Excavations: Where does all that soil come from?

[Newai]

I'm reading some books that describe site excavations. The archaeologists dig layers of dirt up to get to the ancient settlements many feet below. I'm wondering how they get buried in the first place. There are some photographs that look like a town from the ninth century with a series of rooms that look more than ten feet deep. Did people dig cellars or below-ground rooms? Or was the ground level much lower then than it is today? But even closer to the surface, where is all the soil coming from? How do these places get buried?

 

[Jon Richfield]

Look about you; all sorts of things move earth, from earthworms and earthquakes to volcanoes and wind. If you can think of it, some archaeologist somewhere has dug through it. In Pompeii and Herculaneum it was volcanic tephra. In tropical rainforests it has been jungle bioturbation, in mountainous regions it has been landslides, and in valleys, floods. There even are places where human activity was responsible, such as building over old sites.

Actually it was not a bad question; firstly a lot of people take such stuff for granted, and secondly, the nature of the stuff covering a dig may tell a great deal of what we want to know about the place.

Go well,

Jon

 

[Andre]

In addition, don't forget wind blown (aeolian) dust (facies) and sand.

See for instance: http://hirise.seti.org/epo/aeolian_theme.htm

 

[cesiumfrog]

There's a talk up on TED (on the topic of sustainable food, esp. goose-liver) that happens to assert soil is made of dead roots. That is, the grass grows (essentially condensing solid out of the atmosphere for the most part) and the roots push and grow in proportion, then each time the grass is grazed down the roots die back in proportion, stranding material below the surface (whereas material decaying on top might erode faster than it accumulates). The claim is that particular types of agriculture continually improve the soil depth. I'm not sure how accurate this picture is?

 

[Jon Richfield]

In addition, don't forget wind blown (aeolian) dust (facies) and sand.

Interesting link, thanks.
I am relieved to note that I did indeed mention wind though. Sorry about forgetting to mention the term aeolian. I am puzzled however by your wording. Are you suggesting that aeolian dust forms facies and sand doesn't? I assume not!

cheers,

Jon

 

[Jon Richfield]

There's a talk up on TED (on the topic of sustainable food, esp. goose-liver) that happens to assert soil is made of dead roots. That is, the grass grows (essentially condensing solid out of the atmosphere for the most part) and the roots push and grow in proportion, then each time the grass is grazed down the roots die back in proportion, stranding material below the surface (whereas material decaying on top might erode faster than it accumulates). The claim is that particular types of agriculture consistently improve the soil depth. I'm not sure how accurate this picture is?

Hm??

If you think that topic is worth looking at, please give me an url. As stated the proposition hardly makes sense. Certainly the main source of plant food (not counting H2O) is atmospheric CO2, but (ignoring topics concerning H and O) that is only the main source of carbon.
Roots are only a dominant source of soil material in organic-rich soils such as peats, and then other plant material figures as well.

Don't misunderstand me; roots and other humus are very important in many aspects of soil formation, including supporting biological action (worms, grazing etc), trapping wind- and water-borne particles, rock decomposition, storing mineral nutrients, and adding bulk and aeration to the soil, but that doesn't make much sense in the terms you mention.

If you are interpreting what they say correctly, they seem to be putting valid concepts together in misleading, effectively meaningless, perspective. Practically everything to do with soil involves many variously interdependent factors, and it takes a lot of knowledge and experience to make any sense of it. That lot don't seem to be cutting it.

Cheers,

Jon

 

[Antiphon]

There is a small but steady rain of dust from space. Approximately 10 tons per day. Over the eons this builds up.

 

[Jon Richfield]

There is a small but steady rain of dust from space. Approximately 10 tons per day. Over the eons this builds up.

Quite right of course; I had left that one out. But in fairness it is a bit part only. Let's see... roughly ten million billion nanograms per day, spread over 216e12 square metres of planetary surface. If I haven't miscounted (don't rely on it!) that gives us something like 46 nanograms per square metre per day. In other words, about 17 kg per square metre in the last billion years. Lucky for us the first couple of hundred million years of Earth's history were less stingy than that, or we still would be waiting for a planet larger than say, Phobos!

Cheers,

Jon