Researchers have discovered what they think is a new form of plastic pollution: thin films of plastic waste chemically bonded to rocks.

The finding adds to scientists’ growing recognition that plastics have become part of Earth’s geology. In 2020, geologists described sedimentary rocks in Brazil that had plastic-lined bottle caps, plastic earrings and other litter embedded in their layers. They dubbed the rocks anthropoquinas1. Other scientists have turned up “plastiglomerates”, formed when melted plastic glues together rocks, sand and other natural and human-made materials2.

“People in the twentieth and twenty-first century are creating new geological records,” says Deyi Hou, a soil and groundwater scientist at Tsinghua University in Beijing. Hou and his colleagues found the plastic-sheet-coated rocks near a creek in Hechi City, China.

Their report, published in Environmental Science and Technology on 3 April3, is the first to uncover chemical bonds between plastic and rocks in the environment, Hou says. The source of the plastic, he adds, is rubbish that accumulated in and around the creek, including polypropylene films — like those used to make plastic bags — and polyethylene films — like those used by farmers to cover crops.

When the researchers peered at the plastic–rock combos with spectroscopic instruments, they saw that carbon atoms at the surface of the polyethylene films were chemically bonded to silicon in the rock with the help of oxygen atoms. Hou says this bonding might have been driven by ultraviolet light from the Sun, or by the metabolic activity of a thriving community of microbes that the researchers found living on the plastic rocks. The polypropylene films that the team found seemed to be attached to the rocks by physical forces rather than chemical bonds.