On Sat, 2 Aug 2003, Riboflavin wrote:

One of the problems we already face is that we are getting to the point
where we could never recover from a fall because the natural resources
needed to manufacture technology have been degraded to the point where
high levels of technology are needed to extract them.

Aside, of course, from all of those natural resources that are conveniently
being used right now. I think a rebuilding civilization would have a bit
easier time getting iron from old car bodies conveniently located in
junkyards (and whatever happened to the old civilization's cars) than in
trying to mine it from scratch.

True in part.  This question's been discussed here before, though not in
the context of the Fermi paradox IIRC.

It's not a simple issue.  Some elements are so common in the crust of the
Earth (and presumably most "Earthlike" planets, that they simply cannot be
exhausted without taking the whole planet away piecemeal to somewhere
else.  Iron and aluminum and silicon and oxygen are simply impossible to
"use up"--technically feasible ores are everywhere, and many of them are
renewed by natural processes.  Silicon dioxide crystals are constantly
weathered out of granites and conveniently concentrated by streams, green
plants return oxygen to the air, iron ores are renewed (in the form of bog
iron) on a scale of a human lifetime rather than geological eons.

Fossil fuels, unlike iron, really do get used up.  We aren't using them as
chemical elements, but for the sake of the energy in their chemical bonds;
once they burn, that energy is ultimately radiated out to space from the
top of the atmosphere and gone for good, if you believe the laws of
thermodynamics.  While burial and fossilization of biomass does occur
today, we're using fossil fuels at many orders of magnitude faster than
they form--real exhaustion is quite possible here.  Anthracite coal is one
very useful example that's almost unavailable in an economic sense today,
to pick one fuel that's nearing exhaustion.  A future civilization
couldn't count on the cheap and easy abundance of chemical energy we've
had the advantage of, unless you're talking about an empire of
hypercockroaches half a billion years down the line.

There's a third category, where resource exhaustion's also possible.
There are many rare elements that are essentially and economically
unfindable in the earth's crust in general; only a few rare geochemical
accidents have concentrated these elements in a few minable locations.
Exhaust those mines and the metal is simply off the market, and any
technology dependent on it is scrambling for substitutes or simply
collapsed.  For a number of these elements, our use of them tends to
disperse them as waste that's very hard to round up and recycle; unlike
salvaging old cars or copper wiring.  Tungsten is one example of an
important industrial metal that we mine from a very few sites and scatter
in tiny particles across the Earth.

So (assuming a reasonably Earthlike planet) a successor civilization would
find fewer options in the second and third categories.  By way of partial
compensation, its archaeologists could turn up important clues toward
technological progress.  But they may well have to put up with reddish,
short-lived carbon filament light bulbs until they can manage to invent a
flourescent tube or an LED.  They may never have enough fossil fuel for
electric power generation or profligate transportation, and so on.

Conrad Hodson