There are a total of 5 ocean garbage patches in both the Pacific and the Atlantic Oceans caused by the circular ocean currents called gyres. The largest of the garbage patches is the Great Pacific Garbage Patch.
The Great Pacific Garbage Patch is a result of the prolific use of non-biodegradable plastic material entering the water supply. Any plastic waste that ends up in our water finds its way to the gyres in the Pacific Ocean and other oceans. Researchers speculate that The Great Pacific Garbage Patch is approximately the size of Africa (from United Nations Environment Programme, www.unep.org)
This is not a new phenomenon. The gyres in the oceans have collected whatever solid trash has entered our waters from around the globe for as long as they have existed. However, since WWII, a great deal of the waste is from plastics. Plastic bags and water bottles are particularly abundant.
Plastics don’t biodegrade, they very slowly photo-degrade. The process is slower when the plastic is wet. This means when plastics do degrade, they maintain their chemical structure and just get extremely small. The microscopic plastic becomes food for microorganisms and infiltrates the food chain in the ocean. Plastics also soak up toxic compounds such as DDT and PCB which also then infiltrate our ocean food supply. We have yet to really understand the impact this will have on our health, but we have seen disturbing images of the impact on ocean wildlife, as seen in the photo below an Albatross chick died after eating plastic waste.
For a very comprehensive understanding see the article “What is the Great Pacific Ocean Garbage Patch? on the Mother Nature Network site.