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Another issue for the industry: Plastic garbage turning into rocks

By: Don Loepp

June 10, 2014

You remember the types of rocks from geology class, right? Igneous, sedimentary, metamorphic ... and plastiglomerates.

If you haven't heard of plastiglomerates, you will soon.

This comes up now as an offshoot of a problem that the plastics industry has been trying to deal with for years: marine debris. The new twist comes from research by geologist Patricia Corcoran of the University of Western Ontario and Charles Moore of the Algalita Marine Research Institute.

Their research was published by the Geological Society of America's GSA Today.

They found stones that they called "plastiglomerate" on Kamilo Beach in Hawaii.

Here's an excerpt from the study:

"We use the term plastiglomerate to describe an indurated, multi-composite material made hard by agglutination of rock and molten plastic. This material is subdivided into an in situ type, in which plastic is adhered to rock outcrops, and a clastic type, in which combinations of basalt, coral, shells, and local woody debris are cemented with grains of sand in a plastic matrix. Of the 21 sample locations containing plastiglomerate on Kamilo Beach, in situ plastiglomerate was identified at nine. Partially melted polymers adhered to basalt outcrops included fishing nets, piping, bottle caps, and rubber tires. Locally, molten plastic had infilled vesicles in volcanic rock, thereby forming plastic amygdales. The largest surface exposure of in situ plastiglomerate was 176 × 82 cm, which was evident only following removal of 15 cm of beach sediment. Beach sand and woody debris were locally adhered to the plastic surfaces. Based on its location in more sheltered regions of the beach or within depressions of volcanic rock outcrops, in situ plastiglomerate is interpreted to represent campfire debris."

Corcoran and Moore say plastic rock — they call it "anthropogenically influenced material"  — "has great potential to form a marker horizon of human pollution, signaling the occurrence of the informal Anthropocene epoch."

I have a feeling that future geologists will find a lot of other more durable reminders of human activity.

But this is another problem for plastics, to be sure. I expect to see it mentioned in upcoming debates about banning plastic bags, polystyrene food service products, and anywhere else where litter and/or marine debris are issues.