Geologists use the term “pillow basalts” for a specific feature formed in the ocean. Looking at this lovely example of one, I’m hoping you can figure out the origin of the term (sometimes we’re not all that creative).
Pillow basalts form when lava erupts underwater. The water cools the surface of the lava flow rapidly, forming a crust, but the hot lava inside tries to keep flowing forward and downslope. It cracks the crust and surges forward, only to run into more water and cool. Lava flowing under-water thus forms these elongate, somewhat rounded bulbs on the ocean floor that we call pillow basalts.
Pillows have formed probably for more than 4 billion years – basically as long as there has been standing water. They can be recognized in the geologic record by their shape and their texture – they will have fine-grained edges, cooled rapidly by the water, surrounding more coarse-grained interiors that stayed hot slightly longer. Oh, and sometimes recent pillow basalts, like these in the Galapagos, might have the occasional white mussel shell nearby.
Image credit: NOAA