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Mineralogical ClassificationPumiceous conglomerate is correct ?

5th Jul 2019 05:44 UTCAndrea Oppicelli

Hi everyone.

Do you thing the type of rock shown here can be called a pumiceous conglomerate ora is another definition more correct?

The rock comes from a Santorini beach. Some layers of this rock were visible even under water so as to look like a pavement.

5th Jul 2019 11:02 UTCFrank K. Mazdab 🌟 Manager

Volcanoclastic rocks like this can be challenging because they can span a range between being essentially igneous rocks to being essentially sedimentary rocks, and it isn't always obvious, especially without a thin section, which process dominated the formation of the sample.

If the sample is igneous and the matrix is ash, then the rock might be called a vitric lapilli-stone or vitric lapilli tuff. Here vitric refers to the components being dominated by glass (pumice and ash), as opposed to being dominated by rock fragments (then lithic would be the proper prefix) or broken crystals (then crystal would be the proper prefix). Lapilli refers to the dominant grain size, with ash being <2mm in size, bombs being >64 mm in size, and lapilli being in-between. Finally, the root "stone" implies the lapilli make up more than 75% of the rock (with the remainder in this case being ash), whereas if ash were a bit more abundant (between 25% and 75%), then "tuff" is the better root. So this rock might be somewhere on the boundary.

If the sample was completely reworked post-eruptive material, then a sedimentary name might be more appropriate, and as I'm less familiar with sedimentary rock names, I suppose "clast-supported pumice conglomerate" seems like an accurately descriptive name, although there might be a more preferred "official" name, given the specific and unique components that make up the rock.

It's certainly an interesting rock. The pumice in this rock isn't flattened as one usually sees in classic pyroclastic flows, where it flattens and welds a result of the weight of the overlying material and heat-related softening. There also doesn't appear to be any lithics or crystals obvious in the photo. The rock looks a bit more like air fall. But since Santorini is an island and pumice floats, I wonder if after an eruptive event if the sea surface was just covered in a sheet of rounded pumice clasts and finer ash that just washed up like foam on the beach, to be later consolidated, while all the denser materials were lost under the sea. In that scenario, given there would be a "post-eruptive reworking", I suppose a sedimentary name would be more appropriate.

5th Jul 2019 20:00 UTCAndrea Oppicelli

Thank you very much for the careful consideration.

You perfectly hit my curiosity with the due details that led me to think more.

Perhaps over time the pyroclastic event has a new layer of a few centimeters of pumice cemented by the ashes; however I am fascinated by the fact that this pumice may have undergone sedimentation in the intertidal area.

It would be good to look at pumice cementing components.

Do you think that in that beach there are accumulations of magnetic sands ... the dark blue of the magnetite and the beige of the pumice.

5th Jul 2019 20:48 UTCFrank K. Mazdab 🌟 Manager

Hi again Andrea,

Beyond some basic information on nomenclature and perhaps a bit of fanciful speculation, I'm definitely no expert on pyroclastic rocks, especially with respect to the potentially complex interplay between igneous and sedimentary processes that could shape the deposits we see today. But fortunately, there's no shortage of quite detailed references on Santorini geology online, although a quick perusal on my part didn't uncover anything specific on your particular rock.

I have to admit that when I was last on Santorini, indeed actually 20 years ago come August, I largely put on hiatus on my geology interests, and spent most of my time eating, drinking and just awing at the remarkable scenery. Your post reminds me that perhaps it's time for a return visit, and maybe a renewed opportunity to look at a few rocks in between all the food, wine and views... :-)

5th Jul 2019 21:08 UTCAndrea Oppicelli

Certainly Frank.

The island is worth a new visit. This is the third time I've been there, and this time I managed to find wonderful micro-minerals. Moreover the food and the wines as well as the landscape enhance the beauty of these Greek islands that have always exercised an extraordinary fascination on me. I advise you, if you come back, to also take a few days tour of Milos island, also geologically and mineralogically interesting both for the rocks and for the minerals that you can find between a good glass of Assirtiko or Nektari ... and family taverns outside the crowded centers.

When I have classified well the minerals and rocks I found I will insert them.

Thank you again.

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