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To be clear, I'm talking about apocalyptic events that cause devastating long-term effects, such as:

  • An asteroid impact (lets say about 4.5km wide)
  • A hyper-eruption from a super volcano (lets say it erupts 2,500-3,000 km3 of debris)

Not the typical nuclear war aftermath, not particularly interested in that.

Another thing to consider is that humans would not be capable of repairing, maintaining, or salvaging the buildings/cities (at least, not much) because they'd either be dead or underground.

Cities would just be virtually gone. There's almost nothing visually recognizable about them when you go to — for example — Los Angeles, California.

There may be evidence that the cities were there, of course, but what you see initially is either a flatland, a beach, hilly terrain, forests, etc.

So, my main question is: how many years would it take for this to happen after such events?

Side question(s): would the wrecked buildings still be there after, say, 200 years? Would they be gone? Or simply unrecognizable? How unrecognizable would they be?

Here's some cities I'm curious about, if examples are needed:

  1. Seattle
  2. Los Angeles
  3. New York City
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    $\begingroup$ Just one question at a time please as per the help center. At the moment your questions all have answers, the broadest being here, and here. It's worth using the search at the top of the site before asking. $\endgroup$ Feb 24, 2022 at 23:14
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    $\begingroup$ The TV series Life After People answers all your questions. en.wikipedia.org/wiki/Life_After_People $\endgroup$
    – RonJohn
    Feb 24, 2022 at 23:42
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    $\begingroup$ look at the towns around Chernobyl it does not take that long. $\endgroup$
    – John
    Feb 25, 2022 at 0:44
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    $\begingroup$ Your side question, there are plenty of ruins around Europe that have been ruins for far longer than 200 years are still there and are still clearly recognisable as once having been buildings so the real and very obvious world evidence is a resounding yes, it's almost like you didn't give that bit even the most cursory thought 🤗 while others can disappear without trace in less than a hundred years, it depends on several factors including the building materials, local environment, fauna, flora and climate. $\endgroup$
    – Pelinore
    Feb 25, 2022 at 1:59
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    $\begingroup$ Does this answer your question? How long would it take for a forest ecosystem to overtake New York City? $\endgroup$
    – codeMonkey
    Feb 25, 2022 at 15:28

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Welcome I will try to answer this question as best I can in short not long most modern buildings aren't made to last. Well the exact rate depends on the environment and building material of each certainly in most case most of the city wouldn't last the first few decades without human maintenance. Most of rest would be lucky to last the first hundred years.

However older buildings and monuments would last long (how long would depends on each individual case). The longest I know is the Washington monument which is estimated to last 2 thousand years after human existion.

I strongly recommend the documentary life after people. It can give a much better answer. I'll try and find to post the link later

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