pillow lava

pillow lava (countable and uncountable, plural pillow lavas)

1. (geology, uncountable) The rock type, resembling pillows, typically formed when lava emerges from an underwater volcanic vent or a lava flow enters the ocean.
2. (geology, countable) A rock of that type.
   • 2004, Richard Fortey, The Earth, Folio Society, published 2011, page 45:

     Ancient pillow lavas are easily recognisable. I first saw them underneath a lighthouse on the coast near Fishguard in western Wales […]

• lava