Plastic rock: the new anthropogenic marker in the geologic record

How can we measure the influence of human civilization on the story of planet Earth? There are the relics of culture, tools and keepsakes buried in the ground beneath the ruins of thousand-year-old societies, as well as the history of language and art preserved in libraries around the world. But there are also more dubious impacts, such as alterations to the planet's chemical composition that can be tracked for centuries - an atmosphere bloated with carbon dioxide from our puffing power plants - or megafarms that have narrowed the genetic diversity of plants in the pursuit of increasing food production.

These latter stressors to the environment are becoming more numerous as our civilization becomes more crowded and serve as long-term signatures of our planetary dominance. Now, new research¹ in The Geological Society of America has revealed an entirely different marker of our existence: plastic rock. This new form of sediment, borne from the collision between our consumer culture and the natural world, could last for millennia in the geologic record until uncovered by some future society combing the ground for hints about its own history.

Searching 21 different locations along Kamilo Beach on the largest island of Hawaii (see map), Corcoran et al have identified this new type of rock formed from the unlikely union of heated plastic debris and surrounding sediment or volcanic rock. Many of these plastic-sediment hybrids, or 'plastiglomerates', were found in sheltered locations, clinging to rocky outcrops or filling the pores of volcanic rock. This suggests that they were formed from plastic debris heated to high temperatures in campfires and later discarded, where they combined with other sediment while still hot. In other cases, bits of basalt, coral, shells, or wood cemented into the plastic matrix of fragments of plastic containers, fishing nets, or other debris scattered along the coast.

The shores of Kamilo Beach are a perfect location to find such an unusual fusion of the anthropogenic and geologic. Trade winds blow strongly into the coast, continually upturning sand that buries plastics with other sediment to give them time to meld together. Since the beach is 12 kilometers from the nearest paved road, human visitors are rare, and those that do visit usually camp and build fires that produce more heated plastics to create the plastiglomerates. Such remote access also makes clean-up efforts fairly infrequent.

Samples of plastiglomerates look a little like science experiments gone terribly wrong. Plastic elements can stick to the outer surface of volcanic rock, such as the yellow rope or green netting in (A), but netting can also connect two rocky fragments as in (B). Plastic fragments from broken containers, plastic bags, and tubes also cluster together with wood debris and sand (C-D). None of them looks quite natural, with shocks of color and jutting edges that mark the anthropogenic influences.

Beyond their unique look, plastiglomerates may become a part of the permanent geologic record. Plastics normally degrade over hundreds to thousands of years, but this timescale significantly extends if they are buried beneath sediments and protected from ultraviolet light that normally breaks down the organic materials. Many of the samples in this study were found buried beneath vegetation or sand, likely due to the strong trade winds, leaving them nicely protected and in suitable conditions for such preservation.

These unique results of our environmental influence provide an informal marker of the Anthropocene era, when people began burning plastics and altered the geologic history of the planet. Indeed, other anthropogenic markers already exist in the soil record - scientists have observed increases in methane around 5000 BP when they expected a natural decline, likely due to new agricultural practices.² Increased lead concentrations in soil in Norway also occurred due to the mining industry and combustion of leaded gasoline.³ So this is not the first but simply another way that our industrialization will leave its mark on the history of the planet. Whether to be proud of or discouraged by this feat is another matter, but knowledge of these novel geologic creations should at least make us stop and think about where we stand.

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