If soils are formed by decomposing rocks, how come we still find rocks from thousands of years ago?

And how many thousands or millions of years does it take for this to happen?

When I see the hills opposite my flat, was there a time when they were covered in rocks instead of soil and trees?

 

You my little puppy dog now

Look around at all the hills you can see from your flat. You may be able to see the old land level from the last uplift, its obvious in Dorset, lots of flat topped hills with deep valleys between.

As soon as the land gets lifted up by plate tectonic forces (Ripples from the Alpine erogeny are still happening) The weather tries to grind it down to sea level again. Rivers that are suddenly (geologically speaking) uplifted from their outlets at the sea will cut back to form deep valleys all the way back to the source.

As for the time scales, it will happen quickly after an uplift, a landscape can change in thousands of years. Over millions of years, well, thats a longer story.

There was a time when it was just rocks, no plants, no animals.

But in that time the geological processes were wearing down the rocks and forming soils, making conditions ripe for the colonisation of life from the oceans.

 

Mr Zigs is the GC Geologist I think, but while we wait for him, here's my go

Rocks have to be broken down to make soil. This takes many thousands of years and it happens in stages. The weather plays a role, physically wearing the surface. Bacteria also plays a role, consuming bits of the surface, then moss and lichens etc, and so on building up to the more complex plants and fungi.

We still find rock from thousands of years ago because the weathering only works on the exposed surfaces of the rock.

EDIT: Too late, Zigs got in there while I was writing

 

Thanks. This was one of those questions I wanted to ask but couldn't quite work out the right words in which to phrase it...a worrying dilemma for a poet.

 

You got it too Dave

The weather is an important factor, as are the organic components. It also depends on the mineralogy of the rocks, some components are more easily disolved and re distributed.

Take Granite, an acidic igneous rock that forms under mountain chains. Its composed of Quartz, Mica & Feldspar. The Feldspar will break down to form Kaolin or China Clay, the Mica is soft in comparison, leaving the Quartz, which will go on to form a new rock, orthoquartzite and so on.

I'll stop there as i'm getting too much into geowaffle.

There are very few rocks left from the original Earth, some of the earliest are in Scotland up this end, but some earlier ones are in Australia.

Most of the original Earth has been re absorbed from its firey beginnings.

 

So in theory, if these rocks from the original Earth were purposely preserved in a museum or somewhere similar, they would last forever, while the ones left out in the open would naturally break down?

 

Probably in theory, but in the timescales we're on about, the museum would erode away in a short time by comparison.

The oldest buildings in the world are only a few thousand years old. Nothing in geological terms.

 

No kidding Simon, you want to know anything about Earth Geology you just ask.

Oldest bit of rock i've got is about 13.4 Billion years old.

 

They would break down to the elements they are made from yes.

 

...I bet you knew I'd be bound to ask: how were the original rocks formed?

 

soil is formed primarily by decomposition of organic matter.. the decomposition of organic matter is more prevalent than the erosion of rock.

That said I live on a coast which is comprised of mudstone, and it erodes very fast! But that's on the coast and the erosion only really adds to the silt on the marsh chain, or the beach itself. In fact soft rock is almost always the catalyst to start marsh systems.

Rocks on the surface are formed almost entirely by volcanic processes, and rise to the crust via a process called subduction. (even the earth's crust has cycles, like water).

Zigs, you know much more than me though I'm sure!

Just trying to recover the knowledge gained in my Physical Geography/Environmental Management degree!

 

Yep

As far as we know, great big cloud of gas full of elements, probably the remnants of earlier exploding stars, coalesed into a clump that got so heavy in the middle that the clump went nuclear forming our sun.

The remaing debris clumped into lumps forming the planets, some of which heated up internally due to pressure & radioactive decay of the hevier elements & formed a molten core which drives plate tectonics & produces the rocks we see today.

This happened here and may have happened on Mars & Venus, its possibly happening on at least one of the moons of jupiter.

If it stops happening here, we'll end up being waist deep in water. We need plate tectonics to carry on.

 

Up here in Fife below the old Volcano, I expect the earths crust was exploded into the area
I have many large rocks, sort of Sandstone, each year these rocks shed about an eighth of in inch of top surface as a sandy deposit into my garden
I could If I wished smash them up to pebble size and they would be new soil in a couple of years
Now the Granite stones are a different matter, they are there and will never degrade in my lifetime

Jack McH

 

Has he bin using the time travelling helicoper again

 

Saw it happening, awesome