lava

Underwater eruption produces pillow lava.

An excerpt from the article 14 facts about lava

It cools very quickly and splits into ellipsoidal, usually flattened lobes resembling pillows or loaves.

The individual pillows have a spongy structure in the center and a glassy structure on top, and are characterized by a concentric structure. The average size of a lobe is 30-60 cm, but they can also reach several meters.