Fossil hunting in Toe Head
Geology nugget
Earlier this year, my friend Hasti sent these videos from the flanks of the Fuego volcano in Guatemala. A long way from Bandon in County Cork for sure.
A great secret lie under Bandon. A dormant body with its unearthed remains scattered in streams, fields and forests of the local landscape.
And so, a bit of detective began.
I know, I know. It may not look exciting at first, but these are remains from rocks that were once over 1000°C. The photo was taken a bit north of Bandon in County Cork.
Unlike in counties Kerry and Waterford, there aren’t many signs of volcanic activity in the mapped bedrock of County Cork. And so, the records of an extensive rock unit of gabbro near Bandon are almost anomalous.
When I first looked at a geological map of cork and saw a snake-like feature a kilometre or so north of the Cork town of Bandon, I asked an adult (an expert). Very few had looked at it in recent times and it was almost a bit of a mythological tale.
One thing is looking at a geological outcrop on a map, another thing is finding it in the field. Typically, someone would’ve found actual rocks in the field, then later geophysical measurements would help complete the boundary.
The Bandon “volcanic” rocks aren’t actually of a volcano, but rather an intrusion. An intrusion is what creates a volcano – a rising pipe or neck of molten or partially molten rock that may or may not reach the surface.
Calling the intrusion the “Bandon volcano” is obviously to make it more sexy, even though it’s unlikely that it caused volcanism at the surface when it formed. As such, when a volcano doesn’t reach the surface to become a volcano, we can refer to it as an intrusion.
But exist it does, as we found out on a fieldtrip with Cork Geological Association. One of the other members Marc had done an amazing amount of aforementioned detective job by exploring the area and talking to locals. A few of us began to see him like a Cork-version of Indiana Jones, just with geology instead of coloniser archaeology (also remember, archaeology and geology are two separate fields of study).
In advance of the trip, he had gained permission from the locals for us to roam on their land and see the evidence first hand. It became a memorable trip!
White elongated minerals here are feldspar crystals. At higher temperatures, when the rock was molten, calcium-rich feldspars like plagioclase would’ve formed early. Depending on the chemistry of the magma, these feldspars could become more alkaline as the molten rock cooled.
While hard to differentiate here, the darkest of minerals here are very likely pyroxenes. As the temperature of the once molten rock would’ve lowered, pyroxenes crystallised after olivine minerals had formed.
Dark and green minerals in gabbros are often olivine. This would be the first mineral to crystallise early as the temperature of the magma decreased.
After the magma had cooled and the hard rock had formed, secondary minerals would start to alter the rock. These secondary alterations may come from erosion, exposure to fluids other factors. Red seen here could be the iron in the olivines and pyroxenes oxidising as rust.
Magnified closeup of a gabbro rock from north of Bandon. You can click the bubbles to see what the rock is made up of. The section here is about 5 millimetres across.
In general, associating present-day Ireland with violent events like volcanoes can seem far-fetched, but there’s plenty of evidence all over the island. From the basaltic lakes of Giant’s Causeway in the north, to the county-sized granite bodies of Galway and Wicklow and down south to gigantic eruptions taking place in what is now Waterford.
Yup, Ireland has a long history of volcanism. You and I just happen to exist in a time where Ireland is geologically safe and stable.
Most recently, the very creation of the North Atlantic Ocean about 50 million years ago tore the crust near and under Ireland apart, creating numerous volcanoes across the Atlantic Islands (and forming the Giant’s Causeway).
But the “volcanic” rocks near Bandon have a different origin. While cool and hard today, the once molten magma would’ve surpassed 1000°C in temperature passing up through the crust. But as said earlier, it likely wouldn’t have reached the surface like the Giant’s Causeway and Waterford Copper Coast volcanics did.
Most of Cork’s rocks is composed of Old Red Sandstone. It is thus anomalous to find a rock associated with volcanic activity in the County.
My friend Hasti was in Guatemala and got to see this cool event of magma, gas and solid rock getting thrust upwards from the vent of the Fuego volcano. It isn’t a great allegory for the Bandon “volcano” and would better describe the eruptions along Waterford’s Copper Cast.
I posted a video here anyway since she kindly allowed me to use, in the top of this page. Thanks, Hasti!
I don’t like leaving my homebase too often, so it’s nice to able to live through other people seeing such impressive events. While they go and see active volcanoes, it’s exciting enough for now heading to the fields of Bandon to see an inactive “volcano” that didn’t get an opportunity to become an actual volcano.
References:
Horni, J. Á., Hopper J. R., Blischke A., Geisler W. H., Stewart M., McDermott K., Judge M., Erlendsson Ö., & Árting U. 2017. Regional distribution of volcanism within the North Atlantic Igneous Province. Geological Society, London, Special Publications. Vol. 447, PP. 105 – 125. DOI: 10.1144/SP447.18
Brück, P. M., Beese, A. P., Sweetman, T. M. & Wheildon, J. 1988. Alkali Gabbro Sills in the Lower Carboniferous near Bandon, County Cork. Irish Journal of Earth Sciences, Vol. 9, No. 1, pp. 23-37. Retrieved via: https://www.jstor.org/stable/30002501
https://gsi.geodata.gov.ie/portal/apps/webappviewer/index.html?id=f1b130022e7146a7b76e1010c7efb2a9
A bit about me
Thomas Heising here!
I’m originally from Denmark, but now living in Cork, Ireland and working with visual science communication. You may have found this post because you were curious about this page or wanted to know more about the topic at hand – in any case: welcome.
There are more writings, visuals and videos around here about geology, the weather, our planet, Ireland, Denmark and other things that interest me.
