Did my mapping project in Gairloch while at Aberdeen Uni. Just me and the midges. Clive Rice was my supervisor. Loved mapping the Loch Maree Group. Great vid Rob 👌
Hello Prof. Butler, I was hoping you could answer a question I had while watching your beautiful video: I'm wondering about depth of gneiss formation. Without an orogenic event going on in the region, I believe you could start forming gneiss at around 20km depth (please correct me if I'm wrong). In your experience, how much shallower would this process take place if we have now, in fact, an orogenic event, where pressures would increase considerably, on top of burial depth? Thank you!
This is a tricky question. Gneiss is a texture (essentially a banded metamorphic rock) while I think you're asking about the minerals that make the rock. For metamorphosed mudstones (pelites) or, say igneous rocks like gabbros, I suspect you are talking about so-called amphibolite-facies metamorphic rocks... so ones in which amphiboles are stable. Lets say around 500C plus. So it's about temperature rather rather pressure (depth). In old continental crust, these temperatures are likely to be deeper than the Moho (base of crust)... so no active "gneiss" formation there.... In general during orogeny, geotherms are depressed so even deeper, but the crust wil be thicker too... If crust extends then geotherms can be steeper... Incidentally - you get gneiss fabrics in salt/gypsum bodies that deformed (and recrystallised) at temperatures of around 100C - or much less!
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Excellent video! Really enjoyed it, thank you for sharing.
Good fun. Thanks.
Excellent 👍
Did my mapping project in Gairloch while at Aberdeen Uni. Just me and the midges. Clive Rice was my supervisor. Loved mapping the Loch Maree Group. Great vid Rob 👌
Thanks Chris. Midges can be a pest but joyfully very few when I made the film - in 2019!
Hello Prof. Butler,
I was hoping you could answer a question I had while watching your beautiful video:
I'm wondering about depth of gneiss formation. Without an orogenic event going on in the region, I believe you could start forming gneiss at around 20km depth (please correct me if I'm wrong). In your experience, how much shallower would this process take place if we have now, in fact, an orogenic event, where pressures would increase considerably, on top of burial depth? Thank you!
This is a tricky question. Gneiss is a texture (essentially a banded metamorphic rock) while I think you're asking about the minerals that make the rock. For metamorphosed mudstones (pelites) or, say igneous rocks like gabbros, I suspect you are talking about so-called amphibolite-facies metamorphic rocks... so ones in which amphiboles are stable. Lets say around 500C plus. So it's about temperature rather rather pressure (depth). In old continental crust, these temperatures are likely to be deeper than the Moho (base of crust)... so no active "gneiss" formation there.... In general during orogeny, geotherms are depressed so even deeper, but the crust wil be thicker too... If crust extends then geotherms can be steeper...
Incidentally - you get gneiss fabrics in salt/gypsum bodies that deformed (and recrystallised) at temperatures of around 100C - or much less!