Since the mid-1950s, humans have produced more than 6 billion tons of plastic—and at least in Hawaii, geologists are finding that some of it is now fusing to volcanic rocks, seashells, sand, and coral to form an entirely new type of rock. Calling them plastiglomerates, the geologists say they found the stones at all 21 sites they surveyed on the Big Island—the New York Times reports the rocks ranged from as small as a peach pit to as large as a large pizza—and that they've been identified on another Hawaiian island as well. The prevailing theory is that the plastiglomerates form when people burn plastic in beach bonfires or campfires; the heat helps fuse the organic and inorganic materials, according to Science. Some of the plastic is still close enough to its original shape to resemble the toothbrushes, forks, and ropes they once were.
Some of the rocks are likely making their way down to the bottoms of sea beds, where they are more likely to be preserved in the geological record. As one marine biologist tells the Times, "Most conventional plastic is relatively thin and fragments quickly, but what’s being described here is something that’s going to be even more resistant to the aging process." That has one geophysicist not involved in the study suggesting there's a sort of silver lining to this: The rocks "might be a nice marker around the world of when humans came to dominate the globe and leave behind their refuse in mass quantities," he says. As such, one paleontologist argues the plastiglomerates could "be one of the key markers by which people could recognize the beginning of the Anthropocene"—a new geologic era on Earth marked by human's heavy impact on the planet. (On the other end of the temperature scale, plastics have even found their way into Arctic ice.)