Pillow lava

Pillow lava is a variety of rock formed when lava emerges from a volcanic vent that is under water. The liquid lava gains a thick solid crust immediately on contact with the water, which cracks and oozes additional "pillows" as more lava emerges from the vent behind it. Since the majority of Earth's surface is covered by water, and most volcanoes are situated under it, pillow lava is very common.

The other major forms of lava are aa and pahoehoe.