Rock pebbles are common along the shores of the sea and rivers. The perpetual motion of the waves and water currents erode and smooth even the hardest rocks. But along the ocean shores of Cornwall in southwest Britain, a team of British scientists noticed some strange pebbles, which were apparently made not from rock, but some lightweight material.
Chemical analysis showed that those supposed pebbles are composed of plastic 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.
In a paper published in the journal Science of the Total Environment, the research team was able to create "pyroplastics", as they named this new kind of human-made rock, by burning plastic waste. In laboratory experiments the white or colored plastic, if burned, becomes grey or black. The smooth surface of the fake pebbles is like the real ones the result of erosion by waves and marine currents. Chemical analysis also showed that pyroplastic pebbles contain a high concentration of potentially toxic elements, like lead and chromium. The elements derive from the pigments used to dye the plastic material. The British research team also fears that pyroplastics are quite more common and widespread as their survey shows, but not yet recognized in the field as they resemble rocky pebbles.
Such kind of plastic pollution is not new. In 2014 sediments formed by melted plastic and rock fragments were described on a beach on the Big Island of Hawaii. These “plastiglomerates,” most likely formed from melting plastic in fires lit by humans who were camping or fishing. These plastiglomerates and pyroplastics may survive millions of years in the geological record.
