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Wednesday, 20 March 2013

The dating of rock surfaces


This is a nice piece of rhyolite from an erratic (probably not very far-travelled) found on the flank of Carningli in Pembrokeshire.  Note the weathering crust, which is everywhere about 10mm thick and in places up to 15mm thick.  The crust is formed by a weathering process called oxidation, and it will continue to thicken for thousands of years -- maybe millions -- as long as there is free water in the environment.  the rock can be buried or exposed to the atmosphere -- it doesn't seem to matter very much.  Different rock types react to chemical weathering in different ways.

One factor which will influence this weathering crust is the annual rainfall received in the location in question:


This rainfall map of the British Isles shows degrees of dryness in brown and degrees of wetness in white and blue.  It stands to reason that for a particular rock type, weathering rates will be much higher in the wetter areas towards the west, and lowest in the areas coloured dark brown on the map.

This is all relevant because of the increasing use of isotopic / cosmogenic dating techniques which are being used on rock surfaces and even on thickly buried sediments.  There will be more to report on this quite soon......

If you are going to date a rock surface exposed to the atmosphere by measuring the amount of "cosmogenic bombardment" there has been, you need to be absolutely sure that the rock surface has been continuously exposed.  Even on exposed hill summits you can hardly ever be certain of that, for during warmer climatic intervals there may have been peat or other vegetation covering the rock, and during cold periods snow or ice might have covered the same surface -- thus shutting off cosmic rays.  You will get the highest "cosmogenic imprint" close to the surface, and this will decrease with every mm you penetrate into the solid rock.  Sometimes, if an author has not told us exactly how he took his samples, our confidence in his dates may be seriously compromised.......

Another big factor which can lead to the distortion of results is the breaking down of material on the rock surface  -- by the processes of physical weathering.  Frost action, salt spray, mineral contraction and expansion related to temperature changes, and even biogenic effects can break up rock surfaces and lower them by millimetres or centimetres per century.  In carbonaceous rocks like limestone or chalk, solution processes will come into play, and rock surfaces can be lowered even more rapidly, especially if rainwater and groundwater remain aggressive and unsaturated.   If you are dating a surface that has been lowered by any these processes, you will almost always get a falsely young result.

What does all of this mean?  In my view it means that great caution needs to be exercised in interpreting cosmogenic and other dates from "exposed" rock surfaces -- at least until such time that we have a much greater understanding of the correction factors that should be employed in the calculations.  Secondly, wherever possible, at least two different techniques should be employed -- if they bring up broadly comparable dates, this will greatly increase our confidence that we are learning something meaningful.



2 comments:

Constantinos Ragazas said...

Brian

you write, “weathering rates will be much higher in the wetter areas”.

Would the same be true for carbonate coatings on rocks?

Kostas

BRIAN JOHN said...

Kostas -- as I understand it, this would be the case. Groundwater which takes carbonate into solution will only continue to do so until the water is "saturated" -- after that, carbonate is precipitated out as water evaporates or flows on a rock surface, to form stalagmites and stalactites in caves, for example. Alex knows far more about this than I do.......