21 June 2016
Porphyritic rhyolite dike seen on the beach at Cushendun
Posted by Callan Bentley
At the opposite end of the beach at Cushendun, Northern Ireland, we found some outcrops of schist – I’ll be featuring some of them as Friday folds later this week. But cutting across the schist was a pink porphyry, with big well-formed potassium feldspars. I splashed some water from the Irish Sea onto it to increase the contrast:
Here’s a handheld GigaPan image, so you can explore it for yourself. Find a euhedral feldspar! Find a zoned feldspar! Find a beach fly!
Link GigaPan by Callan Bentley
Here is a shot showing the contact between the rhyolite (bottom) and the schist (top):
You can probably tell that schist has a strong potential for featuring Friday folds. Indeed it does. Stay tuned!
Question: In the GigaPan, why are so many of the small porphyritic grains hollow in the middle? What happened here?
It’s differential weathering – the cores of these presumably zoned feldspars have/had compositions which were/are less stable chemically than their rims. So they weathered out more rapidly.
Grrr… Some local redneck with a tractor was out here last week stealing shingle and turned a lot of the big boulders over.