Formation Mechanisms of Anthropogenic Rock Coasts

Humans are depositing ever-increasing volumes of sediment on the Earth’s surface, much of this in coastal areas. In this contribution we show how some of this artificial sediment is rapidly changing into rock on interaction with coastal and marine processes, and creating new anthropogenic rock coasts. At a case study site in West Cumbria, UK, ferrous slag (a by-product from iron and a steel making) was deposited from ~1860 to 1980 at the coast, forming a bank ~2.5 kilometres long and up to 30 metres high. Over time, wave action from the Irish Sea has eroded the seaward side of this slag bank releasing material onto the foreshore. This material has been reworked by wave and tidal processes, and then deposited on the foreshore, before subsequently undergoing rapid lithification. The present-day foreshore is thus made of a rock platform composed largely of this eroded slag – an anthropogenic rock coast.

The rock platform is a form of conglomerate, with clast analysis showing that the clasts of slag are dominantly sub-rounded to rounded. Scanning Electron Microscopy analysis revealed the slag clasts are cemented together with calcite mineral cements, and occasional other minerals such as goethite. The highly chemically reactive nature and high calcium concentration of the slag resulted in leaching of calcium, making interclast porewaters hyperalkaline, resulting in ingassing of CO₂ and precipitation of the dominant calcite cement. Prior to slag dumping, this coastline was a soft coast. Industrial activity, and human deposition of anthropogenic geomaterials, can be shown to dramatically change the physical and hydrodynamic properties of the coast, resulting in the rapid change from a soft coast to a rock coast at the case study site.