…how long would it take before there was no visible (surface) sign that we had ever been here?
and would there ever come a time when the buried archaeological remains would have decayed beyond recognition?
As far as I know, the most durable structures built so far are the Giza pyramids. They should last for at least another hundred thousand years or so, unless they get buried by a heck of a lot of blowing sand.
Plus, we’re probably going to leave at least a few fossilized human skeletons around. Paleontologists have recovered animal fossilis half a billion years old, so hopefully at least one of the 6 billion human bodies alive today will be lucky enough to get covered with silt in the right manner for its bones to fossilize and last approximately as long.
So it would take something really really cataclysmic to wipe out all evidence? - maybe as extreme as the earth’s crust melting? - is there anything that could cause this, like an asteroid strike or something? (I realise that the earth is destined for the torch anyway, regardless of whether you listen to cosmologists or fundamentalists)
Visible sign: A long time. 10,000 years plus? Possibly closer to 50 or 100,000?
It’ll have to do with your definition of “visible”. As in some guy walking around where America used to be, not even finding an odd outcropping of concrete from where an overpass used to be, or happening across a patch of blacktop in the middle of the Nevada desert?
Or as in not readily visible, say from an aerial survey, or what we consider “modern” satellite scans?
Forested areas would be reclaimed faster, due to the constantly-falling leaves and needle buildup. I have stuff that’s only been in my backyard a few years and they’re half-covered in humus already.
Dry areas like Utah or Nevada… geez, the remnants of a building would likely be visible for millenia. Only wind and earthquakes would be knocking 'em over. There’s gold-rush-era towns and buildings in California and Nevada that are still in great shape and easily habitable even today, over a hundred years later. And these are wood-framed huts; a modern steel-reinforced concrete building would be around far, far longer.
No “Detectable” sign? As in no fossilized remnant, no fragment of steel or bone?
Impossible.
We’ve made our permanent mark on this planet, and signs of it will still be here to be found and puzzled over by passing Vogons in five billion years when the Sun expands into a Red Giant.
As mentioned above- fossilized bone half a billion years old can be found today. That’s bone, turned to rock. Think of our modern stainless steels, things made from Titanium. Think of massive blocks of concrete like Hoover Dam. Smaller chunks like overpasses and house foundations may eventually disappear by erosion, but think of how long the Great Pyramids have been around, and consider how much concrete is in Hoover.
What about that underground salt-dome somebody converted to storage? Supposedly it’s been stable for 100 million years. Even if it collapses, the low-moisture salt would preserve even organics for far longer than they would be in more conventional conditions.
Like in “Planet of the Apes”- they found glasses, a plastic doll and… what, an artificial heart valve?
I think they’d have found much more- if the doll was in good enough condition to still talk, they’d have found the heart-valve-recipient’s skeleton, the remains of the car he was driving, a section of the street the car was on, a mailbox on the sidewalk, two streetlights and maybe the remains of a hot-dog vendor.
Well, unless you’re limiting this to the surface of the Earth, we have a few artifacts that might be still around after the sun goes nova. For example, the Voyager space probe, the one with the disk on it, is hurtling through interstellar space right now, and will be unaffected by anything that is local to our planet or solar system. The satellites we have in high Earth orbit might last a while, and I know the robot probes we sent to Mars and the junk we left on the Earth’s moon will last for a few million years.
And then there’s Howdy Doody hurtling through space at the speed of light in the form of NTSC-format electromagnetic radiation. 
As for the surface of the Earth, I think there will be some evidence of human presence for a long time to come. For example, the half-life of plutonium-244 is almost a hundred million years. So if you count evidence of nuclear power plants and weapons evidence of intelligent life on Earth could last into the billions of years.
A comet/asteroid strike, provided that the object was dozens/hundreds of miles wide.
When the sun may engulf the Earth 5 billion years from now.
Other than that, it seems like there will always be a fossil to find.
Yeah, the oldest human structures seem to be about 5,000 years old. So, there’s a lower limit anyway. My WAG would be that 100,000 years sounds about right to completely erase all surface structures.
Good point about the Voyagers. Those will be around for billions of years - - long after our sun “dies”.
By the way, the sun will not go “nova”. A nova is a sudden flaring up of a star when it accretes new mass from a companion star. Our sun will swell up to a red giant, create a “planetary nebula” (poorly named - - it has nothing to do with planets), collapse into a white dwarf, and then spend billions of years cooling off toward the background temperature.
What Phobos said about the Sun’s eventual demise.
There is the possibility that the Sun’s red-giant phase will extend the solar atmosphere far enough out to cause the Earth the spiral into the Sun. Mercury is definitly doomed to that, and Venus probably is. If that happens to Earth, the heavy atoms will sink to near the core of the Sun and pretty much everything else will end up as small molocules and atoms churning around in the Sun’s outer layers. Our atoms will end up as part of the pretty planetary nebula.
But everybody seems to have forgotten something - the Earth’s crust is made up of plates that move. Granted, it will take hundreds of millions of years, but eventually all of the current dry land (and wet land too for that matter) will end up down a deep-sea trench and enter the Earth’s mantle. The temperature and physical stresses from that will pretty much erase all visible evidence of our existance. As somebody said, a sufficiently advanced civilization might find small pockets of enhanced radioactivity and guess that we had nuclear power…
So, if we humans were to die off today, our trash’s days would definitely be numbered, although it would be a pretty high number.
So does anyone have an educated guess as to how long it would take for all of the earth’s current surface to be recycled by plate techtonics? It seems like a long, long, long time, since we’ve found evidence of life 3.5 billion years ago. If the rate of surface subduction (am I using that term correctly?) was brisk, wouldn’t those extremely archaic fossils have been drawn deep into the earth and destroyed?
Tectonic movement won’t necessarily lead to the subduction of all of the present day earth’s crust. Slightly less dense continental crust “floats” on oceanic crust, so it is primarily the relatively young oceanic crust that is subducted. Much of present continental crust will probably not be subducted before the earth loses enough heat for tectonic movement to halt.
That’s not to say the surface will remain unchanged; erosion, vulcanism, continent-continent collisons, thrust faulting, etc. will all be changing the neighborhood.
Actually, the average orbit for most satellites is only a few years before they fall down. At most, the best orbitting onces will only be there for a century.
So basically, the Empire State Building would outlive them.
However, there is a lot of junk floating our there way out of reach of Earth’s gravity. If anyone here saw the 3D map published in Wired magazine a while back, you’d know what I was talking about.