Scientists report plastic-rock formation from India’s western coast

In May 2024, a research group set out along the shores of Diveagar beach in Maharashtra’s Raigad district, in search of unusual rock formations. As they walked, they collected samples by carefully chiselling out pieces of rocky outcrops where plastic waste had been trapped, later transporting these to the lab for analysis. One of these samples had a green plastic net embedded within the rock, which  the team recently reported as the first evidence of ‘plastistone’ — a plastic-rock formation — from India’s western coast.

Plastic-rock structures have been documented globally, including from the shores of Andaman and Nicobar islands and Tamil Nadu. Earlier formations found in India were largely plastiglomerates and pyroplastics that are formed by human-induced burning of plastic. In contrast, researchers suggest that plastistone has formed through natural processes, where the bonding of plastic debris with sediments and biological components was brought about by UV exposure, high temperatures and waves.

The study, published in Discover Oceans, also found that the plastic filament protruding from the rock was polyamide in nature, suggesting that it originated from a discarded nylon fishing net.

Plastistone is likely a “novel form of sedimentary rock”, said Punyasloke Bhadury, professor at the Indian Institute of Science Education and Research (IISER), Kolkata and the study’s corresponding author, who added that that this is the first plastistone formation to be reported from the Indian subcontinent.

A 2024 study found that India emits an estimated 9.3 millions tonnes of plastic pollution each year, equivalent to nearly one-fifth of global plastic emissions. The widespread generation of plastic is driving the formation of such plastic-rock complexes, which will persist in ecosystems for thousands of years, Bhadury explained. “We are witnessing an increasing record of human signatures on geological structures,” he told Mongabay-India.

When plastic fuses with rock

Bhadury was part of the team that found plastiglomerates in the Andaman archipelago in 2023. The team later turned their attention to Maharashtra — a state known to have a high coastal pollution load due to rapid urbanisation, tourism and fishing activities. A study from 2020 found higher accumulation of micro and macroplastics on Maharashtra’s beaches, compared to Karnataka and Goa.

The research team that reported plastistone collected samples along a 10-kilometre stretch of Diveagar beach. These samples were screened and analysed using microscopic and spectroscopic methods. By magnifying the sample, researchers confirmed that the plastic filament had integrated with the rock. Then, infrared radiation was passed through the plastic to determine its chemical signatures, explained Nirupama Saini, a PhD student at IISER Kolkata and the study’s lead author.

This chemical analysis revealed the plastistone was made of polyethylene, polypropylene, polyethylene terephthalate (PET) and polyamide polymers. “All of these, somewhere or the other, have (health) effects on the human body. So, there’s a biomagnification angle that needs to be understood,” shared Bhadury.

Further, the sample showed no signs of burning and was found in a low-tide zone where there is little chance of human-induced fires, suggesting a natural formation process.

During this process, biogenic material (substances originating from organisms) — in this case, mollusc shells, barnacles and single-celled organisms called foraminifera — is deposited on plastic waste that has washed ashore, slowly integrating with the rock. This three-way biological and chemical interaction among the plastic, biogenic material and the rock is what makes plastistone different, explained Bhadury. “Because there’s a direct biological interaction here, the magnification in terms of the additional damage would be far more,” he added.

Upon further observation, a section showed plastic filaments bound with sediments and biological components, likely reflecting the early stages of lithification — the process by which sediments turn into rock. “[This] suggests plastistone formation is an ongoing and progressive process, making this study highly relevant for understanding evolving coastal sedimentary systems,” said Anish Kumar Warrier, professor of geology at Manipal Institute of Technology. Warrier, who was not associated with the study. He added that plastistones may act as indicators of environmental degradation.

Warrier also noted  that this formation “highlights a critical transition where plastics are no longer merely pollutants, but are actively participating in sedimentary processes.”

The ghost gear problem

The study also spotlights the alarming levels of marine plastic pollution from abandoned, lost or discarded fishing gear (ALDFG), earlier estimated to make up around 10% of global marine plastic waste, though some researchers suggest this figure is higher. Often known as ghost gear, it comprises nearly half of the Great Pacific Garbage Patch. “The findings in this (plastistone) paper strongly align with what we observe in the field: ghost gear acting not just as debris, but as a structural and geochemical agent within coastal ecosystems,” said N. Pravin Kumar, a senior programme coordinator at WWF-India, who was not involved with the study. These field surveys have repeatedly documented ghost gear accumulated along rocky ocean bottoms and trapped in reef crevices.

“What is particularly concerning is that these accumulations are not transient,” said Kumar. Modern fishing gear is made of high-durability polymers that are designed to be resistant to degradation. Only a part of this material breaks down into microplastics and enters the food web. The rest remains intact and continues to interact with the environment over long timescales, he explained.

Plastistones may act as reservoirs of microplastics and toxins which slowly leach into their surroundings, Kumar added. According to Saini, a rock’s chemical composition and heat retention capacity change when plastic is incorporated into it. Plastic tends to trap more heat, which may lead to a rise in the rock’s temperature, potentially impacting the breeding behaviour of marine organisms like crabs and turtles. Research also points to plastistone altering microbial communities in its surroundings.

While the damage these formations may cause is only speculative at present, it will not remain localised, warned Bhadury, emphasising the need to investigate its ecological impact.

The need for more awareness

There is a need for managing plastic waste and robust coastal health monitoring,  Bhadury said, adding that sensitising and involving citizens remains key to the process. “These learnings about plastic-rock complexes are very critical to have effective implementation and policies in place. These problems also help us to find new innovative ways of technologies to address them.”

The researchers have also been charting ways to communicate with the state government on this matter. They have started reaching out to local communities in Raigad and NGOs, civil society and other organisations working along the coast, across India to inform them of this phenomenon and  encourage them to report similar findings.

Bhadury’s team is also planning to expand their research along the coastline as well as inland. “My hunch is that we will encounter these a lot more and they would not be restricted to the marine environment. I’d not be surprised if we end up getting it in freshwater ecosystems,” he said.

Banner image: A plastiglomerate, the first plastic-rock hybrid structure found in 2014 from Kamilo beach, Hawai’i, on display at Museon in The Hague, Netherlands. Image by Aaikevanoord via Wikimedia Commons (CC BY-SA 4.0).

Read more: Plastic-rock hybrids found on the Andaman Islands