No One Can Agree on Whether a New Kind of Rock Is Actually a Rock at All

Hawaii’s Kamilo Beach is also called “plastic beach” due to its high volume of trash. Patricia Corcoran, a geologist at Western university, went there to study not the sand, not the trash, but something in between: a material that formed when plastic melted in campfires along the beach, mixing with other material, and hardening into a novel sort of mass.

She called the resulting “rocks” plastiglomerates, a name she coined in 2014. “It’s a mudstone-siltstone-sandstone conglomerate, but the matrix holding everything together is plastic. It’s the perfect word,” Corcoran says.

Today, the strange conglomerate “rocks” have been found in more than a dozen locations around the world. But no one really agrees on what to call them. Corcoran’s original plastiglomerates phrase has been used by other geologists, to some extent. They have also been dubbed “plasticrust,” “plastistone,” and “anthropoquinas,” a nod to the fact that the rocks form due to human activity. It’s not just the technical name that’s under dispute. Geologists can’t even agree on whether these “rocks” are rocks at all.

The assumption that rocks have to be old and hard is an “urban legend,” says geologist Jan Zalasiewicz. He is a former head of the Anthropocene Working Group, which has been pushing for the declaration of a new geological age, the Anthropocene, in which humans have become the most powerful force reshaping Earth.

Zalasiewicz points out that some rocks originate as liquid, as when lava cools and solidifies into igneous rocks; these rocks form quickly. Others, like shale, can be peeled apart by hand; these rocks are incredibly frail. No geologist would dispute that shale and igneous rocks are rocks.

Scientists should consider plastic, now a ubiquitous feature of the ocean, a kind of mineral, Zalasiewicz says. He compares plastic to amber, a rock formed from fossilized tree resin, which he calls its close geological equivalent. Though amber is chemically different from most plastics, it is also made of complex long-chain organic molecules that can survive for millions of years when buried in the ground.

Also on team “they’re rocks”: Fernanda Avelar Santos, a Brazilian geologist and marine biologist who coined the term “plastistone” in 2022. Santos, like Zalasiewicz, says plastistones are part of a “paradigm shift” in how we think about geology in a heavily polluted world. The name she picked aligns with other rock types, like sandstone, mudstone, and limestone. A research team at Tsinghua University in China also calls plastistone “an emerging type of sedimentary rock.”

Other geologists are not convinced. Those other stones all notably formed from naturally occurring sediment (which plastic, notoriously, is not). When it comes to “plastistones,” or whatever you wish to call them, “I don’t know if it’s even reasonable to call them rocks,” says Steven Earle, author of the textbook Physical Geology. “Rocks have to be consolidated, and they should stand the test of time.”

A material like amber is formed over tens of thousands of years, when the heat and pressure of overlaying sediment causes a complex chemical reaction resulting in the resin-like stone. Plastic has only been around for about a hundred years and became popular in the mid-20^th century—not long enough for it to become a permanent part of the rock record.

Because we don’t know how plastic will behave thousands of years from now, Earle says, they may be a less permanent part of our geological record than some geologists suggest.

“I refuse to call it a rock and would correct whoever asked me,” says Corcoran. Her reasoning: a rock by definition is made up of naturally occurring minerals, which plastic is not. “Oftentimes, people—especially young scientists—want to call old discoveries by new names to sensationalize them,” she says.

For Zalasiewicz, though, calling a plastic rock a rock is a matter not of sensationalism, but of accepting a new reality for what it is. Plastiglomerates are a signal of the ubiquity of plastic, a sign that we are entering a truly new era of the planet in which the old rules do not always hold. “The surprise for geologists,” Zalasiewicz says, “is how quickly plastic has become part of geology, not just part of the visible world around us.”

Plastics are enmeshed in our world, now. Microplastics have been found in even the most remote corners of the Earth, such as the Mariana Trench, the deepest oceanic trench on the planet. An estimated 170 trillion plastic particles are floating in the ocean and could outweigh fish by 2050.

But whether plastic “rocks” will end up as a layer in the permanent record of the Earth, whether geologists thousands of years from now will be able to dig them up, is an open question. Corcoran hopes to explore that question by placing plastiglomerates under “metamorphic conditions” in a lab—with heat, pressure, and a bath of mineral-rich fluids, all the factors that help true rock become solidified into the rock record.

She does agree that plastic rock hybrids, however they behave in those conditions, and whatever you want to call them, are indeed a sign of the impact that humans and our plastics are having on the world around us. “It’s all just semantics,” she says. “I don’t let it get to me, because the message is what’s important and not really their nomenclature, right?” The fact that the rocks exist is horrifying in and of itself. It’s a little concerning that we have to have this debate at all.