Assume a volcano erupts in an uninhabited area and then goes back to sleep, and poisonous fumes, ash, all the good stuff spills out. How long will it take for the debris and clouds to settle and allow people to walk around in the area, and perhaps settle there?
You have to define what safe and going to sleep mean to you. Mt St Helens had erupted at various intervals over the last few thousand years, sometimes being quiet for a few 100 years, other times being active nearly continuously for periods. Volcanos rarely spring from nowhere and then go back to sleep.
By safe I mean people can walk around with minimal protective gear. Sleep means it’s not currently burping smoke and magma.
Depending on the type of volcanic activity, people walk around active lava fields without protective gear.
Interesting! I’ve seen photos like that but I wasn’t sure how recent the eruption had occurred vs the cooling of the lava.
Safety often depends more on the type of eruption, rather than the recentness of it. Some types of lava advance so slowly that any halfway healthy person can keep ahead of them simply by walking. On the other end of the spectrum are pyroclastic flows.
re, the people walking on the solid lava flow looking at glowing hot liquid lava… well they aren’t totally safe. It seems to be Kilauea which is shield volcano, and anyway the rift zone vent is below the mountain, and the magma is oozing out of the mountain into the rift zone due to gravity (not much due to gas pressure.) and its oozing slowly as its cool and relatively thick (rather than watery) lava generally at Kilauea too. Its a whole set of things that allows approaching the lava so casually …
The rock they are on has been cooling quite a while, the lava is flowing in a well defines track down to the left, its an old vent that hasn’t been watery or pumped for quite a while since the erruption and its known the pressure is definitely reduced. (the height of the lava lake in the mountain indicates the pressure…)
I took a tour of White Island in New Zealand. Apparently it’s closed now.
Supposedly for sensitive areas, there is seismic monitoring which can give a warning that a problem is imminent. Without certain warning rumblings of activity, the assumption is there is a lot less likelihood of ash, lava, or hot gas suddenly erupting. Of course, in the case of White Island, they were wrong the one time. But when I was there, the guide mentioned that they’d come out with a tour one morning a while before, only to find the crater completely rearranged, that something had spewed during the night and nobody had seen any warning before or during.
@Isilder - I agree, that sure appears to be Kīlauea.
Some areas of a volcano may be unsafe even if an active eruption hasn’t occurred for some years. This is from the hazard analysis section of the Kīlauea Volcano Update page:
Other significant hazards also remain around Kīlauea caldera from Halemaʻumaʻu crater wall instability, ground cracking, and rockfalls that can be enhanced by earthquakes within the area closed to the public. This underscores the extremely hazardous nature of the rim surrounding Halemaʻumaʻu crater, an area that has been closed to the public since early 2008.
As it happens, there is an eruption of sorts (nothing overly dramatic at the moment, just a persistent lava lake with occasional breakouts around the edge of the crater) at Halemaʻumaʻu now. But as indicated above, the area was deemed unsafe even in the absence of an active lava flow.
In many poorer regions (Indonesia, Philippines) people deliberately live near volcanoes because of the highly fertile soil. And they are often in a rush to return after an eruption because they have no other place to live or means of earning income.
This article describes people staying away for four months due to increased activity, only for several to be killed shortly after they were allowed back in.
This article describes people returning to their homes only a couple of weeks after ash fall.
Specifically to
That depends on the weather. If there are high speed winds, then it will happen quickly. Areas downwind, not so much. If rain events happen shortly after, then the area air will clear out fast too. But will have problems with denser sodden ash layers on the ground to clear. More risk of structure collapse too. Optimal situation would be winds clearing ash elsewhere. That could be as little as a day.
During the eruption the very local weather situation can be overwhelmed by the heat and explosive volumes. But after the eruption has settled, overall weather, wind is the biggest factor for clearance of ash and gasses.
Heavier debris settles almost immediately, within an hour at most. Ash takes longer, but not as long as the several days or weeks that pure particle size would suggest, because they tend to accrete in the volcanic clouds. So from a pure ash & debris sense, a couple of days ought to do it.
I think there will be different answers to th OP depending on what type of volcano were talking about. As the photo above shows, hot lava can be approached fairly close from a shield volcano, as long as the wind is at your back. I believe there are still dangerous fumes coming from the slow moving “aa” as well as the fast moving streams of “pahoehoe” lava from Kiluea. As soon as it solidifies and cools, it can be walked on and manipulated like any other rock pile, so probably a few days as long as no more new lava arrives.
More explosive composite volcanic eruptions like St Helens will generate hazardous conditions from volcanic ash/fine particles, gases and fumes, as well as other hazards like lahars, which will make an area dangerous to tread for some time, maybe weeks, even after the dust settles and fumes are blown away/dissipated.
With any significant lava flow, that soil won’t be arable for many years.
Not sure how long after a major ash fall one could start farming again.
Where the magma itself flows and subsequently solidifies into rock, there’s no soil, true. But that won’t be most, or even a large fraction, of the affected acreage.
I recall on bit I saw on TV a while ago (was it a commercial?) where they are walking around near the Hawaiian lava flow and the one guy says “My running shoes are melting.” The bottoms were starting to stick to the rock.
This. I wasn’t meaning to suggest they wanted to return quickly to be able to farm on new lava or fallen ash. They just want to go home. And their home is where it is because of fertile soil from past eruptions.