Thursday 23 May 2019

Crummock Aureole at Bowness Knott

I collected the rocks on the right from the summit of Great Borne in Ennerdale. They are made of Ennerdale Granophyre. Granophyre is an intrusive rock with a similar composition to granite (hence the pick colour) but is said to be "subvolcanic" rather than "plutonic" meaning that it crystallised closer to the surface - at depths of less than 2km. This means it would have cooled faster than deeper rock, I think. It gets hotter as you go deeper so that would make sense. Hence the crystals are smaller. As we came out of Rake Beck and down towards Bowness Knott, I found a very different rock, shown left. It seems much harder with no obvious crystals. It occured to me that an intrusion ought to have a metamorphic aureole, and that the rock I found was likely to be a hornfels. And then I found this which confirms what I thought http://jncc.defra.gov.uk/pdf/gcrdb/GCRsiteaccount296.pdf
Here's the granophyre on the summit
And here is Rake Beck which seems to be the boundary of the aureole