Garbage floating in the Indian Ocean gyre is complicating efforts to find Malaysia Airlines flight 370, when searchers mistake rubbish for airplane wreckage. So what are ocean gyres and why do they carry so much garbage?

Any circulating water or air current can be considered a gyre. However, the term usually refers to an oceanic vortex. Five major gyres swirl the planet’s oceans. In addition to the Indian Ocean gyre -- which rotates between Madagascar and western Australia -- the North and South Atlantic and Pacific oceans each have a gyre.

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Ocean gyres form due to winds, atmospheric pressure and the rotation of the Earth. As the Earth spins on its axis, a physical force known as the Coriolis effect causes the gyres to spin in different direction depending on their locations. The northern Atlantic and Pacific gyres spin clockwise, while the Indian Ocean gyre and southern Atlantic and Pacific gyres turn counter-clockwise.

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All five gyres trap tremendous amounts of trash in their circulations because the debris never washes ashore. Tiny bits of plastic floating near the surface make up most of the trash, along with larger items like chunks of Styrofoam and fishing tackle.

The marine debris can prove deadly for animals that swallow the plastic, either accidentally or after mistaking the bits of plastic for food. The trash gyres also harbor bacteria and chemical pollution.

Illustrations: NOAA