Suppose for a moment that every human being on the planet were to die tomorrow. I don’t mean in some catastrophe such as an asteroid strike or a nuclear holocaust, but just a gigantic coincidence – everybody simultaneously suffers a fatal heart attack and dies. Or if you prefer, everybody is teleported to Alpha Centauri by a race of giant lobsters to work as slaves in their krill farms. The point is that suddenly there are no humans on the Earth. Other animals and plants are unaffected, and our towns, cities and infrastructure generally are still standing.
A million years pass by. Would any evidence still be visible (to an alien visitor) that we had ever been here? No archaeological digs allowed – the ruins or whatever have to be in plain view. If a million years is too long (or too short), how long would it take for the forces of nature to remove all visible traces of our existence? And what would be the last human-made structure to survive?
In a million years, we’re likely to have had another glaciation or two, which will likely destroy all man-made structures in much of Europe and North America. That might even destroy Mt Rushmore. I wonder if the Great Wall of China might survive in some form? Undoubtedly not intact, but enough to be able to tell that there was something there?
So we’re really looking for something significant in warmer climes made out of something durable - granite or similar - where there’s going to be minimal erosion. Actually, last time around, the glaciation stopped just short of Stonehenge, so that might qualify, but they might be toppled by an earthquake. The pyramids are made of sandstone and won’t last.
Sorry, this doesn’t count. My question is about natural processes on the Earth.
Besides, how would the alien visitors know these craft came from the Earth? After all, we’ve sent craft to Mars. Hypothetical Martians could have sent probes to our Moon. What I’m looking for is unambiguous evidence that a civilization once existed on Earth.
Large cement structures will eventually crumble, and structures in jungles will get overrun, but large stone structures free of wild growth will last a heckuva long time.
In particular, the Pyramids, especially the big ones at Giza, aren’t going to get overgrown and they’re already in a pretty stable state, so they’re not going to fall down. They don’t get much rain there. They’ve already lasted something like 4500 years, and they’ll last many times that and still be obviously created.
Newgrange and similar tombs have collapsed, and are only obviously creations since being rernovated, but stone circles like Stonehenge are obvious Works of Man.
Others? The Pont du Gard, maybe. Stone jetties built to form harbors all over the world. Modern roads built across the deserts.
Mount Rushmore, fer cryin’ out loud. Not to mention Crazy Horse, Stone Mountain, and other such monuments.
All that construction on the island of Manhattan. Nothing natural carves out rectangular holes in gneiss.
My guess is tens of thousands of years at least, and probably longer. Depends on how laxly you define your terms about how obvious it is.
Your question was “How long would it take Nature to destroy the evidence of human existence?”
Nothing in the original premise restricted it to only the earth. I know Im being a smart ass though, and you werent really intending off earth objects.
“Besides, how would the alien visitors know these craft came from the Earth?”
Because its the only easy life supporting planet in the solar system and its right next to the moon. A swath of other stuff still orbitting the earth would probably be around for a fair while as well, eg geosynchronous satellites maybe?
Australia has been free of significant geological change for millions of years. If and when the solar chimney gets built, it might last a heck of a while.
I’d think a million years would be long enough to wipe all traces of present human habitation off the surface.
I saw an article in 2001 that pondered a similar issue - it actually centered on New York and how long skyscrapers like the WTC would last (written before 9/11) and they concluded the structure could stand for about 6,000 years in the best case scenario without repair. I think it was either in National Geographic or the New Yorker, but I’m not sure.
Think about how little we know of what the Earth’s surface was like one million years ago, how much erosion will take place - I really doubt Mt. Rushmore or the Great Wall of China would be recognizeable or even findable after a million years.
Give them hundreds of thousands, and they would probably be able to find something, but a million is giving human engineering way too much credit. Even if Mt. Rushmore erodes only 1 inch per 10,000 years the gradual rate of erosion will almost certainly be disrupted by a major catastrophe over the time scale of a million years.
Quarries, underground mines, and the excavations for large buildings are going to leave their mark for a very long time. I’d have no doubt that, wiuth the normal biological and geological processes that are going on at present, some of those will be around for a long time. I’d also expect a few subway systems to be still traceable.
“So, Professor, how do you explain these long strips of iron oxide in what look like collapsed tunnels?”
“My guess is that Earth was the home of giant iron-eating worms.”
Stone Mountain will show obvious signs of tampering a million years hence.
Using CalMeacham’s erosion numbers, a million years will erode away 2.5 meters of material. The sculpture in the side of the mountain is much deeper than this. Even if its human and equine shapes aren’t exactly recognizable in a million years, it should still be apparent that some sort of sculpture was carved into the mountain.
And then there are the building foundations carved into the top of the mountain.
As for plants overrunning the mountain, I can’t see it happening. The mountain has been mostly naked for millions of years.
There’s an accidental experiment on this going on right now. The town of Pripyat is located a short distance from the Chernobyl nuclear plant in Ukraine. Its entire population was evactuated the day after the accident in 1986. Now it’s slowly being reclaimed by the Russian forest.
On the other hand, they’re large, and could be recognized as artificial (being make of hewn blocks all the way through) until they erode all the way down to nothing (as opposed to something like Mount Rushmore – abrade off the top few dozen feet and they’re ambiguous, abrade off a bit more and it’s nothing-to-see-here).