A shaft connecting an underdark cavern to a mountainside?

[Seharvepernfan]

I was thinking about this recently; what if there was a large underground cavern, something like a mile long/wide and a thousand feet high - it has a large circular shaft in it's ceiling that runs up through the earth, potentially a couple miles, before opening up high on a mountainside?

What kind of atmospheric/wind effects might this have? Would wind blow up it at a certain time of day, and blow back down it for the other half? How strong would the winds be? Would snow (likely melting on the way down) fall all the way into the cavern?

How would this work?

[Rhynn]

Can't give you specifics (I'm not a speleologist, and that stuff is obscure), but this reminded me of a tidbit I'd saved from somewhere...

A quote from James M. Tabor's Blind Descent, the Quest to Discover the Deepest Place on Earth, on Chevé Cave:

Caves breathe. The diurnal pressure changes from solar heating, as well as larger system-related barometric pressure shifts, account for air movement through caves. Little caves sigh. Big caves blow. Supercaves roar, sometimes with hurricane-force winds. The bigger the cave, the bigger the blow. With it's gusty breath, this one had just given Vesely and Farr the best Christmas gift either could have imagined: the kiss of depth.

Probably the answer to your question would be very complicated and not determined merely by the shaft, in which case the grand tradition of anyone writing anything for a RPG is to make up what they think is coolest.

[Seharvepernfan]

make up what they think is coolest.

I know, but I always try to make it as realistic as I can first.

[Slipperychicken]

Is it an epic-scale missile silo?

I could see people using it as the most awesome execution ever. Catapult (or just throw) criminals into The Pit.

[Seharvepernfan]

Is it an epic-scale missile silo?

I could see people using it as the most awesome execution ever. Catapult (or just throw) criminals into The Pit.

Yeah, that's definitely possible, but I'm considering this from the bottom up. What's it like from the cavern?

[Jay R]

In Moria, Tolkien called it the Endless Stair.

I don't know how hard the winds would blow, but based on a trip to Carlsbad Caverns, I suspect that the horde of bats flying out of it at dusk would be spectacular.

[TheStranger]

Also not a speleologist, but I would certainly expect wind to blow in/out pretty strongly depending on conditions outside. After all, you probably have a large pressure differential between inside and outside. Time of day would affect that, but I imagine it would also be sensitive to changes in atmospheric pressure outside. If there are other entrances, you could see other effects, such as a one-way flow of varying intensity. In theory, you could probably generate a strong enough gale blowing up the shaft that somebody who jumped in would never actually hit the bottom (Somebody please do the math on this, because I will steal it if it's at all plausible).

As for snow, precipitation might reach the bottom if the shaft is perfectly vertical, but it seems more likely that the wind in the shaft would blow it against the sides on the way down. Also, I imagine that very little light would reach the bottom unless the shaft was enormous; you would be able to see the circle of sky, but that light wouldn't make it down.

[Gravitron5000]

I could see people using it as the most awesome execution ever. Catapult (or just throw) criminals into The Pit.

Yeah, that's definitely possible, but I'm considering this from the bottom up. What's it like from the cavern?

Probably half-filled with bodies ...

[Xeratos]

How large of a shaft are we talking about? I could see this ending up getting clogged up with wood, dead animals, vegetation, or just plain dirt very quickly if it was small enough. If people are actively throwing things into it, maybe even faster.

As far as weather goes, how straight is this shaft? If it's a straight shoot, like you spit over the side and it splatters on the floor, then you've probably got some sort of small lake directly underneath it where falling rain/melted snow has eaten away the cavern floor and the water had pooled. Depending on the structure of your cavern, you might have an underground river that this is contributing to.

Next question: what is the composition of the earth? Is this a dirt hole? If so, it's probably collapsing on itself with every rain storm. If it's bored through stone, then it's eroding away slowly and getting larger, probably forming a sinkhole-like structure around the opening at the top.

As far as weather changes go, think of it like having a hole in the roof of your house. You've got water and raccoons coming in, and maybe it's a bit colder, but overall, your house's environment isn't radically effected.

TL;DR: Your hole is probably clogged up, and if it's big enough, may have an ever-growing stagnant pool of water in the cavern underneath it or be feeding an underground river or aquifer.

[Seharvepernfan]

Spoiler

Show

The shaft is red, obviously.

The shaft is something like 30 or 50ft. in diameter. Not positive, but something like that. It empties down into the roof of a very large cavern, with hundreds, possibly more than a thousand feet to the cavern floor. The shaft is perfectly cylindrical, it was created by dwarves. The cavern below the shaft was a dwarven city, and directly under the shaft was a city plaza.

I imagine the composition would be pure rock.

The thing about stuff coming in - the opening at the top is way up on a mountainside, as in, snow and retardedly cold/strong winds up there, most of the time. So, yeah, you've got snow coming down, but I figure it'd melt if it made it all the way down, assuming that wind doesn't blow up constantly.

[Rhynn]

Some water would fall down, some would end up on the walls and form flowstone, some would flow down the walls.

Winds would probably blow both ways; caves have to breathe both ways, I should think, because it can't just be air coming OUT, it has to come in somewhere, too, and pressures and air temperatures woud presumably shift in both. Granted, the air's probably mostly going to the warmer in the cave than over the shaft, so air is going to be coming up and out a lot.

Avalanches would be a problem: if there's a lot of snow on the mountainside, there's going to be avalanches eventually, and that could actually block up the shaft.

[Greylond]

You might want to check out the Hollow World campaign setting for D&D for ideas;

http://en.wikipedia.org/wiki/Dungeon...s#Hollow_World

http://www.dandwiki.com/wiki/Hollow_World

[TheStranger]

With only a 50 foot diameter at most, your shaft is likely to get blocked up. Wind + cold + moisture = ice caked on things. Over time, your shaft could easily get blocked with snow and ice, not to mention trees and other debris. A good-sized tree is well over 50 feet, so it could easily get wedged in the shaft and catch other stuff. I'm not sure what your minimum size to stay clear is, but I'd guess several hundred feet in diameter. With a length measured in miles, that's still very narrow compared to its length.

I'd be more worried about rockslides, though. All that ice freezing and thawing is going to break rocks loose, and those will fall to the bottom of the shaft. Your dwarves would have to check the integrity of the shaft on a regular basis.

As for the wind, it does need to blow both directions. Either direction would tend to blow water against the sides, though; a few miles is a long way to fall without moving more than 25 feet to the side. However, both precipitation and groundwater would tend to drip down the sides of the shaft and fall into the cavern.

The mechanism for the wind to blow only or primarily up would be to have another entrance (or multiple entrances) to the cave system, preferably at lower elevations. If the pressure and temperature differentials work out right, you could get a mostly one-way flow of air, rather than an in-and-out flow. I think. Maybe. Again, somebody please do the math for me.

[Seharvepernfan]

Over time, your shaft could easily get blocked with snow and ice, not to mention trees and other debris.

We're talking waaaay above the tree-line, here.

I can't see a 50ft. diameter shaft getting clogged with snow and ice, but the rockslides are a definite thing that would have to be dealt with.

[Rhynn]

We're talking waaaay above the tree-line, here.

I can't see a 50ft. diameter shaft getting clogged with snow and ice, but the rockslides are a definite thing that would have to be dealt with.

Avalanches of relatively tightly packed masses/sheets of snow could block it, and especially up near the mouth moisture will just keep freezing over the sides, then over the ice, possibly eventually blocking it.

[Seharvepernfan]

Avalanches of relatively tightly packed masses/sheets of snow could block it, and especially up near the mouth moisture will just keep freezing over the sides, then over the ice, possibly eventually blocking it.

Derp. I hadn't thought of avalanches. I still don't think ice would block it by gradual accumulation, I think the ice would give way under it's own weight before completely choking a 50ft. wide hole.

Anyway, let's assume for the point of this discussion that the dwarves keep the tunnel from being blocked. The cavern underneath connects to the underdark, so it's not just one big cave. Which way or ways does the air flow, how fast does it flow, and what qualities would the wind have? These were my main questions.

[Fouredged Sword]

I don't know caves that well, but here is my understanding of fluid dynamics and thermo.

Well, a lot of that depends on the temperature defferential between the sides of the shaft, and if the bottom side of the shaft has no other outlets.

The top of the mountain is cold, and the cave is probably around 60 degrees F.

IF the city bellow is connected to the base of the mountain it will turn into a chimney. As the ground around the base of the mountain heats, the air will be puled into the cave and up the shaft. With a significant temperature differential (and you could easily have an 50-100 degree differential.) you would get a light breeze in the city and a roaring torrent of air up the shaft. The city would be warmed by this hot wind.

At night the air on the side of the mountain would cool and the air flow could reverse and cold air would fall down the shaft. The temperatures in the cave would drop quickly and cold air would seep out the sides of the mountain through the cave network. This airflow would likely be slower, as the temperature differential would be weaker and moving counter to the direction of the pressure gradient.

IF the city bellow is sealed, then you will get turbulent flow of air both up and down the shaft at the same time. The city would act as a airflow sensor to the wind blowing across the mouth of the shaft. High winds would decrease the pressure in the city. Depending on the surface conditions (gusting winds or shifting storm fronts) you could get sharp pressure variations that would be decidedly uncomfortable on anyone's inner ear.