Sedimentary rocks comprise around 75% of the stones on the Earth's surface. They are also the type of rock which interacts most with surface processes such as erosion and water movement. It turns out sedimentary rocks are also greatly affected by pollution, as shown by the emergence of plastic stones, which come in various shapes and forms.
(Photo: Wikimedia Commons/ Aaikevanoord)
An Emerging Type of Sedimentary Rock
Scientists have encountered a strange new type of rock mainly formed from plastic created and discarded by humans. This illustrates the extent of plastic pollution worldwide, posing an impending danger to human health and ocean sustainability.
So far, plastic rocks have been discovered both on the coast and inland in 11 countries across 5 inhabitable continents. These include Portugal, Peru, Japan, Italy, India, Hawaii, China, Brazil, the Spanish Canary Islands, and the UK.
The scientific community disagrees on what to call these deposits. Various terms have been proposed, some indicating plastic rocks formed in specific ways. Some terms include plastiglomerate, plastitar, plastisandstone, plasticrust, plastistone, and anthropoquinas.
Geologist Patricia Corcoran discovered the first example of a plastic rock in 2013 on Kamilo Beach, Hawaii, and termed it a "plastiglomerate." Researcher Deyi Hou reported that molten plastic, clasts from rock, wood debris, sand grains, corals, and shells were held together after a campfire burned on the beach. Since then, they have been frequently found on beaches on mainland coastlines and islands.
In 2019, scientists made a disturbing discovery when they found plastic rocks in Brazil's volcanic Trindade Island. They were located by geologist Fernanda Avelar Santos while working near a protected nature reserve called Turtle Beach.
This discovery is alarming because this area has an untouched landscape. The only evidence of human presence on this island is a small Brazilian military base and a scientific research center. Santos suspects that ocean currents have swept an abundance of household waste and other plastic trash from around the globe to the island.
READ ALSO: Microplastics Discovered in Abundance in Long-Closed Missouri Cave System: How Did It Get There?
How are Plastic Rocks Formed?
Plastic rock combines rock and plastic polymers from human waste that have been compressed together. In a recent paper, Hou and colleagues explained that plastic rocks may form through various methods.
The commonly proposed mechanism is burning, a process in which plastic debris melts during a campfire or waste burning and is then held within the mineral matrix upon cooling down. As waves crash against intertidal rocks, plastic debris from maritime sources can physically adhere to the rock's surface.
Another method is through oil leaks, which contain significant plastic and reach a beach. According to Hou, this material can firmly attach to the host rock matrix and undergo partial evaporation and solidification.
Finally, the study also proposed that chemical binding can contribute to the generation of plastistone in an inland region. It is assumed that sunlight irradiation leads to plastic oxidation, which in turn leads to chemical binding.
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