How plastic turns into rocks and ends up on our beaches

Plastic rocks
Plastic rocks
Image: Daniel Wolfe
We may earn a commission from links on this page.

A detour to avoid the film set for Hobbs & Shaw was the only reason I happened across this special beach.

In December 2018, on vacation on Kauai in Hawaii I was following a tip from a co-worker to see giant tortoises sunbathe along the sandy Mahaulepu beach. The shoot forced us to take a different route, which is where I spotted a solitary gray pebble against the pale beige sand. I was struck by how unusual it felt.

An image of a hand holding a gray piece of what looks like a rock. There's handwriting on top of the photo that says: "I first thought it was granite. It felt soft and light, like a crayon."

Not too far from the same spot, there was an off-white rock that looked more like a fragment from an animal. Relics like that are common on the Pacific coast beaches where I walk my dog, so I leaned in to pick it up.

It's a photo of a white and pot-marked piece of plastic that looks like a bone. There's handwriting over the photo that reads: "It looked like a whale bone to me."
Image: Daniel Wolfe

Again, I was struck by how it defied my expectations. The object certainly looked like a whale bone, but as soon as my fingers touched it, my skin recognized it as plastic. It was potted and porous like animal remains bleached in the sun. In my hand, its sheen and smoothness more closely resembled the top of a bottle cap than a fragment of a vertebrae.

Looking around the beach, the vast number of plastic rocks became apparent. There were many variations. My brain tried to sort the plastic rocks into something that belonged on the beach.

This is the third photo of a hand holding a plastic rock. It's bluish in color. There's handwriting over the photo that reads: "Behold the bluish coral I thought I found. It had rounded edges like polyps in coral. The color added to the illusion."
Image: Daniel Wolfe

Plastic that’s been melted by thermal vents along the sea floor, forest fires, active lava flow, or campfires are called “plastiglomerates by geologists. As the material cools it might bond with other debris or rock. The end result can be beautiful other-worldly creations.

Three illustrations are next to each other representing the ways high temperatures in nature could change the form of plastic. The first is an illustration of thermal vents in the ocean. The second is a drawing of an erupting volcano with lava flow crossing in foreground. The final illustration is of a campfire.
Image: Daniel Wolfe

The misshapen plastic on the beach could have degraded in other ways. According to Megan Lamson Leatherman, program director at the Hawaii Wildlife Fund, “humans are more in the picture than thermal vents and lava flows.” The physical weathering could be the result of UV radiation. Plastic exposed to the sun breaks down and releases greenhouse gases, while creating ever smaller fragments.

This is a drawing of UV radiation impacting plastic waste on beaches. An angry looking sun emits radiation towards a lone plastic cup on a sandy beach.
Image: Daniel Wolfe

Leatherman also pointed to wildlife as a potential culprit for the faux-rocks. Over 15% of plastic collected by the Hawaii Wildlife Fund’s marine debris recovery efforts had bite marks.

This is an illustration of animal bites changing the shape of plastic. Suspended in the ocean is what looks like a thermos while a dolphin inspects it, quizzically.
Image: Daniel Wolfe

Whether gnawed or super-heated, the plastics wind up in nature, part of our geological record, washing up on our shores.

“The Hawaiian archipelago acts like a sieve,” says Leatherman, “the way that the currents and wind patterns work, the stuff tends to accumulate on our shorelines.” Trade winds and warm water currents  deposit tons of plastic waste along once pristine beaches.

This year, volunteers from environmental-protection nonprofit Surfrider have removed 68,308 pounds of plastic waste from Kauai’s beaches.

As curious as these objects may be, scientists already point to the appearance of plastics in this form as a signpost for the beginning of the Anthropocene epoch, an age in which humanity’s imprint on the natural world is indisputable.