Piles of elongate basaltic lava pods, having the general appearance of a stacked accumulation of discrete stone pillows, often many hundreds of metres in thickness. Each ‘pillow’ is surrounded by a chilled, fine-grained, lava skin and sags into the pillows below it. The pillows are rarely more than one metre in diameter and in cross-section each one has a convex upper surface, radial and concentric fractures, and commonly a central cavity or tube which once fed lava to the front of the advancing finger. The morphology indicates that the pillows continued to behave as fluid bodies after the chilled carapace had formed. This provides good evidence of submarine eruption: lava entering water acquires a glassy outer skin as heat is conducted rapidly from the surface. Because water absorbs heat more readily than air, with little increase in its own temperature, the rapid surface cooling allows the molten plastic state of the pillow interior to be maintained longer than it would be in air. Pillows have been observed forming under water from lava entering the sea off Hawaii. See also hydrovolcanic processes.