of Stromboli.
