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Lava / Pillow Lava 

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Pillow lava USGSWhen basalt erupts under water it extrudes in pillow shaped lobes.As the the basalt emerges an outer skin forms as the hot rock makes contact with the cold sea. This crust breaks as more basalt is pushed out like toothpaste.The skin of the pillow lava is rapidly cooled and has no crystal structure, it is glasslike.If pillow lava s are found in the field then geologists know that the area was once covered by water. On Hawaii most of the submarine part of the island is made of pillow lavas.