Here’s another image of the deep Pacific floor, taken, like image 1, from one of the small portholes of the research submersible Alvin. The rounded forms you see are referred to as pillows, for obvious reasons. If basalt lava erupts slowly on the seafloor, it cools and becomes very viscous. Its surface freezes quickly into solid rock, but that creates an insulating barrier against the cold seawater, and the molten lava within the pillows continues to flow, slowly, like thick molasses, creating a series of interconnected pillows. If you look carefully at this picture (again, you can click on the image for a larger version), you can see the connections and visualize the process.