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Pillow lava basalt: island arc volcanism and subduction

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Colour image of brown rock with white spots
Pillow lava basalt: island arc volcanism and subduction. Ordovician. Belledune-Point Verte. Collector: H.E. MacLellan, c. 1980. New Brunswick Museum (NBMR 1278). Image width 15 cm. Basalt forms structures known as pillow lava, when molten magma erupts under water. The rapid cooling of the exterior creates a margin of fine-grained crystals while the interior cools more slowly. Bubbles created by escaping gases streaming to the margin of the pillow are filled with crystals.