h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

When we see archeological programmes, the experts always talk about "digging down to an earlier period". Does this mean that the Earth was smaller then? I've asked this question before and was told that the extra layers were due to us dumping rubbish etc then building on top ... but this can't be correct - the same "digging down to an earlier age" is true when they are digging in times before humans, in places where there was no vegetation, etc. So are we having a constant rain of dust from space? Or what is the answer?

Redmoss

Just because the area they are digging in now doesn't have vegatation doesn't mean that it didn't have vegetation a few hundred, or thousands or more of years ago. Matterial is constantly being dropped on to teh surface; bird droppings... rain; each droplet of rain has a foci of condensation in its core, a grain of dust... sure its going to take a lot of single grains of dust to add up, but given time....
Then of course there is the wind, blowing sand, dust, silt etc.
Plus once your considering any time in the past you have to think about where the waters were; rivers change course, disappear reappear, the same so for lakes, seas and glaciers, Glaceiers can have quite an impact in some areas, flattening some and taking matterial away, dumping miles thickness of matterial in others...
Also connected with waterways, is flood plains, and when they flood and it goes back, it leaves behind quite a build up of mud and silt and suchlike, and human kind often set up settlements near water supplies for obvious reasons, so certainly in terms of archiology the digs I guess are often in these settlements which by their nature must of at the time been near a source of clean drinking water

Yes, but nearly all the sources of "infill" is stuff that would have come from another part of the planet - resulting in an overall equalisation .....

I think the mountains are getting shorter and taller at the same time (erosion vs tectonic lift).

We're losing topsoil in many places, not sure where that's going when the wind and rivers take it away but I guess stuff is being moved around alot. It's not like the boffins are digging everywhere at once.

I think there are just places where the ground is being eroded, and re-deposited in the places where archeologists are looking. You sometimes see on Time Team (and its ilk) thay dig only an inch or two before finding Roman stuff, while in others they dig shafts that go metres down, just to reach Victorian buildings.
Add to that, I'd imagine the places where erosion and not deposition is happening are pretty inhospitable places scoured by the elements, and so not that suitable for the kind of human habitations archeologists are interested in.

Yes, that is what is happening. Bits disappear, bits appear. Just look at the UK coastline; go under the North sea between the Suffolk/Norfolk coast and the continent and you often find big land mammels from way way back, as well as more recently the dozens and dozens of villages and towns which have vanished and indeed far enough back you could walk across to the continent pretty much...
On a geological scale you've got sublimation of rocks going into the mantle from the crust, and then you've got mountain formation where new rock is being formed up from the mantle, its all just getting moved about...

things get deeper and deeper untill they get so hot they melt and then the earth settles down on this pool of liquid

remember we dont live on a solid shell but different floating 'islands' of solid rock floating on a sea of liquid rock

so the world is not getting bigger, your bit might get higher but will then sink back down again

Also, I gather the Earth is indeed gaining weight due to asteroid fall, but at a rate that's negligible over human history.

Yes, more or less what others have said earlier.

In some places stuff is deposited overall, which is equalised by stuff being eroded overall in other places. There are probably many places subject to erosion where ruins have disappeared. And even in those places where deposition is occurring, much of the original structure of the ruins has disappeared, such as the wood walls and roofs, and will be incorporated into the subsoil, so only the stone structures remain eg floors and foundations.

And over geological time even those structures will gradually be incorporated in the soil.

There is only a certain amount of 'stuff' that composes planet earth, and over geological time its a continual process of circulation, plus whatever small percentage of stuff added by meteors.

And over vast aeons of geological time some stuff goes back into the inner core of earth on the edge of tectonic plates, and melts again, and some stuff re-emerges such as via volcanoes.

Human history is such an infinitesimal amount of time in earth's history that it's impossible to see the bigger picture overall.

And the bits at the bottom, as was said earlier, are settling, becoming part of the inner, hotter part of the planet. It's a long old way down but everything's settling, compacting and breaking down, moisture is often being pressed and leached out of things that are fairly dep underground (compression, cappillary actions etc) so you get layers, yes, but they are much, much thinner than they probably were when they were laid down. Compacted, compressed and shifted around.

It's all pretty exciting really

Space debris is in fact a major contributor to earth mass in time scales beyond normal human perception.

But plants and critters contribute the most. It may be argued that these 'soil builders' are (like the wind and waters) merely recycling pre-existing materials, but 'growth' is not a strictly quid pro quo equation. Nitrogen from the atmosphere is likely the greatest contributor as it is converted from gas to matter. Carbon of course goes both ways.

http://www.hbci.com/~wenonah/new/e-worms.htm

And don't forget how deep we have to drill to get at those gazillions of plankton that have turned into huge petroleum deposits.

And the molten lava that erupts to form land masses actually expands as it emerges and cools and takes on new forms.


~jwf~

And we're sucking up the liquefied flora and fauna and extracting gaseous carbon which we spread all over the atmosphere! (Not to mention the plastic *islands* we make in the oceans).

As most people said, there are tectonic movements and erosion but the net amount is the same, just moved about.

jwf. Nitrogen also goes both ways. If it didn't, we'd quickly run out of atmosphere (Nitrogen being about 75% of the air).

I believe Darwin wrote a whole book on this topic.