Plastistone: An emerging type of sedimentary rock

Sedimentary rocks are the dominant rock type found on the Earth's surface, and they are highly susceptible to influence by human activities. In recent years, a number of plastic-rock complexes have been reported across the globe, such as plastiglomerate, plastitar, plasticrust, and anthropoquinas. Despite these findings, a holistic comprehension regarding the diagenesis and ultimate fate of these innovative plastic forms within the framework of sedimentary geology is lacking. Here we contend that these novel plastic forms meet the criteria of a sedimentary rock. Consequently, they merit recognition as a distinctive type of sedimentary rock. In this context, we propose the adoption of an existing term “plastistone” with a revised definition to collectively describe these novel plastic forms. This term aligns with the nomenclature convention for other sedimentary rocks, such as limestone, dolostone, sandstone, and mudstone. Plastistone is formed when plastic and clast from pre-existing rock are lithified together. Plastistones have been found on a global scale, both in coastal and inland regions. The polymer types most frequently found in plastic debris trapped in plastistones is polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP), and their origins are mainly from domestic waste, such as packaging and containers, or as a result of maritime activities. Plastistones can form through a variety of means, including campfire or plastic waste burning, wave action, evaporation, or chemical bonding. Plastistones have been shown to alter the microbial communities of the surrounding environment and can generate significant amounts of microplastics and nanoplastics. This new type of sedimentary rock provides compelling evidence of how human activities can act as a powerful exogenic geological process that reshapes the geological record of our planet.