My last new rock? The peperite.

When you begin to learn geology you are asked to become familiar with an enormous new vocabulary. Rock types, time periods, structural features, and locations, from anticline to zeolite (or apatite to zircon if you decide to be a thermochronologist). I started reading scientific papers in a class I took as a sophomore, and my memories of those early papers involve a lot of circling words and looking up definitions. Depending on the paper you are reading just becoming familiar with the rock names being used can be a chore, it seems like there was a period in geology where any slight variation in plagioclase anorthite content required an entirely new rock name. This was part challenge but also part awesome, the more words I became familiar with (poikilitic) the more I felt like I was a member of the club of professional geologists. But of course at some point you start to become saturated, and new words appear less and less frequently. The general decline in newness is of course punctuated by new experiences, starting graduate school, writing a proposal, going to your first hydrogeophysics department seminar, etc, but in general, the new terms you meet are from outside of your field (e.g. hyporheic zone, PETM). Therefore, by the time you’ve been a “hard-rock” geologist for 20 or so years (roughly me), it is rare to run into a new rock name. Up until 2010, the last new rock name I’d learned was haplogranite, which is kind of lame because it is most often used to refer to a synthetic mixture used in laboratory experiments.

Then, in the fall of 2010, I went on the pre-meeting field trip just before the 2010 International Conference on Thermochronology in Glasgow. The field trip was a fantastic week-long excursion on and around the Isle of Mull with 25 or so other thermochronologists and a Scotland Geology expert.

It was on this trip that I met the peperite.

Carraig Mhór Peperite on the south coast of the Isle of Mull

Carraig Mhór Peperite on the south coast of the Isle of Mull

The Carraig Mhór Peperite to be exact, on the southern coast of the Isle of Mull. Peperites aren’t necessarily rare, but they aren’t rocks that have ever come up in areas I’ve worked in. Previous bloggers have written on them (see Highly Allocthonous for example), but for me this was a new one.

Peperites are formed when basaltic (low-viscosity) magma mingles with wet sediments. The hot magma is basically disintegrated due mainly to the steam generated upon contact, and what you are often left with is a net mixture of baked sediments and hydrothermally altered basalt. Lava flowing into a swamp or stream valley, for example, could create a peperite. – oddly enough, if you google the term peperite you also find a variety of creationist/young-earth nonsense websites that claim peperites are proof of the biblical flood, that somehow they require biblical catastrophism, which makes no sense, but come to think of it I don’t know why I am surprised by that.

Boulder of Carraig Mhór peperite on the Isle of Mull with greenish altered basalt and dark baked mudstone.

Boulder of Carraig Mhór peperite on the Isle of Mull with greenish altered basalt and dark baked mudstone.

Deskcrop of Carraig Mhór Peperite with many geolgist hand for scale. Note the pale greenish altered basalt, dark baked mudstone, and 60's era surplus property wooden desk.

Deskcrop of Carraig Mhór Peperite with manly geologist hand for scale. Note the pale greenish altered basalt, dark baked mudstone, and 50’s era surplus-property wooden desk.

I doubt that peperite will be my last new rock, I am sure there are bajillions left for me to learn. I just hope that each time I learn a new one it is on another wonderful field trip.

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