Strumble Head pillow lavas 4

Another photo of the Ordovician age pillow lavas on Strumble Head, Pembrokeshire. This photo shows the slaggy texture of the pillows. Unfortunately no scale as this particular exposure was not readily accessible to allow the placing of the lens cap or similar. However, the individual pillows are around 20 cm - 40 cm long.

Pillow lavas are formed when molten basalt lava is erupted onto the sea floor. If the water is deep enough, the hydrostatic pressure stops the lava from exploding the sea water into steam. Instead, the liquid rock and liquid water co-exist together in an uneasy truce. The lava is extruded like toothpaste into successive oval globules, resembling pillows or sacks, hence the name. See the video clip here:

www.youtube.com/watch?v=y7CtJNO14YI