How are pillow lavas formed? and where could I find some in the \UK?
Dave Hassell
Hi David,
Pillow lavas are essentially the underwater equivalent of pahoehoe.
They form from low effusion-rate eruptions of fluid basalt lava.
They have a rounder form than pahoehoe toes, mainly because of the
ability of water to help buoy them up (gracity doesn't flatten them
out so much). Also in contrast to pahoehoe toes, pillow lavas tend to
have thicker skins of glass (because they are quenched more quickly),
less vesicular skins (because even shallow water pressure is able
to prevent bubbles from expanding very much), and generally radial
fractures (in contrast to the generally concentric flow banding
seen in pahoehoe toes). Nevertheless, it is often difficult to
tell the two types of lava apart in exposures. The only way to
absolutely know that the flows you're looking at are pillow lavas
rather than pahoehoe toes, is to find submarine sediments (such as
hyaloclastite debris formed from the violent reaction of lava ans
water) between the pillows.
I had to ask some British volcanologist friends the second part of
your question: Andy Harris of the Open University said the
following not-quite UK example:
I know there are some excellent Pillow outcrops at the Cyclops
Islands (Aci Trezza, Sicily). These were formed during the earliest
volcanism at Etna (~300 000 BP) by the eruption of tholeiitic lavas
into a shallow bay on the site now occupied by Etna.
Dave Rothery (also of the Open University) came up with some truely
UK examples:
Strumble Head, Pembrokeshire and Anglesey
Hopefully these places are within your reach.
Sincerely,
Scott Rowland
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