Global Volcanism Program | Image GVP-03399
Submarine extrusion of lava produces a characteristic "pillow lava" morphology, as seen here on Loihi seamount SE of the island of Hawaii. Thin streams of molten lava are extruded in a form that resembles toothpaste squeezed out of a tube. The outer surface of the lava is quickly chilled by the water and solidifies, while the interior remains molten and continues to flow, lengthening the tubes.
Photo by the Hawaii Undersea Research Laboratory (University of Hawaii).