Bryn Celli Ddu RevisitedPosted on: 29/06/2015
Monday June 29th 2015
Away from wildlife here and back to Standing With Stones. I thought it important to share this as many of you have expressed an interest.
I was contacted last night on Twitter by Ffion Reynolds who had been to Bryn Celli Ddu with a geologist who identified the 'tree' pillar as blueschist - i.e rock, not fossil.
Now, the interesting thing about blueschist is that it is a metamorphic rock which only forms in extremely particular circumstances, best explained by this quote from the United States Geological Survey's website:
For many years geologists have been able to relate individual facies to the pressure and temperature conditions of metamorphism.
But they had no satisfactory explanation for the geologic processes that form metamorphic rocks, that is, until the theory of plate tectonics emerged.
One good example is this relatively rare metamorphic rock called "blue schist."
Experimental work had shown that the minerals in blue schist form only under very unusual metamorphic conditions.
These conditions are a pressure range equivalent to a depth of 15 to 30 kilometers in the crust and a very cool temperature, only 200 to 400 degrees centigrade.
That's the approximate cooking temperature of a kitchen oven or toaster.
At a depth of 15 to 30 kilometers, however, the temperature is normally about twice as hot, 500 to 750 degrees centigrade.
So the only way that rocks can be metamorphosed to blue schist facies , is to be quickly shoved down to those extreme depths and then rapidly brought back up before the rocks have time to heat up completely.
And that's exactly what happens where two tectonic plates are colliding in a subduction zone.
In fact, blue schist bearing rocks normally occur in long linear zones that mark ancient plate subduction boundaries.
One of the principal reasons I clung to the fossil theory was the cylindrical tree-trunk shape which had clearly not been cut, despite all previous descriptions of it being 'carefully worked'. That was ill-considered twaddle by people who weren't using their eyes.
Finally we have a plausible theory for the monolith's unique form. The formation of blueschist could allow a seam of cooling rock to literally roll up between the tectonic plates rather like a piece of seaside rock.
This leaves us with two, rather lovely points:
One: The builders of Bryn Celli Ddu would know no difference, it was still a magical stone tree.
And Two: Metamorphic rock does not form in conveniently sized pieces. There must be more.
My very grateful thanks to Ffion for getting in touch... now to find the anonymous geologist...