October 8th, 2016

Let us start with an assumption that most of us already suspect to be true: that civilization will not survive more than a few centuries into the future. If that sad assertion be true, then what will the earth look like in the far future? There was a television show some years ago entitled “Life After People”. It did a good job of showing how the artifacts of civilization would decay, erode, disintegrate, and disappear. What’s surprising is that most of the stuff won’t last more than a few centuries. Our big cities, freeways, bridges, skyscrapers, and so forth will be untraceable within a millenium of the collapse of civilization. What will survive for longer?

Well, the pyramids will last a long, long time. If there were more rain in Egypt, they’d erode away in a few tens of thousands of years, but no matter what, they’re going to be here for a long, long time. Most of our other stuff, however, won’t last a thousand years.

Except for marbles. Yes, the pretty little glass spheres that children play with. Marbles will last for many thousands of years; a few might well survive as long as a million years. Why?

First, marbles are made of glass — silicon dioxide. Quartz is just silicon dioxide with a few impurities in it. This stuff is extremely durable. The sand that stocks beaches all over the world is quartz, ground down into small bits a millimeter or so in diameter.

At this point, I have to explain how rock is broken down into fine particles. Big rocks are broken down by weathering — the combination of ultraviolet light from the sun, extremely weak acid from rain combining with carbon dioxide in the air to form carbonic acid, and lichen. This produces smaller rocks that are slowly pushed downhill by occasional heavy rain.

Those smaller rocks continue to be attacked by sunlight and rain, breaking down into smaller and smaller rocks. Once they get below about a millimeter in size, they are ingested by earthworms who survive by extracting microscopic amounts of organic compounds mixed in with the soil. While inside the gut of the worm, a particle of rock is subjected to more acid, which breaks it down even further. Much of our soil is manufactured by worms.

But silicon dioxide is immune to all this. It is one of the most inert chemicals on the planet. There is no natural chemical reaction that breaks down silicon dioxide; have you ever noticed that we store the most corrosive acids in bottles made of silicon dioxide? Quartz does break apart, but that breakage is due to the chemical impurities, which make weak spots in the quartz. Quartz can also be broken through mechanical erosion, most of which takes place in flowing water. A little piece of quartz is tumbled downstream by the force of water, and crashes into another piece of quartz, which causes a little piece of the quartz to break off. This happens zillions of times as the quartz tumbles downstream to the sea. Bit by bit, the quartz is broken up into smaller and smaller pieces. 

Other kinds of rock aren’t so tough, so they tend to be broken down into finer and finer soil. But not quartz. 

Now for another bit of physics: the smaller something is, the stronger it is. For example, it’s easy to make a short bridge with just a piece of paper: merely fold the paper along its length and place it over the gap. If you want a footbridge over a creek, the folded paper won’t work; you’ll need something like wood. A few planks of wood stretched over the creek will suffice. But if the creek is bigger, you’ll need a more elaborate design. And for spanning a really big gap, like the entrance to San Francisco Bay, you’ll need steel. So the smaller something is, the stronger it is.

Apply this concept to a piece of quartz tumbling down a stream. A big chunk of quartz that crashes into another rock will surely break off a piece. But as the piece of quartz gets smaller and smaller, the breakage becomes less and less serious. By the time it reaches the size of a grain of sand, the impact simply can’t break it down any further. That’s why beaches are composed of grains of silicon dioxide — it’s the only thing on the planet tough enough to withstand all the banging around.

So now let’s go back to our marble. It is completely impervious to any chemical attack, so it will never be harmed by anything until it is washed into a creek and starts its long journey to the ocean. That marble will last forever if it is not washed into a creek.

This is why I occasionally dig a deep hole — perhaps two feet deep — on my land, drop a marble into it, and cover it up again. I always dig such holes on flat land halfway between the slope and the creek. The soil erosion here is slowest. For many years, the rains will slowly move dirt down the slopes toward the creek. On this flat section of land, the process will be very slow, and the loss of dirt to the creek will be matched by the gain of dirt from above. But eventually the former process will outperform the latter process, and dirt will start eroding away from above the marble. Eventually, all the dirt over the marble will be washed away and it will be exposed. 

This will not happen for tens of thousands of years. I expect that Homo Sapiens will still be around, but not in any advanced technological society. Perhaps the people fifty thousand years from now will be living at the state of the Indians who populated this area just a few hundred years ago. But one day in the far future, I hope, somebody will notice one of my little marbles, pick it up, and wonder what it is, how it was made and even — if we’re very, very lucky and tales of a glorious civilization somehow persist that long — who put it there.

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