Pillow Lava

Pillow lava is basalt erupted underwater that solidifies into stacked, pillow-shaped lobes, the diagnostic top layer of oceanic crust formed at a mid-ocean ridge. Each pillow forms when a tongue of about 1,200 degree C lava meets seawater, chills a glassy skin in seconds, then inflates and buds the next lobe; the rapid quench traps a glassy rind and radial cooling cracks. Pillow stacks make up the uppermost extrusive layer above the sheeted dikes in the oceanic crustal section and in ophiolites. They are observed forming today at ridge eruptions and at shallow hot-spot vents off Hawaii.