Today’s Stone of the Day from Gemstone Beach contains veins, going in different directions, that speak of stages of displacement as various stresses have cracked it and pushed it around.
I found this stone on Gemstone Beach during my second-last Southern Sojourn fossick there, on Friday 28 April. It features as the third find in this Post – and it does not appear to have been changed much at all by tumbling.
Stone of the Day #15 is probably a form of rhyolite, an igneous rock. It has some similarities to ignimbrite stones found along this southern coast. It is the presence of tiny gaseous pockets (which look to have been infilled by some mineral) that tells us this is an igneous rock. Ignimbrite originates as the deposit of a pyroclastic flow, which is a hot suspension of particles and gases flowing explosively and rapidly from a volcano. New Zealand geologist Patrick Marshall originally came up with the term in the 1930s meaning “rain of fiery rock dust”. Some ignimbrite can be very loosely deposited, with lots of pumice in it. If it is buried at depth, it becomes compacted, gases and liquids are squeezed out, and it becomes solid and fine grained and glassy, like much of the material in Stone #15.
See this Post for more information on this kind of stone.
Go to Stone of the Day #16.
Go to the beginning of this Stone of the Day Series. Go to the Series Index.
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