More than 20 years since they went overboard in a storm, thousands of plastic ducks - part of a Chinese shipment of bath toys destined for the US - are still washing up on the world's shores. Once yellow, now bleached white, the toys have become a boon for oceanographers who have been tracking them to learn more about ocean currents. Thousands are still expected to make it to shore intact; but many will have a much longer legacy.

Plastic is becoming part of the world's geology. Researchers at the University of Western Ontario and the Algalita Marine Research Institute in California say they've discovered a completely new type of rock, formed when discarded plastic softens and combines with volcanic rock, sea shells, sand and corals.

Camp fires on beaches form a particularly dense variety, but any discarded plastic will do: examples found by the team derived from fishing nets, piping, bottle caps and rubber tyres. The plastic becomes incorporated into rock mainly in the form of 'confetti': tiny particles formed as larger items break down. The result is analogous to sedimentary rock such as limestone, says Patricia Corcoran of Western University: "the plastics I see as grains of sediment ... because they move on a beach in the same way, comtrolled by wind and water."

Much of the plastic isn't even visible. "Basically, there are probably more microplastics out there than there are larger particles - we just can't see them," says Corcoran. "So, do plastics break down to the point where they don't exist? No. they've been shown to exist in a form that is a monomer, so they do still go on, so there is a process of organisms ingesting these microplastics."

Some of these particles come from the most surprising places - who'd have thought, for example, that body scrubs could be damaging the planet? But, in the US, there's now a move to ban the type that uses tiny plastic microbeads - already outlawed in Illinois, and with New York considering following suit. Incredibly, a single tube of facial scrub can contain as many as 330,000 beads, which aren't removed by standard sewage treatment systems.

In 1997, it was estimated that a staggering 5.8 million tonnes of waste was reaching the oceans every year; and in 2005, the United Nations Environment Program (UNEP) concluded that there were over 13,000 pieces of plastic litter floating on every square kilometre of ocean. The amount is rising, with Ocean Conservancy predicting that 'peak plastic' won't occur until the next century.

Many people have heard of the Great Pacific Garbage Patch; what's less well known is the fact that this is only one of five rotating ocean currents, known as gyres, all of which are collecting massive quantities of floating plastic - as many as 30,000 pieces per square kilometre. Earlier this month, an expedition by the Ocean Research Project set off from California for Japan, using a high-speed trawl net to gather samples of these ocean plastics in an attempt to quantify the problem.

"The media likes to sensationalise stories, and at some point five or six years ago some media outlet came up with the story of an island of trash, and the concept went viral," says the project's Matt Rutherford. "The truth is there is no island of trash in any ocean. If that was the case the problem would be much easier to solve. If the trash was all in one place we could just go there and clean it up. The reality is much worse than the fairy tale: the ocean is full of plastic trash, microplastics."

Ideas for cleaning up this mess are never short on the ground. The latest, devised by nineteen-year-old Dutch student Boyan Slat, involves a device anchored to the sea bed with a number of V-shaped arms, which take advantage of natural ocean currents to catch pollution at the surface while allowing living organisms to slip under the floating barriers. The idea has been hailed by some as miraculous. But, says Stiv Wilson, policy director of the ocean conservation nonprofit 5Gyres, it's offering false hope - just like all the others that have preceded it.

"I find debating with gyre cleanup advocates akin to trying to reason with someone who will argue with a signpost and take the wrong way home. Gyre cleanup is a false prophet hailing from La-La land that won’t work – and it’s dangerous and counter productive to a movement trying in earnest stop the flow of plastic into the oceans," he writes. "Every time a gyre cleanup proponent has shown me a design for addressing the problem, the first thing I ask is, 'do you have the money to make 20 million of those doo-hickies?' They look at me with a puzzled look, and I just mutter, 'The ocean is really, really, really, big'."

Nor is recycling the answer. In North America, the annual 'consumption' of plastic is over 148kg per head. And the vast majority of this can only be recycled once, before heading for landfill - where, like plastiglomerate, it will remain for thousands of years.

Plastic is by no means the only 'anthropogenic' marker showing man's impact on the planet - others include raised methane concentrations in ice cores and improved fertility in soils. It is, though, one of the most enduring - and may one day be one of the most obviously visible to archaeologists. As Kelly Jazvac, assistant professor of visual arts at Western University says, "People are putting their imprint on the earth in a way that can't be changed - it's irrevocable; it's permanent."