You might think that plastic pollution on land and sea is old news by now, but it's taken on a new form we've only just noticed: Researchers have identified a crust of plastic particles building up on shoreline rocks.

This 'plasticrust' isn't just a worrying symbol of the garbage piling up in our oceans. The coating is a potential threat to the organisms that live and feed on the rocks, and could be yet another way that plastic is entering the food chain.

Since 2016, a team from the Marine and Environmental Sciences Centre (MARE) in Portugal has been monitoring the build-up of plastics along the shore of the volcanic island of Madeira, assessing their subsequent impact on the local ecosystem.

"[The crusts] likely originated by the crash of large pieces of plastic against the rocky shore, resulting in plastic crusting the rock in a similar way algae or lichens do," marine ecologist Ignacio Gestoso told Jake Buehler at Earther.

(Gestoso et al., Sci. Total Environ., 2019)

The plasticrust looks as if someone left a chewed up piece of gum or a squeeze of toothpaste on the rocks. The shape does resemble natural organisms that crust rocks in similar ways, making this a sober reminder of how plastic is now literally embedding itself into the environment around us.

It's still early days for plasticrust research – how it forms, the effects it has – but Gestoso and his team have now taken several samples of the stuff to investigate further. Chemical analysis has revealed the crust is indeed made of the widely used polyethylene, the material found in plastic bags and food packaging.

And the polyethylene clinging to the shoreline now covers nearly 10 percent of the rocks' surface, according to the researchers. The team also found evidence that algae-eating winkle sea snails (Littorina littorea) were equally at home on plasticrust as on rock – suggesting they might be sucking up plastic as well as algae.

Further research should tell us just how widespread the problem is and how much of an impact this might have on nearby wildlife, but for now the scientists just want to draw attention to the problem.

If you're not already thinking about ways of reducing your plastic usage, perhaps the sight of beach-side rocks getting covered in a plastic coating will bring home the seriousness of the problem we've got.

Sadly this isn't the first time that human-made plastic has tied itself so closely to natural rock – cast your mind back to 2014 and the discovery of plastiglomerates, rock-like substances made from melted plastic and organic debris.

It's now got to the stage that experts think we're going to leave a noticeable plastic marker in Earth's sedimentary record – something for future generations to look back on and realise just how much we rely on plastics at this point in history.

"As a marine ecologist researcher, I would prefer to be reporting other types of findings, and not a paper describing this sad new way of plastic pollution," Gestoso told Earther.

"Unfortunately, the magnitude of the problem is so huge that few places are free of plastic pollution."

The research has been published in Science of the Total Environment.