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!
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.
Question: In the GigaPan, why are so many of the small porphyritic grains hollow in the middle? What happened here?
Hey Margot!
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.
C
Grrr… Some local redneck with a tractor was out here last week stealing shingle and turned a lot of the big boulders over.