DR KEN MACDONALD / SCIENCE PHOTO LIBRARY DR KEN MACDONALD / SCIENCE PHOTO LIBRARY
Ocean floor fissure. This metre-wide fissure is due to volcanic activity at a mid-ocean ridge. A mid-ocean ridge forms when magma rises from inside the Earth due to spreading tectonic plates. The magma erupts as lava and cools to form new oceanic crust. As the plates spread, the crust is stretched, causing fissures to form. Seawater flowing into the fissure will be superheated and expelled at hydrothermal vents or "black smokers". The currents caused by this process have attracted the crabs seen either side of the fissure. Photographed on the East Pacific Rise off Central America, at a depth of approximately 2600 metres.
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