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How does trash in the ocean disappear?
Category: Earth Science
Published: August 20, 2019
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Much of the trash in the ocean does not disappear, and this is becoming a big problem. Biodegradable materials such as paper, cardboard, and food quickly disintegrate, degrade, and are eaten by microorganisms in the ocean. However, materials such as plastic, glass, and metal do not degrade as well. Glass and metal tend to be heavy enough that they sink to the bottom of the ocean, out of sight. On the other hand, most plastic objects float on the surface of the ocean. Over time, ocean currents either wash this floating trash ashore or sweep it into big patches of garbage in the middle of the oceans.
There are many oceanic garbage patches across the globe. The biggest one lies in the northern Pacific Ocean and is called the "Great Pacific Garbage Patch." Oceanic garbage patches are formed by the tendency of circulating ocean currents to gather floating debris into the center of the circulating pattern. The Great Pacific Garbage Patch stretches from Japan to California and is densest at two central regions: at a point between Japan and Hawaii and at a point between Hawaii and California. Most of the floating, non-biodegradable trash that makes it to the Great Pacific Garbage Patch stays there. For this reason, the garbage patch is continually growing.
With that said, it's not like there are mountains of recognizable objects sitting in the ocean. The churning motion of the ocean, temperature fluctuations, and the high-energy components of sunlight all tend to steadily break down non-biodegradable trash into small bits. These bits typically range in size from microscopic specks to marbles. They mostly disperse across the surface of the water, forming a thin layer. Some of the plastic bits are also pushed down below the surface by the churning of the water. In addition to the large number of plastic bits spread out across the ocean's surface, there are occasionally larger objects such as sneakers, toothbrushes, and bags that have not yet broken down to smaller bits.
What collected all this trash into a patch? The ultimate culprits are global ocean currents that are forced to flow in a circulating pattern by earth's rotation. The rotation of the earth creates a force called the Coriolis force. This force acts on everything, but it is usually too weak to notice. Since the Coriolis force is so weak, its effects only become significant when applied to a very large fluid system such as the atmosphere or an ocean. The Coriolis force causes the ocean currents to travel in giant circles; clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. A giant region of circulating ocean currents is called a "gyre" and is somewhat like a giant, slowly-moving whirlpool. The continents tend to get in the way, deforming and weakening these gyres. Being the largest span of uninterrupted ocean, the northern Pacific Ocean has one of the largest and strongest patterns of circulating ocean water. The pressure differentials that naturally arise in the circulating water currents of a gyre tend to push floating bits of trash toward the center of the gyre, where the trash accumulates. This trash is harmful to marine life and can even reach humans through the food chain.