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I am working on a survival suspense story, where a sizeable group of surivors is trapped in a tropical island (somewhere in the Ring of Fire) by a volcanic eruption. They find shelter from the erpution in an old mansion built in a high-rise cliff on the edge of the island.

The cliff is sturdy and the mansion is stonemasonry, so there is little danger of fire from cinders or a collapse.

The eruption I am visualizing would have a Volcanic Explosivity Index of 4 to 5. Most of the inhabitants and tourists of the island died because of the pyroclastic flow and other eruption hazards. Only the few that climbed the hill leading to the cliff survived because the pyroclastic flow goes downhill, and there is a sizeable valley between the volcano and this hill.

They are cut off from rescue for a long period of time (so I can play with isolation, food shortage and so on), so I need the mansion to survive the eruption and remain in the extreme end of habitable (temperature, etc - almost inhospitable). Sickness from inhaling ashes and heat strokes are hazards that will claim some survivors, but a handfull must survive to the end.

The lava flow would reach the ocean before climbing the cliff, but isolated the group from the rest of the island, and is too close for confort, but they can still venture outside the manor for short periods of time.

How close can I put the mansion from the lava flow and/or the volcano caldera so not to push the suspension of disbelief?

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  • $\begingroup$ You may want to pick a specific eruption event as your baseline to get better answers here - there can be a ton of variation just between eruptions of the same volcano, and that will impact the safe distance. $\endgroup$ – Dan Smolinske Apr 7 '15 at 14:43
  • $\begingroup$ Pompeii is a good lesson to avoid volcano, but the fertile mineral filled land near volcano and still attract many farmers in Indonesia despite the warning from their authority. $\endgroup$ – user6760 Apr 7 '15 at 14:58
  • $\begingroup$ @DanSmolinske I added the info on the kind of volcanic eruption I am envisioning $\endgroup$ – Mindwin Apr 7 '15 at 15:05
  • $\begingroup$ @user6760 the island is a touristic spot, and nobody believed the eruption warnings / the government supressed them so the tourists would not be scared away. But the story takes place during the eruption, so the before or after is not a concern now. $\endgroup$ – Mindwin Apr 7 '15 at 15:11
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It depends first on the eruption. Mt. St. Helen's type eruption is devastating for many miles in whichever direction it blows.

Hawaii type is mostly medium to slow lava flows making the island bigger is another matter. They could easily have some fast flows and cut off the people in the mansion. If it was a little bigger and went in other directions too, then any emergency response would be focused else where leaving the far reaches to fend for themselves for a while.

Lava flowing through someone's backyard

enter image description here

EDT: Taking into account the size of the eruption, you might want to have an uneven one, like Mt. St. Helens. The main resort side is in the direct path of the eruption and most of the flow, leaving the other side (more of a remote location) to be 'safe' and anyone who was staying there or just touring the 'back' of the island would be the survivors.

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  • $\begingroup$ I almost feel sorry for the poor guy/gal who just lost their extremely expensive plot of land to the lava flow. $\endgroup$ – the_OTHER_DJMethaneMan Apr 7 '15 at 15:46
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    $\begingroup$ @DustinJackson But think of all the nutrients the lava deposited on the earth! $\endgroup$ – Mindwin Apr 7 '15 at 17:22
  • $\begingroup$ Do you get slow lava flows on high VEI? $\endgroup$ – Taemyr Aug 27 '15 at 13:52
  • $\begingroup$ @Taemyr probably not, at least highly unlikely, but this is for a story... :) $\endgroup$ – bowlturner Aug 27 '15 at 13:56
  • $\begingroup$ Uneven flow and not being on the wrong side seems nice. accepted $\endgroup$ – Mindwin Oct 28 '15 at 16:19
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Looking at a variety of VEI 5 eruptions throughout history, I think 20-30km is often immediately survivable, regardless of terrain. That's not to say the house might not catch fire or collapse in the resulting earthquake. Another factor that might let you get closer is that not all eruptions are directly upwards. To take Mt. St. Helen's again, that blast was most severe towards the north, where the mountain sloughed away a 27km landslide, followed by a pyroclastic flow in that direction that covered an area 37km by 31km.

The biggest problem your hapless tourists are going to face is asphyxiation from the immense ash cloud that is the very definition of a VEI 5. That, and the searing heat of the potential shockwave. You might consider going for a smaller VEI number, as that has much less to do with the amount of lava as the total amount of all ejecta combined.

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  • $\begingroup$ The manor is sturdy stonemasonry, there is nothing flammable on the exterior... (plus some mistery features) $\endgroup$ – Mindwin Apr 7 '15 at 17:24
  • $\begingroup$ Don't forget the blast of superheated gasses traveling hundreds of miles an hour! $\endgroup$ – Paul Chernoch Apr 7 '15 at 17:27
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    $\begingroup$ @Mindwin Then yeah, they'd have reasonable odds of surviving through the first explosion. Make sure that someone in the group knows how to deal with the choking ash cloud. I have no idea if a wet towel on the face would be enough. $\endgroup$ – Emmett R. Apr 7 '15 at 18:11
  • $\begingroup$ Is this image wrong; en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens#/media/… - Because I am having trouble seeing a 37 by 31 km coverage of pyroclastic flow. $\endgroup$ – Taemyr Aug 27 '15 at 13:45

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