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I'm looking at a science fiction scenario similar to Lovecraft's At the Mountains of Madness where buildings are exposed to the elements for millions of years. I have a feeling they wouldn't be quite as intact as Lovecraft had it, so I want to try to figure out what's realistic.

As in Lovecraft, the location is a geologically stable plateau that is very dry and permanently below freezing point so no earthquakes, volcanoes or water.

The buildings are sturdy construction, reinforced concrete.

What would be left after one, ten, a hundred million years? For the purposes of the story, I don't need them to be usable, just to have enough remains for investigation by archaeologists.

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    $\begingroup$ The buildings would collapse before they eroded. Very few Roman-era structures have not collapsed. Take the timeline out to 10,000 years and everything will have collapsed except the Pyramids and Great Wall (and maybe even them). Once they collapse they are just a pile or rubble; they will trap loose soil, and get grown over, and turn into a little hillock with unusually high iron content before they erode much. $\endgroup$ – kingledion Mar 1 '17 at 21:53
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    $\begingroup$ You may want to read Deep Time: How Humanity Communicates Across Millennia (1999) by Gregory Benford. He’s released a free PDF now, IIRC. He covers many of the issues mentioned in Answers, in great detail. $\endgroup$ – JDługosz Mar 1 '17 at 22:07
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The main agent of erosion in your scenario would be wind, so you could have some interesting wind-sculpting of buildings. You'll need light winds and conditions that never freeze or thaw; and also never get much direct sunlight.

Erosion rates in this scenario can go as low as 0.1mm per kiloyear.. or 10 centimeters per million years - assuming your building is made of solid rock.

Of course, if you want to preserve buildings in a less contrived manner, then you can simply have them rapidly buried by flood or volcanic ash. Then have a geological uplift that allows the weaker material to be eroded out. That could give you remains out to the billion year mark.

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It depends on a lot of things, like climate and how long the building is exposed vs buried. nothing is going to survive a million years of exposure but a cut stone building like the pyramids might survive a million years buried in the right climate, a million years exposed not a chance, the the effects of night and day light fluctuation will destroy it. A billion years can grind several miles off a mountain so a building does not stand a chance especially considering how mich the climate might change in that amount of time, Antarctica was a jungle 60 million years ago.

Consider this chart, you want to keep yourself in the "slight weathering" area. enter image description here

Keep in mind steel reinforced concrete will destroy itself with its own chemistry in a hundred years. Concrete decay,types and causes

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There's concrete and there's concrete. The Romans used a form of concrete in some of their buildings which is still in place and in-tact today. Modern concrete, by contrast, can start to crumble within decades.

Reinforced concrete may make it stronger, but it also adds another failure method: If air or water gets into the structure (eg through a minor crack), it can cause the steel rods to rust. This can dramatically weaken the structure, and also trigger further cracks in the surrounding concrete as the rusting metal will expand as it reacts with the water and air. This process will be a lot slower under your described ideal conditions, due to the temperature stability and lack of water, but it will still happen.

The time it would take for this to happen will also depend on the quality of both the building materials and of the construction techniques, but it will happen sooner or later. Once it starts, it will accelerate: any damage to the structure will aid further decay.

I would predict that even under your ideal conditions, a typical modern reinforced concrete structure will be a heap of rubble within a few hundred years at most, if left unmaintained. Even being generous, add a couple of zeros to that, you're still only looking at tens of thousands of years; a long long way short of the millions of years you're looking for.

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    $\begingroup$ Well, as I noted at the start, Roman concrete is known to be able to last orders of magnitude longer than modern concrete. That might be a good starting point. Also, you'll want to drop the steel reinforcements: stronger, but not longer lasting. Finally, consider switching away from concrete entirely: A drystone wall (ie built with interlocking stones, so no cement or mortar required) may be a good answer: use a durable stone and good building techniques and it could last a very very long time. Parts of the Great Wall of China are built this way, as are the Pyramids of course. $\endgroup$ – Simba Mar 1 '17 at 16:21
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    $\begingroup$ Of course, the problem with dropping steel reinforcement is that you'll find it hard to build more than a few storeys high (unless you go solid and tapering like the pyramids). Probably ten storeys max. $\endgroup$ – Simba Mar 1 '17 at 16:24
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    $\begingroup$ BTW, for reference, Roman concrete was made of volcanic ash. $\endgroup$ – Simba Mar 1 '17 at 16:25
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    $\begingroup$ Re your question about digging down to the basement levels... of course, the basements would also have been constructed with reinforced concrete, so would also have collapsed. $\endgroup$ – Simba Mar 1 '17 at 16:28
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    $\begingroup$ steel reinforced concrete decays much faster than concrete by itself. concrete produces a charge which speeds up the decay of the steel and the decaying steel makes the concrete decay faster. one of the reason roman concrete decays slower is it has no metal inside it. Although this is by no means the only reason it lasts longer. $\endgroup$ – John Mar 1 '17 at 16:29
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Building for deep time is difficult.

  • Metals need to have surfaces that are self healing, or be coated with materials such as gold, platinum, iridium.

  • You may want to have it as in essence a self maintaining machine. A building doesn't have to be very tough if you have critters slathering a quarter inch of mud on it whenever it gets thin. You also have to be able to repair the critters.

  • Can you postulate future changes in building technology: E.g. a foobar field that repels rain and wind borne particles. This slows down erosion.

  • Large scale transmutation would allow massive use of non-corrodible allows.

  • A building that skips through time. If it is present only one day per millenium, it would easily last a million years.

  • Instead of being on the surface of a planet, put it on an airless planet or in orbit.

  • Embed into a stable cliff. The surface opening keeps eroding away. Some limestone caves are at least hundred of thousands of years old. Mind you, I don't know what fraction survive this long.

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  • $\begingroup$ Mesa Verde is exactly what you are looking for $\endgroup$ – Paul TIKI Apr 2 '17 at 5:36
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Unless the area is protected from the wind, erosion would sandblast all the features off.

On the other hand, that city was created by a race that can create artificial biological(?) constructs that can last that entire time. So the buildings were probably built from some indescribable (insert a paragraph of description) building material.

If I recall, the survival of the buildings was part of the (Lovecraftian: weak minded) characters' horror.

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