Strumble Head pillow lavas 3

Another photo of the Ordovician age pillow lavas on Strumble Head, Pembrokeshire; a close-up view of one of the pillows. Radial cracking/jointing is visible, as is a hint of a finer-grained chilled margin on the edge of the pillow.

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