Depends on what you mean by “structure”. I’m willing to bet that road cuts made through solid bedrock (fairly common for highways going through the Canadian Shield) will last, and remain obvious to space visitors, for millions of years, depending on glaciation. They would long outlast pyramids and dams.
I doubt that the Hoover Dam would last all that long. I read in Wkipedia: "The large spillway tunnels have only been used twice, for testing in 1941 and because of flooding in 1983. Both times, in inspecting the tunnels after the spillways were used, engineers found major damage to the concrete linings and underlying rock. The 1941 damage was attributed to a slight misalignment of the tunnel invert (or base), which caused cavitation, a phenomenon in fast-flowing liquids in which vapor bubbles collapse with explosive force. In response to this finding, the tunnels were patched with special heavy-duty concrete and the surface of the concrete was polished mirror-smooth.The spillways were modified in 1947 by adding flip buckets, which both slow the water and decrease the spillway’s effective capacity, in an attempt to eliminate conditions thought to have contributed to the 1941 damage. The 1983 damage, also due to cavitation, led to the installation of aerators in the spillways. Tests at Grand Coulee Dam showed that the technique worked, in principle.
I suspect from this, that once water started to flow through the spillways in large volumes, and for an extended time, they would fail and bring the dam down.
When Gutzon Borglum surveyed Mount Rushmore he found several cracks extended deep into the rockface. He redesigned the Memorial to account for the cracks. That’s why the faces are not all lined up together, but have Teddy’s face deep in the rock and Abe’s face off to one side.
When Borglum found out the granite of Mount Rushmore erodes at a rate of one inch per 100,000 years, he added an extra foot to the design for Washington’s nose. Paraphasing he said, “It will give George another million years.”
Hoover Dam is not solid concrete. It was built using numerous concrete pours. A continuous pour to create a solid block was not possible because of the physical impossibility of transporting fresh concrete as a continuous pour, the heat generated would never allow the concrete to set properly (if at all) and unavailability of raw materials for a continuous pour. The dam is made from thousands of interlocking blocks, embedded tubing for cooling during setting and grout. The water pressure actually strengthens the dam structure, and the concrete mixture used grew strong because of the water.
Well, sand and other items in the water would certainly ruin the “mirror-smooth” finish in short order. After that, I expect that the spillways would begin to erode at a fairly steady rate.
Only in the comparative sense, I mean - to earthwork dams. Obviously if it were just solid concrete it wouldn’t generate much power…
This is what I was thinking as well. Once the drawing of water from behind the dam for human use ceases, and then maybe a couple of wet winters upstream, and it would not be long before the spillways were engaged, and not too long after that before they started to be damaged, and straight-up erosion commences. They could also clog with debris (that present day humans help control), and then the lake would breach the dam over the top - a really bad thing for dams. I would guess Hoover Dam would last only a few hundred years before being reduced to rubble.
I agree with the previously mentioned Great Pyramids of Egypt. But, what about some of the craters left from nuclear bomb testing - most are in dry, stable areas. Meteor Crater (natural, not nuclear) in Arizona has been there for about 50K years. Granted, craters like that are not “structures”, but they are man-made. What about things sunk in the oceans?
Road cuttings through rock are more obviously human-made than craters, and I think they would last longer.
Are there any monuments anywhere made of bubblegum? If anything’s eternal. . . . Although I suppose it would ultimately “flow” under its own weight.
As to the Hoover Dam… reinforced concrete really isn’t that great a material for maintenance free longevity.
Once spalling begins, or wearing of the concrete, water can get to the steel reinforcing. Then that rusts, which further spalls the concrete.
I’d give it a few hundred years, tops without maintenance.
This book, which I highly recommend, also suggests that Mount Rushmore just might be the longest-lasting human artifact: http://www.amazon.com/The-World-Without-Alan-Weisman/dp/0312427905/ref=sr_1_1?ie=UTF8&qid=1373314899&sr=8-1&keywords=earth+without+us
So, what happens to the Panama Canal thousands of years after we’re gone? If the locks all gradually rust out, would we have a constant flood of water going through the canal, or would the canal get all clogged up?
Remember that the Panama canal has locks on both ends, and there is a large lake in the middle, which is 85 feet higher than the locks on either side. At best, you would have a lake flowing to two oceans for a while.
Because USA! USA! USA!
The ‘spillways’ at Hoover Dam are tunnels located at the sides of Hoover Dam. Water flowing thru them might erode and collapse the tunnels, but I don’t think that would compromise the dam itself.
Eventually the underwater ports that feed the turbines and spillways will clog up with silt and other crap and the water level will rise and begin to pass over the top of the dam rapidly eroding it. That’s if the clogged ports don’t cause it collapse just from the unrelieved weight of the water. Assuming of course that it continues to fill with water.
Ankor Wat would be eroded last, being made of laterite.
Which very nearly happened at Glen Canyon Dam in 1983.
How big does it need to be to count?
As I understand it, any copper statues are going to hold out a very long time.
Here’s what you said last:
Hardness has nothing to do with it. I’m taking about resilience to weathering and erosion. My mistake was using the the term laterite when i should have used metal oxides. Oxides erode the slowest on the earth’s surface because they are already products of erosion. :rolleyes:
Just not sure how much of Ankor is laterite bricks and how much is sandstone. Maybe you can tell us.
Diorite, like granite, has large crystals, some of which erode faster than the others, which is why you eventually have pitting in a lot of quarry facings. I wouldn’t use it. Meta-sediments (not marble) are my pick.