Plastic first became widespread in the mid-20th century. Since then, about 6 billion tonnes have been manufactured. Much of that has ended up as trash, and nobody knows what will become of it. Now, researchers have discovered an unexpected way that some plastic waste is persisting: as a new type of stone.
The substance, called plastiglomerate, is a fusion of natural and manufactured materials. Melted plastic binds together sand, shells, pebbles, basalt, coral and wood, or seeps into the cavities of larger rocks to form a rock-plastic hybrid. The resulting materials, researchers report in the journal GSA Today, will probably be long-lived and could even become permanent markers in the planet’s geologic record.
“Most conventional plastic is relatively thin and fragments quickly,” said Richard Thompson, a marine biologist at Plymouth University in England, who was not involved in the research. “But what’s being described here is something that’s going to be even more resistant to the ageing process.”
Plastiglomerate was discovered in 2006 by Charles Moore, a sea captain and oceanographer at the Algalita Marine Research Institute in Long Beach, California. Moore was surveying plastic washed up on Kamilo Beach, a remote, polluted stretch of sand on Hawaii’s Big Island.
Like other southeastern shorelines in the Hawaiian archipelago, Kamilo Beach accumulates garbage because of how currents circulate. Spotting the odd plastic-covered rock assemblages, Moore took a few photographs and collected some specimens.
The significance of that discovery was not realised until 2012, when Patricia Corcoran, an earth scientist at Western University in Ontario, invited Moore to give a lecture about plastic pollution. He included the plastic conglomerates in one of his slides, although he had no name for them. Intrigued, Corcoran decided to fly to Hawaii to see the strange anthropogenic stones for herself. At Kamilo Beach, she and a colleague sampled 21 sites on a 2,300-foot strip. They collected all plastic-rock specimens with a diameter of about an inch or more.
Most of the melted plastic was hard to identify, but traces of nets, ropes and lids appeared in some stones. They collected 205 pieces, ranging from the size of a peach pit to the diameter of a large pizza.
At first, Moore hypothesised that lava from the nearby Kilauea volcano created the plastiglomerates, but flows have not approached the beach for at least a century. Interviews with local residents revealed a more likely explanation: bonfires. Kamilo Beach’s sand is laced with degraded pollutant particles called ‘plastic confetti’. This makes it virtually impossible to find a bonfire spot free of plastic waste, Corcoran said. She also heard that some locals intentionally burned plastic in an effort to get rid of it.
Kamilo Beach’s plastiglomerates are probably not unique. Plastic is found around the world, as are bonfires. And in some developing countries, people regularly burn garbage to dispose of it. “I’m sure people have seen plastiglomerates in other places and just haven’t reported them or given them a name,” Corcoran said.
Because scientists define rocks as things formed by natural processes, she prefers to label the new materials as stones. This distinction does not affect plastiglomerates’ longevity, however.
Many scientists believe the planet has entered a new geological era, the Anthropocene, in which human activity is leaving a vast and durable imprint on the natural world. Along with building materials, tools and atmospheric signatures, plastiglomerates could be future markers of humanity’s time on earth.
“Plastics and plastiglomerates might well survive as future fossils,” said Jan Zalasiewicz, a geologist at the University of Leicester in England, who was not involved in the discovery. “If they are buried within the strata, I don’t see why they can’t persist in some form for millions of years.”