While hiking in Santorini I stumbled upon an interesting rock. What first striked me was the size. It is slightly more than one meter in diameter and weighs several tons. This area is covered with pyroclastic rocks but most of it is ash mixed with fist-sized or smaller clasts of pumice and scoria.
This apparently andesitic block doesn’t seem to be aerodynamically shaped, so it probably isn’t a volcanic bomb. If it was ejected from the crater at all, it was likely already solid or mostly solid. What also puzzles me is the lack of deformation in the tephra beds below the rock. I try to imagine such a giant landing in high speed on a soft bed of ash and pumice and it doesn’t make any sense to me.
Is it possible that the current location of the rock is not where it originally landed? Maybe it rolled downhill to the current location? Or maybe signs of original deformation are somehow lost? I’d like to hear your interpretation.
Update
Gareth Fabbro responded quickly to my question. You can see his answer in the comments thread. So this rock is neither volcanic bomb nor is it a block thrown out of a volcano. It is most likely fragment of an old lava flow that is removed from its original location by a volcanic eruption and associated pyroclastic flows 3600 years ago during the Minoan eruption which most likely also destroyed the mythical Atlantis.
I know exactly where that rock is, I’ve seen it myself… There are loads of others just like it all over the island (like this one http://www.flickr.com/photos/fitzgabbro/5283467057/ ). They are fragments (if you can call something that size a fragment) of pre-existing andesitic and dacitic lava flows that were destroyed by the Minoan eruption ~3600 years ago. It is quite possibly a block of one of ‘my’ lava flows, one of the Therasia lavas I’m working on.
The lack of deformation does suggest it wasn’t thrown there. It is part of an ignimbrite, a deposit from a pyroclastic flow, so I assume it was this flow that transported it and placed it there. Every time I came across one of those bocks I had to stop and stare, it really brings home how powerful the eruption must have been.
Thanks, Gareth. Good to know that help is so near. I just need to post something to get answers.