When basalts erupt underwater, they commonly form elongated «pillows». First, a flexible glassy crust forms around the newly extruded lava, forming an expanded pillow. Next, pressure builds until the crust breaks and new basalt extrudes like toothpaste, forming another pillow. When pillow basalts are found in ancient rocks, it can be concluded that the area was once under water.
Contact between deposits of pillow lava (left), and tephra (right, orange), tilted about 60 degrees by tectonic forces; near Aci Castello, Sicily. Note person at lower left for scale (photo M. Fulle). Inset: Diver examines recently extruded lava pillow off the coast of Kilauea volcano, Hawaii (photo R.D. Grigg).
Close-up of Aci Castello pillow lava. Note radial structure caused by stress induced during the cooling process. Pigeon at lower left for scale ;-)