Sue Heap of Dunedin asks :-

I understand there are two large masses of rubbish vortexed in the Pacific and Atlantic ocean. Could each rubbish accummulation be compacted together to form an island or into blocks to use as a protective reef for an island endangered by tsunami? What glue could hold them together and what process would make them heavy enough to sink?

Ross Vennell, a physical oceanographer at the University of Otago's Department of Marine Science, responded.

A really interesting question! Floating material accumulates in the middle of large oceanic circulations known as "gyres" created by the wind. These gyres occur in all the major oceans of the world. The strong northward flowing currents of the Gulf Stream form the western side of the North Atlantic Gyre, with weak southward flowing Canary Current forming the eastern side. There is a similar North Pacific Gyre bounded by the Kuroshio and California Currents.

Within the strong currents of the gyres the force of the wind combined with the Earth's rotation, i.e. the "Coriolis effect", creates weak ocean currents towards the middle of these large gyres in the top 200 meters of the ocean. The weak currents cause any floating material on the edges of the ocean to move in a large spiral motion over many years towards the center of the gyre into areas known as "convergence zones". A bit like bath toys spiraling inwards in the vortex above the plug hole. Floating plastic being washed, blown or dumped into the ocean accumulates in the middle of the ocean gyres, but takes many decades to breakdown.

It would be great to make use of this accumulated trash. However the accumulation of floating plastic is very patchy and spread out over 1000's of square kilometers, so may be too expensive to collect, given the cost of ships and fuel. So the best solution is to collect it before it gets into the ocean! Making floating plastic heavy enough to use as a protective reef would require lots of dense material like rocks, possibly with concrete to bind or glue it all together.