Thread: Where's all this Top Soil coming from?

Because I am a great fan of Time Team on Foxtel, I am constantly puzzled by the amount of digging and removal of soil that is required to reveal ""recent "" History. I know there is a rapid build up of dust in my home on a daily basis. I often tell people that this dust in my home is from the Sahara Desert, having been drawn upwards in weather events over this desert, and then entering the Jet Stream in the Stratosphere to fall back to earth in my Latitude. When we have vast Dust Storms in Australia it is not uncommon for the Alps in the South Island of New Zealand to receive a coating. So Australia goes to New Zealand. We are all Anzacs, new Zealanders come to Australia to escape the dust. But say, when Time Team go looking for Iron Age or Stone Age, they expect to remove built up soil, sometimes averaging three to six feet of soil. Surely our surface soils to not break down or accumulate at the rate that appears to be happening in England etc? If this was the case our houses would visually be seen to sinking below foundation level. Wind moves sand at a rapid rate under certain conditions, hence our northern railways often required clearance in the 20th Century before Trains could continue. But surely these conditions do not occur in England and Scotland? How come the, to me anyway, rapid soil deposits on what is now farmland ( in most cases ) in these Countries? Would appreciate some enlightening on this Subject. westwind.

Earthworms.

Believe it or not, the continuous turnover of soil by the action of earthworms will bury anything over a period of hundreds of years.

Wow. I can imagine.


In British Columbia we don't have much soil to support earthworms, but we have natural fertilizer. Each year puny salmon smolts leave their birth streams to fatten up in the Pacific. These return to spawn and die heaped up in shallow streams, from which scavengers disperse the meat and bone throughout our forests.

Originally Posted by westwind

Surely our surface soils to not break down or accumulate at the rate that appears to be happening in England etc? If this was the case our houses would visually be seen to sinking below foundation level.

If you look at old churches in England, they very often are doing just that. One common factor is burial of human remains in the ground around the church - eventually, they add up to a fair amount of dirt, and the church is lower set than it used to be.

In Iowa and nearby tall grass prairie regions, there are places where the accumulated soil since the last glaciation is ten meters and more deep.

(None of that was from earthworms, btw - the earthworms in the northern tier of the US are imports from Europe and Asia. Recent glaciers removed the native worms, and they haven't had time to recolonize that far north. ).

I think what skeptic ment in detail is better known as Bioturbation and not only done by earthworms. I believe that this could be right because since the humans began to settle down and run agriculture are those regions losing soil, for example in bavaria in germany a soil loss of 30 centimetres per year is estimated and I think in some regions like the "dust bowl" this number is even greater.

First, 30cm a year as an average seems a little excessive for a non desert area, but, maybe so.

Second, aerial transport happens all the time, Mongolia, as mentioned the Sahara, or the American west.

See: http://www.tomkennonphotography.com/...0823master.jpg

The amounts of missing sediments are huge.

It might be worth considering that you will only get preservation in areas where there is rapid sedimentation - a body which falls onto hard rock and is not covered over, will not be preserved. It therefore stands to reason that archaeological finds are often underneath thick sediment layers.

Depends where you are in the world, clay forms a decent part of it which is a major ingredient of granite, the grandfather igneous rock, where it all starts, once it is worn down by chemical and physical erosion. Most of it today comes from the decomposed remains of the fauna that onced lived above it, this is why in areas of conifer forest (eg. taiga) the soil is very acidic, as the pine needle leaves of the trees that live there are acidic. Entire ecosystems evolve to live only in acidic soil like this and wouldn't stand a chance in a more alkaline soil further south. There is very little top soil in the tropics as the heavy rain washes it into the sea and tall trees with shallow roots have to form butresses in order to not fall over in the wind, they're basically balancing on bedrock.

Modern day agriculture to feed the ever increasing 7 billion is utterly dependant on it yet hasn't seemed to realise that it is unsustainably exploiting hundreds of thousands of years of dense forest decomposition which, now that it has been felled is becoming shallower over time the more nutrition we extract from it.

There will become a time, possibly all too abruptly, where we find that we can take much less from the land than we could in previous years.

Drowsy Turtle and Mr. Monkey have both implied, but not absolutely stated a very important way in which the soil builds up. It moves into place from somewhere else. I have a tarmac driveway at the rear of my house. Every year I have to remove a couple of centrimetre depth of soil that has been carried there by rain.

Mr. Monkey - a complete side point. I don't think we can rightly call granite the grandfather igneous rock, since it is ultimately derived from the mantle rocks, peridotite, dunite and eclogite. (I'll be happy to deabte the point, should you wish, in a new thread. )

Edit: much of our understanding of the role of earthworms came from the work of Darwin, who wrote "The Formation of Vegetable Mould through the Action of Worms with Observations on their Habits". This article is a fascinating commendation of the work written about the same length of time after it was published as we are after the review.

Originally Posted by John Galt

Mr. Monkey - a complete side point. I don't think we can rightly call granite the grandfather igneous rock, since it is ultimately derived from the mantle rocks, peridotite, dunite and eclogite. (I'll be happy to deabte the point, should you wish, in a new thread. )

well there are a couple of things to add:
1. Perhaps he rather ment that granite very good represents the chemical composition of continental crust
2. Is it really derived from the mantle rocks? In most cases it's earth crust which is generating the raw material for granite and not earth mantle (think of S-type granite), ok so you can argue that this is also a product of melting the mantle but I say no, you can only get this chemical composition by erosion (I'm curious waiting for your answer)
3. I don't know if you realized the range of that question about the granite genesis, this questions ends up in the question: how have the "modern" continents been created.
4. I think that there is only under +3 Ga old continents a layer of eclogite, so this is really a exception. Also there are discussion about adakitic melts (molten subducted slabs so this would be eclogite)

so do we want to open another thread?

Terrestrial planets generate basalts. Continental crust evolved, lets simplify, via Bowen's reaction series to produce granites. Mr. Monkey described granite as the grandfather of igneous rocks, but what then are its children? The odd rhyolite here and there?

Certainly most granites today, and for some time, have arisen from reworking, but that still leaves them as descendants, not ancestors.

These are minor points of phraseology. I would be far more interested in a discussion on the subject of granites and primeval crust formation. I have a spade and we can even dig up H.H. Read.

Originally Posted by John Galt

Continental crust evolved, lets simplify, via Bowen's reaction series to produce granites.

1. Very simplified, I know that this is not working like this, even when you've got very less percent of molten mantle you get basaltic melts like in Hawaii
2. the Bowen series is also working for erosion the most basic parts erode at first

I would be far more interested in a discussion on the subject of granites and primeval crust formation. I have a spade and we can even dig up H.H. Read.

hmm... granites and primeval crust... I don't know how old are the oldest known granites? I think that there should be a border in time, with earths mantle being much hotter than today there shouldn't be granite, should be? What do you mean by H.H. Read??

I think this merits a new thread. I'll start one shortly. H.H.Read wrote a book, I think over half a century ago called, The Granite Controversy. He addressed the debate as to whether granites were magmatic or metamorphic. It was very important in its day.
There is evidence, from zircons, that granitic continental crust existed from an early time.

Originally Posted by John Galt

I think this merits a new thread. I'll start one shortly

waiting impatiently

Well we can always go back a bit further to find that everything originated from something else, ultimately taking us to the big bang. Considering granite as the "grandfather rock" is quite useful in illustrating the formation of basically everything on the surface of the continents with the exception of the organic contribution, particularly in the context of top soil as clay and sand, and all of the sedimentary rocks that are derived from them are basically all the products of eroded granite.

If that is what you had said I would have been in agreement with you. However, your phrase was the not the "grandfather rock", but "the grandfather of igneous" rocks. That strongly implies granite as the parent magma of many (most) igneous rocks, which I know you know it is not. Therefore I stand by this small side point and objection: it is misleading and incorrect to describe granite as the grandfather igneous rocks. Let's move on.

was meant to be interpreted as "the grandfather rock that happens to be igneous" guess I'll take a hike back to English school..

Originally Posted by Mr. Monkey

was meant to be interpreted as "the grandfather rock that happens to be igneous" guess I'll take a hike back to English school..

Probably not necessary. Just avoid smart ass, pedantic oldsters in future.

I've started a thread in Earth Science on the formation of continental crust. You might wish to look in.

Ok you're right, all of the non organic surface particles smaller than 2mm comes ultimately from an eroded initial rock which could have been igneous, sedimentary, or extraterrestrial without effecting the overall process.

You missed out metamorphic.