Underwater Lava
French geologist Denis Westercamp* observed “pillow-lavas” on the coastline of Pointe Arago.
Pillow-lavas or cushion-rocks are what hot volcanic lava can turn into after slowly oozing into the ocean.
More specifically, pillow-lavas are bulbous, round, or tubular lobes of lava. They form during volcanic eruptions with a slow extrusion which gives enough time for a thick crust to form on all sides of a pillow lobe, preventing individual pillows from combining into a sheet.
Pillows are not usually hollow and tend to solidify all the way through. They often have marks on them, lines or scrapes, that form during extrusion.
*Denis Westercamp (1946-1989); French geological engineer at the Bureau of Geological and Mining Research (in 1985).