These are some notes from an essay I wrote last year so the citations are probably not working. If you need them, though, ask and I'll hunt them down.
Calling these garbage islands is highly misleading, as they're not solid expanses of plastic you can walk on. The plastic collected in the confluence of ocean currents called the North Pacific Central Gyre, for example, is in the form of very, very small pieces, almost invisible to the naked eye. One study of the gyre found 334,271 pieces of plastic per square kilometer of ocean surface, but these pieces weighed only 5,114 grams altogether – that's about 11 pounds. The average piece of plastic they found weighed only about 15 milligrams, or about the weight of a grass seed. The great majority of these particles are less than 2.8 millimeters in diameter, or a bit more than a tenth of an inch.
MYTH: There is a giant island of solid garbage floating in the Pacific.
FACT: There are millions of small and microscopic pieces of plastic, about .4 pieces per cubic meter, floating over a roughly 5000 square km area of the Pacific. This amount has increased significantly over the past 40 years.In reality, Goldstein said, most pieces of garbage in the Pacific are "about the size of your pinkie fingernail." Though she and her team have found some larger pieces of plastic, like buoys and tires, most are microscopic.
The patch is not easily visible, because it consists of very small pieces that are almost invisible to the naked eye.[7] Most of its contents are suspended beneath the surface of the ocean,[8] and the relatively low density of the plastic debris is, according to one scientific study, 5.1 kilograms per square kilometer of ocean area (5.1 mg/m2).[9]
http://oregonstate.edu/ua/ncs/archives/2011/jan/oceanic-“garbage-patch”-not-nearly-big-portrayed-media
http://www.straightdope.com/columns/read/2893/why-dont-we-ever-see-pictures-of-the-floating-island-of-garbage
http://io9.com/5911969/lies-youve-been-told-about-the-pacific-garbage-patch
Check Wikipedia sources before using them:
https://en.wikipedia.org/wiki/Great_Pacific_garbage_patch#cite_note-moore2001-9