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I'm at the mall and Yellowstone erupts - My, and everyone else's car is covered with ash, fire, and maybe even lava. Lets say it was a big shopping day too, like Black Friday or Christmas Eve, just to ensure the parking lot is packed -- That is a lot of steel in one place.

Now, everyone is dead, and our cars are covered in chemically reactive ash and chemically reactive fire. Also they're in a wasteland. Also, most everyone in the continental US is dead (or dying).

After a few decades, the US will be covered in plants, and in few centuries it will be covered in animals. Underneath all of that of course, perhaps a few dozens of feet, is me, the mall, my car, and my Christmas presents.

How long will it take for my and everyone else's car to look like a regular iron vein? Will it ever happen? What about the weird man made chemicals, like paint, motor oil, or antifreeze?

Also, Merry Christmas!

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    $\begingroup$ "in few centuries it will be covered in animals." That sentence makes me think it will look something like in this xkcd what-if $\endgroup$ – Mrkvička Apr 10 '17 at 17:33
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    $\begingroup$ @Mrkvička I'm pretty sure What if should be required reading for participation in this stack. =D $\endgroup$ – Sidney Apr 10 '17 at 17:58
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    $\begingroup$ You are assuming too much of a catastrophic event. If Yellowstone erupts, only in close proximity to the site it would be hot enough to melt iron. Everywhere else, it would be just ash, which would preserve cars so in thousands of years new archaeologists can learn about car design of early 21st century. $\endgroup$ – Alexander Apr 10 '17 at 18:21
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You could get something that might fool an alien geologist, at least briefly. Imagine that the mall gets blown out by a particularly large lava flow, one superheated to the point that all the cars in the parking lot get completely melted. Rendered down into a fully liquid state.

Of course your Christmas presents will be vaporized by this

If it happens fast enough, the metals may not even oxidize much until everything has a chance to cool down into a solid state. That should leave threads of metal throughout the strata.

Earthling geologists would know that this is not likely, because we've had time to study the earth for a long time. Aliens, might take a while longer to figure it out. They might send prospectors down to mine out the valuable metal ore.

Obviously, I'm not a chemist, so I don't have any idea if this is what could happen, but it sounds plausible to a layman.

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Never

For one thing, to my knowledge, no veins of Iron have ever been found in volcanic breccia so large deposits of steel (iron) found in one would immediately be suspicious. Additionally, cars are made up of many materials that over such a short time would not decompose. More surprising would be finding the concentrations of aluminum and platinum, let alone the plastic and glass. It would never be mistaken for a vein of iron.

Edit: When I say vein of iron, I really mean Red Bed which are deposits where you find iron in the form of oxides.

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    $\begingroup$ In fact, I don't think that any veins of iron have ever been found, period. (Other than iron meteorites.) Iron is pretty reactive, and most steels should corrode in a few hundred years. $\endgroup$ – jamesqf Apr 10 '17 at 18:52
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    $\begingroup$ @jamesqf You are right. I did not feel like going into a discussion about red beds and all the various iron containing ore. If the car is really buried in ash then over time water intrusion will turn them into large rust stains in the strata, but the iron is still there. $\endgroup$ – Joe Kissling Apr 10 '17 at 18:56
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    $\begingroup$ +1 for mentioning glass. That stuff lasts almost forever, barring deep metamorphosis. The original CocaCola(TM) glass bottle was specifically designed to be recognizable even as a fragment. Since archaeologists often name a culture after characteristic artifacts (such as Folsom points and pottery styles) this raises the entertaining prospect of future archaeologists mapping the spread of CocaCola Man. And Budweiser Man. $\endgroup$ – WhatRoughBeast Apr 11 '17 at 21:47
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No, it will never happen that cars buried in the ground will revert to iron ore.

The scenario you described already happened, on a smaller scale, in Pompei. Tool used by the people, even bread, fruits or wine jars have been found back. No iron, copper, lead, silver or gold veins found.

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    $\begingroup$ They found cars on a parking lot in Pompei? $\endgroup$ – Ghanima Apr 10 '17 at 21:09
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    $\begingroup$ @Ghanima: "on a smaller scale". They just found 1 car in a driveway :) $\endgroup$ – njzk2 Apr 10 '17 at 21:22
  • $\begingroup$ Yes. And on a longer time-scale the car will turn into a fossil. Groundwater will dissolve the metals and deposit some other mineral such as calcite or quartz. Tens, even hundreds of millions of years hence, a subsequent intelligent species will find abundant evidence that they are not the first posessors of technology on this planet. $\endgroup$ – nigel222 Apr 11 '17 at 9:25

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