pillow lava

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Lava that forms from an underwater eruption and is characterized by pillow-shaped masses.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geology, uncountable The rock type, resembling pillows, typically formed when lava emerges from an underwater volcanic vent or a lava flow enters the ocean.
• noun geology, countable A rock of that type.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun lava that hardened in rounded shapes suggestive of pillows; believed to result from underwater eruptions