“It’s a sedimentary basalt”

These were the words to pass the lips of an undergraduate in my care on a field trip two weeks ago.  The thing is, just as the geology parts of my brain were about to collide with this idea in some kind of geology-antigeology annihilation, and my concern increased that nothing I’d been explaining to them that day had been listened to, I thought about what they had said…

A Photograph of Brent Tor

Image via Wikipedia

We had spent the day introducing these first years to what an igneous rock looked like.  We observed some granite up close and talked about mineralogy and crystallinity, we looked at the contact between the Dartmoor granite and the surrounding Meadfoot Formation, and we showed them the difference between dykes and sills and the contact margins associated with them.  Finally, we took them to the spectacular Brent Tor.  It was a glorious day, and the Tor (not a true tor by the way) provided stunning views across to Exmoor, Bodmin and of course Dartmoor.  It was at this location we introduced them to some 350 million year old submarine volcanics.

To be fair to them, if you don’t know what you’re looking at and you’ve only being doing geology for a few weeks it’s a pretty tough locality.  The rocks are very weathered, and it’s a designated SSSI so you can’t hammer it to get a fresh surface.  Essentially, what you see are a sequence of dipping sedimentary layers made up of basaltic – often vesicular – clasts.  The majority of the material is 0.1-50 mm in diameter, with a few very large rounded clasts up to about 0.3 m in the long axis (some field photos to follow).

The larger clasts are clearly fragments of pillow basalts, with the finer material comprising hyaloclastic material – fragments of basaltic material which are generated as a hot (~1000 degree C) lava is ejected into cold (~2 degrees C) water.

If it were core seamount material we would expect much more competent and connected pillow basalt sequences, and certainly not the highly broken up fragments of pillows which suggest a significant level of transport.  The steep (~20 degree) dipping sedimentary structure is likely to be largely natural rather than tectonic based on other similar aged structure in the area, and the monogenetic clast material tells us that the source region must be entirely volcanic in nature.

The best interpretation of this structure is as part of a talus slope at the foot of a submarine volcano.    This material accumulates up the side of the seamount as it grows, and results in what may (apparently) by some be called a sedimentary basalt (I feel dirty typing that by the way).

Now of course that’s a misconception that I knocked on the head pretty quickly, but it’s a nice example of the application of terminology by non-experts to generate new terms which can be descriptive, informative and misleading all at the same time.

Anyway, the reason I posted this is I got pointed in the direction of some new footage this afternoon which is being hosted by New Scientist, showing exactly the kind of submarine eruption which would be able to generate the material found in the Brent Tor talus.  It’s an exceptional bit of film recorded in about a kilometer of water depth, just off the shores of Tonga. I recommend clicking through and watching it:


For a Victorian view of the geology of Brent Tor, there’s a really lovely article available here: http://jgslegacy.lyellcollection.org/content/36/1-4/285.full.pdf+html

For an idea of why this kind of geology is relevant – exactly this kind of submarine activity is what’s going on currently at El Hierro in the Canary Islands.




About Pete Rowley

Earth Scientist with a background in volcanology and sedimentology. Enjoys a good rant, beer, and games. Dislikes reality TV, crowds, and unreasonable people.
This entry was posted in Education, Geology, Science, Sedimentology, Volcanism and tagged , , , , , , , , , . Bookmark the permalink.

One Response to “It’s a sedimentary basalt”

  1. Pete says:

    Pete, how about posting the video link on the gl1100 moodle site for the South Devon trip with a brief explanation that they were not. “sedimentary basalts”?

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