A Very Deep Valley: How Weird a Climate Can I Get?

[WayneLigon]

I have this idea for an extremely large, extremely deep valley. Roughly the size of the Mediterranean, bounded by very high mountains on all sides. The interior will be cold desert; moslty just a wastland of stones, crevices and the like.

I had the idea of making it very, very much below sea level. Like, 2-4 miles below. The lowest spot on Earth is 1362 feet below sea level, on the shores of the Dead Sea. This would be at least 10,000 feet below sea level.

Never mind how for right now, just go with the flow.

So, would a large land area that far below sea level, bounded on all sides by miles-high mountains, have a breathable atmosphere at all? Would be be too dense? Would there be a Venus-like runaway greenhouse effect, or... would it just be normal, if maybe a little murky?

 

[trollwad]

thoughts

A cold winter day in your valley might be like the hottest and most humid summer day in Houston. 10,000 feet might be excessive, maybe just a mile deep. call it the valley of sweat

 

[painandgreed]

The deeper you get, the hotter the region will be. If you want cold, make it a plateau surrounded by mountains which come down into a valley that is 5,000 feet above sea level or so.

 

[Hand of Evil]

Weather cycle would have a lot to do with it, water from the mountains would rush into it, the air thick and humid but I don't think pressure would be too much of a concern, water that went in would convert to rain and cycle back in. Yes, it would be a green house but nature always seems to balance itself.

 

[The Shaman]

The rough estimate for temperature change with elevation is that is decreases 3.5-deg. F for every 1000' of elevation gained - this would translate to roughly 35-deg. warmer at the bottom than at sea level.

Then again, given that this is a fantastic landscape, it could be a balmy 74-deg. all the time because the gods will it to be so...

 

[The Grackle]

Maybe it's just really cold everywhere except in the valley. If there was less atmosphere than Earth, a 4 mile deep canyon might be the only hospitable place. I think there was a Larry Niven novel w/a planet like that.

Although, that sounds kinda like the inverse of what you're going for.

 

[Nyeshet]

Of course, latitude also plays its part. If the valley is around, say, the arctic circle, then perhaps being a couple miles deep would bring it up to a nice sub-tropical warmth. Its a nice idea, really, but my first question (as a world builder that strives for realism) is why is this massive valley present? I can think of two good (and non-magical) reasons off hand:

1) Collapsed magma chamber from a supervolcano that erupted millions of years ago. There are regions on the earth today that - while not so deep as that - are thousands of feet deeper than the surrounding region due to this. Eruptions at such sites tend to occur every half million to million years.

The depressions formed by such are not even necessarily round regions. Look up "Long Valley Caldera" on google (the first link to appear is this one. Its an oval valley about 15 by 30 km and about 150 m to 1000 m deep (depending on which end you are at). However, the entire thing is about 6500 m above sea level, thus the reason it is not a contestant for deepest surface on earth.

Just take this, and multiply it about five to ten fold (in size) and it will be what you want.

2) Rift Valley. Occationally the crust bulges and then rifts, usually in a Y shaped pattern. Consider the Rift Valley in Africa and the Red Sea (two prongs in that Y-shaped Rift). The region - when it cuts into a continent - is often steep-sided and relatively flat. Parts of it will have hot springs and bubbling mud flats due to geothermic activity, and earthquakes are about as common there as one might expect to find in California or Ice land or Japan, to use some examples. There are also likely to be some volcanoes nearby - but not necessarily in the rift itself.

Suppose such a rifting occurred not next to a continent, but in the center of one? Suddenly we have a vast Y-shaped crevesse in the center of a continent, slowly dividing it into three continents. Now suppose we are near the arctic circle, and the rift occurs in a vast plateau. Now we have a rift that - if deep enough - provides (warm?) temerpate (rather than sub-arctic) temperature throughout the year, and is surrounded by very high steep slopes that likely have volcanoes dotting its edges - or forming on the outskirts of the plateau beyond the immediate border of the rift.

- - - -

Now in regards to your deep valley, note that depth does not directly affect humidity or percipitation. If the rift is in a dry region, then likely there will be little humidity, while if it is in a wet region the humidity will be as terrible as might be found in a tropical swamp.

If you go with my idea of having it near the arctic circle (not above it, as that would cause several months of darkness, which would prevent trees and other temperate / tropic plantlife) then it is likely that during the periods of snow any snow that falls into the valley will become rain by the time it hits the valley floor. Furthermore, if the valley is deep enough, it may have its own clouds forming, perhaps where the temperature meets some critical balance between the chill of above with the warmer below - forming vast mid-air fog/cloud- banks that would percipitate to the valley below.

This is actually a very nice idea. I might have to make use of it myself.

 

[tarchon]

Oddly enough, there was one of those, back when the Med basin mostly dried out in the Miocene, during the "Salinity Crisis". Wasn't quite that deep though. The Grand Canyon is a mile deep or so too, and it has a very substantial climate change from rim to floor.
http://www.nationmaster.com/encyclopedia/Messinian-Salinity-Crisis

 

[Graf]

(With the caviate that I don't use any more physics in my games than strickly speaking necessary).

A deep crevase in the ocean or earth that is a blisteringly cold wasteland is a cool idea. I would say go with it. A sort of blasted desert area with a permenent swirl of chilled frost hanging out a foot off the ground is a cool image.

I really wouldn't go overboard on the distances however unless there is some kind of pressing reason. Have a rough idea how deep the thing is in terms of getting in and out (i.e. you can fly at 60 with perfect mobilityand be up and down in 4 hours... a skilled climber can do it in 12 hours. A less skilled team can make it down in two days with one night camping on ledges) and just fend off player attempts to find out how deep it is.

Heck I think a "cold desert" is a cool idea period.

 

[Nyeshet]

Air pressure should not be a major concern until around 4+ miles deep (perhaps a little deeper). As for the flora, consider an inversion of the classic altitude / latitude presentation (using a mountain in the tropics, showing slow diffusion from tropic to desciduous to boreal forest, then to tundra, and finally to ice). Assuming the same (look here for an example), we find that:

feet      temp*     biome
0k - 4k   30 - 42   ice, frozen waste
4k - 8k   42 - 54   tundra
8k -12k   54 - 66   boreal forest
12k-16k   66 - 78   desciduous forest
16k-20k   78 - 90   tropical forest
* This is the average temperature.

Of course, by the time you are 20k deep the air pressure is becoming a bit of an issue - much like climbing 20k feet will be a bit of an issue. (It is called the 'Dead Zone' by climbers for a reason.) Actually, by the time you are 8 - 12 ft deep you face a bit of a problem, as the air pressure is a bit more than double the norm (for sea level). Of course, if you start at a higher altitude then descend, then the resultant air pressure would be about normal (or at least less than double normal).

I would suggest starting around the arctic circle in an area perhaps 4k to 8k ft above sea level. The region would normally be tundra with trees not too far to the south (several days journey on foot, perhaps). There create a valley about 10 - 14k ft deep (~6k ft below sea level, so only about 1.25x to 1.5x sea level air pressure). Now we have a mix of boreal / desciduous - mostly desciduous in the deeper regions and mostly boreal in the 'higher' regions. The rain from snow fall that passes over the valley surface combined with the steam from geyseurs, hot springs, mud flats, etc give the area a slightly more humid feel, allowing for more plantlife than typical (as tundra biomes are not exactly known for their percipitation). The clouds that rise from intra-valley evaporation do not rise high enough to escape the valley lip, so most of the moisture within does not leave. Some does, but it is eventually replaced with snow fall (that becomes rain as it falls into the depths of the valley).

The bottom of the valley will be difficult to breath if one is not used to such. I would use the rules for altitude adjustment in the DMG (or SRD) in reverse as one descends (or ascends, if they are attempting to leave). Once used to it they would have to adjust again upon leaving. Treat the valley floor as a forest prone to occasional mists (esp during transitional periods, such as morning / evening and autumn / spring) Some flat treeless areas would exist, but such would often have mudflats, geyseurs, etc.

Hot springs and a few seasonal streams (more likely, waterfalls) from the mountains surrounding the valley lip would be the source of the streams and (very few) minor rivers present - which eventually form lakes. These lakes would partially dry up during the autumn / winter (when meltwater from above is not sliding into the valley) and fill during the spring / autumn. This shift could lead to agriculture along these streams / minor rivers and around the lakes they eventually form.

Another question to consider: do you use the concept of ambiant magic in your world? If so, how does such a rift (or collapsed supervolcano) affect local magic? If based on ley lines, such a rift (or sv) might be particularly potent in magic, suggesting magical creatures (and beings and classes) being more common.