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Let’s suppose that there is a canyon, several hundred kilometers in length, stretching from Leipzig to Geneva and reaching the depth of 30 kilometers. It is up to 60 kilometers wide, cutting through the Thuringian Forest, separating Franconia and Swabia in half and forming an abyss between the Alps and the Jura Mountains. Its presence is the only distinction from the real world; all geomorphological features not affected by it exist in their real forms. There is a sea at the bottom; its level is 27 kilometers below absolute sea level. Hygromagnetic Handwavium-Phlebotium(TM) rock magically removes all water surplus to that threshold; rivers and creeks exist in the canyon. The slope is fairly smooth, beset by some cliffs.

My question is:

  • How would the canyon look from space? What would be the climate inside it and how would it affect the climate of the world; how would vegetation and animal life look (and what would be the lowest height of survival, if applicable)?
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  • $\begingroup$ Are we to assume that the canyon suddenly appeared, or formed gradually? How long has it been there? $\endgroup$ – Alex S Feb 3 '18 at 18:40
  • $\begingroup$ If you handwavium magic material to magically do stuff, why would you bother asking what the climate would be, as you simply handwave that as well ? $\endgroup$ – StephenG Feb 3 '18 at 18:55
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    $\begingroup$ @StephenG On this site we assume the scenario in the question and don't use any magic/handwaving beyond that stated in the question. It is up to the person posting what and how much handwaving they want to do. $\endgroup$ – Bellerophon Feb 3 '18 at 19:17
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    $\begingroup$ Geneva is not in Central Europe, as it lies west of Italy; if Geneva is not in western Europe then I don't know what is. The Rhine would have to cross this canyon... The Kola superdeep borehole reached a depth of about 12,260 meters; at that depth the rocks had a temperature of 180 °C. $\endgroup$ – AlexP Feb 3 '18 at 19:30
  • $\begingroup$ Related: How could a Giant Chasm be created? worldbuilding.stackexchange.com/questions/96000/… $\endgroup$ – Dubukay Feb 3 '18 at 21:24
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The first major factor is air pressure. Air pressure at sea level is one atmosphere, and density will increase going further down; the exact formula is something like

$\frac{P(z)}{P_0} = e^{-\alpha(\Delta z)}$

With a $\Delta z $ of 27 km I expect a pressure to the tune of 12 atmospheres, which makes most organisms unviable due to oxygen toxicity (oxygen partial pressure is 2400 millibars instead of ~210). This assumes an air temperature like STP, which as we'll see is not verified. Actual pressure will be somewhere between 4 and 6 atm.; still too much. It's about 45-75 psi above standard atmospheric pressure; pressure cookers do not usually exceed 15.

Also, sunlight at the bottom is little and weakened by atmospheric absorption, so there is no mechanism to remove carbon dioxide, which is heavier than air and would flow downwards.

Considerations such as the temperature come a close second. At a depth of 27 kilometers you may expect a temperature around 600 °C, which neatly explains why the water level isn't rising. At that temperature, even with the pressure, water boils anyway.

There is very little to bother about climate inside what is - to all intents and purposes - God Almighty's own pressure cooker, or an approximation of the surface of Venus (hold the sulfuric acid rains).

Other considerations about the breath of spring that's to be found at the bottom:

  • outgassing from rocks would include additional CO2 from weathering of carbonates
  • the rocks would also outgas significant quantities of radon gas, making the local atmosphere mildly radioactive.

In some places, the chasm might "delve too greedily and too deep": in places in yellow you don't really want to go deeper than 25 km, and that's without any great safety margin. A lava spout is a distinct possibility.

Distance to the Mohorovicic Discontinuity

I'd need to run realistic calculations but I think that with things standing the way they are, over 27 kilometers of air there wouldn't be too much liftoff - you'd get a hell of a cloud cover, but no roiling incandescent vapour inferno from the depths. Rather, a sort of flat white surface like a sea of clouds, with occasional kilometer-wide plumes. (Don't trust me on this, please).

The whole setup would mean that most of Europe would run on hydro energy from the German Bore (or Der Abgrund). Drive a conduit several kilometers inside the Bore, harnessing whatever waterway was falling in previously, and reap huge hydropower output at the bottom.

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  • $\begingroup$ "most pressure cookers I believe work in the neighbourhood of 15 psi" Atmospheric pressure is right around 15 psi. Did you mean an overpressure of 15 psi, for a total pressure of around 30 psi? $\endgroup$ – a CVn Feb 3 '18 at 19:59
  • $\begingroup$ You missed the biggest: Earth's rock can't support a 30km hole. It would close up. $\endgroup$ – Loren Pechtel Feb 4 '18 at 1:15
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I assumed along with your Hygromagnetic Handwavium-Phlebotium(TM) you had some patent-pending SuperRock-Handwavium to make this thing not collapse in on itself and to plug any cracks into the Earth's mantle. So, my best ideas for your questions:

Overall, this canyon would be a mostly dark scar on the landscape with extreme weather near the bottom but few effects on the rest of the world, a normal ecosystem near the top and underwater but with nothing living in the middle.

-How would the canyon look from space: If the sides are very steep, then it would look like the Grand Canyon, except more than twice as wide as the Grand Canyon's widest point. If the sides were more sloped, then it might look more like some of the canyons in the Himalayas. However, the biggest difference in appearance is that this thing would be dark trench in the landscape. At ~30km deep, sunlight would touch the bottom of the canyon rarely. The Grand Canyon is less than 2km deep, and even at that depth the parts along the river get a good amount of shade. Sunlight definitely will reach the bottom for at least part of the day, however much of it will only get a brief glimpse of sunlight reflecting tens of kilometers up the side of the canyon.

-What would be the climate inside of it: Temperature throughout the canyon is tricky. As altitude increases temperature decreases because there is less air to hold the heat(snowy mountaintops), so it makes sense that temperature would be high in the canyon since the air pressure is higher. This gives a rough estimate of 6.5c per km of altitude change, however the lack of sunlight can negate much of that. So, depending on the amount of sun the canyon will have areas that are boiling hot and areas that are freezing cold. According to calculations that use too many letters, pressure at 30km below sea level would be about 15 atmospheres, where 1 atmosphere is sea level, if it was a comfortable 15 degrees Celsius. Pressure can get closer to 25atm in the colder parts, or 7atm in the hot parts. This will cause extremely chaotic storms where these pressure differences meet.

-Effect on world climate: Thankfully, probably small. Wind from the Great Canyon Storms will definitely blow out of the canyon, but most of the churning atmosphere will be contained down below. A 60km wide, 30km deep, 500km long canyon would be 900,000km of air missing from the atmosphere, however the pressure will mean that even more than that would fall into the canyon. The exact amount can probably be calculated, however the pressure differences from depth and temperature results in an equation beyond me, so let's assume this canyon sucks in 5-10 million cubic kilometers of air as a very, very rough guess. There is roughly 4 billion cubic kilometers of air, so this trench will bring the edge of space closer to the earth but thankfully air pressure at sea level will remain basically the same.

-Canyon Critters: 0-3km down: Much the same as the Grand Canyon. Birds, adventurous goats, and very adventurous people, as well as hardy trees, bushes, moss, etc. 3km-5km: In areas that are warm enough to ease the atmosphere pressure, but shady enough to not cause the critters to boil, will likely also look like the Grand Canyon. 5km-'sea level': Extremely high pressures, extreme temperatures, extreme storms... nothing but hardy bacteria until the bottom. Underwater: Plenty of life. I'd imagine the sea would look much like the bottom of the actual oceans: tough critters huddled around hydrothermal vents. Since the bottom of this canyon will be like 5 feet from the Earth's mantle, I'd assume these vents will be plentiful. Water is really hard to compress and air is light, so the pressure here is likely less than what it actually is at the real bottom of the ocean.

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It would render a good portion of Europe uninhabitable as it became the center of a spreading rift center causing massive flood basalts erupting up this valley which would then likely cause a global extinction event, so from space there'd be a massive red and black scar ripped across the continent surrounded by a dead or dying world.

Also, not much plant or animal life in it, what with the magma ripping through it and all.

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