Plastiglomerates, a new kind of rocks made partially from burned plastic, were described for the first time in 2014 from a beach on the Big Island of Hawaii. Most likely, the plastiglomerate there formed from melting plastic in fires lit by humans who were camping or fishing on the beach. The burned and partially melted plastic incorporates pebbles of lava rock from the beach, forming a human-made conglomerate.

Example of a newly named rock, called plastiglomerate, made of lava rock, green and red netting, yellow rope and melted plastic. PATRICIA L. CORCORAN

Another human-made and plastic-based rock are pyroclastics. Described in August 2019 from the shores of Cornwall in southwest Britain, pyroclastics 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.

Pyroplastics, human-made pebbles collected on the coasts of Cornwall.ROB ARNOLD

Chemical analysis of the pyroclastics collected in Cornwall 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% 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 plastic pollution worldwide and it is likely that plastic-rocks are widespread along the shores, yet not recognized in the field. New research, describing the third kind of plastic-rocks, may support this prediction.

A paper published in October 2019, describes plastic rusts from the Portuguese island of Madeira and a recently published online report records plastic rusts from the island of Giglio, an Italian island situated in the Tyrrhenian Sea, off the coast of Tuscany. Plasticrusts form when saltwater chemically corrodes plastic debris, and the motion of the waves smashed the plastic into tiny fragments. The weathered fragments stick onto the rocks, forming a thin crust of polyethylene on the cliffs along the shores.

Plasticrust observed on a rocky cliff of the island of Madeira.IGNACIO GESTOSO

The researchers are unsure of the impact of plastic-derived rocks on coastal habitats and animals feeding on rocks in the tidal zone. Burned plastic can contain high concentrations of potentially toxic elements, like lead and chromium, derived from the pigments used to dye the plastic material.

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