Researchers reported in a news article by Sergio Queiroz for Reuters the discovery of plastic-rocks on Trindade, a remote island located 1.140 kilometers (708 miles) off the coast of Brazil.
Trindade Island is one of the world’s most important conservation spots for the endangered green turtle, or Chelonia mydas, with thousands of animals arriving each year to lay their eggs. The only human inhabitants on Trindade are members of the Brazilian navy, which maintains a base on the island and protects the nesting turtles.
The plastic rocks are found along the beach and form as plastic debris washed ashore breaks down and mixes with the island’s volcanic rocks.
“We identified (the plastic debris) mainly comes from fishing nets, which is very common debris on Trinidade Island’s beaches,” Fernanda Avelar Santos, a geologist at the Federal University of Parana, explains. “The (nets) are dragged by the marine currents and accumulate on the beach. When the temperature rises, this plastic melts and becomes embedded with the beach’s natural material.”
The discovery shows how widespread pollution by plastic debris has become and is evidence of humans’ growing influence over the Earth’s geological cycles, so the researchers.
Plastic-rocks or plastiglomerates, a new kind of conglomerate made partially from plastic, were described for the first time in 2014 from a beach on the Big Island of Hawaii. The plastiglomerate there formed from melting plastic in fires lit by humans.
Another human-made and plastic-based rock are pyroplastics. Described in 2019 from the shores of Cornwall in southwest Britain, pyroplastics form from burned plastic waste. In laboratory experiments with white or colored plastic pieces, if burned, the plastic melts and forms a gray or black mass, resembling at first glance a rocky pebble.
A chemical analysis of the pyroplastics showed that the plastic-pebbles are composed of materials like polyethylene and polypropylene.
Polyethylene is the most common plastic. As of 2017, over 100 million tons of polyethylene resins are produced annually, accounting for 34 percent of the total plastics market. Its primary use is in packaging like plastic bags, plastic films, containers including bottles, etc.
Polypropylene is similar to polyethylene, but it is slightly harder and more heat resistant and often used in packaging. In 2013, the global market for polypropylene was about 55 million tons.
Most plastic is disposed of in landfills, a small amount recycled, but about 8 million tonnes of plastic enters the sea every year. Oceanic currents distribute the plastic pollution worldwide and it is likely that plastic-rocks are widespread along the shores of the oceans, yet not always recognized in the field.
Another two papers published in 2019 described plasticrusts from the Portuguese island of Madeira and from the Italian island of Giglio.
Plasticrusts form when saltwater chemically corrodes plastic debris, and the motion of the waves smashes the material into tiny fragments. The weathered fragments stick onto the rocks, forming a thin plastic crust.
The researchers are yet unsure of the environmental impacts of plastic-rocks.
Burned plastic can contain high concentrations of potentially toxic elements, like lead and chromium, derived from the pigments used to dye the plastic material.
Buried in the ground, plastic has the potential to survive millions of years and even enter the geological record.