If you wanted to sum up the Anthropocene in a single picture, this might be it: a stony surface marked by a sliver of baby blue crust dubbed a "plasticrust". An image providing indelible proof that our throwaway plastic culture is creating a geological mark on planet Earth.

The photo was captured by Ignacio Gestoso, a marine ecologist at the Marine and Environmental Research Center (MARE) in Madeira, Portugal, and his team. Now, the film-like substance has been analyzed and the results published in the journal Science of the Total Environment.

Plasticrust was first spotted in 2016 and thought to be a one-off. But returning to the island in 2017 and 2019, Gestoso and his team found not only was the substance still there, but it had spread from just one sighting to close to 10 percent of the island's rocky surface within that three-year period. 

Later, chemical analysis of the substance revealed it was an extremely common type of plastic called polyethylene, used in everything from packaging to construction. And while plastic pollution has received a lot of press since that "Blue Planet" episode, plasticrust – the researchers say – is an "entirely new type of plastic pollution". 

"As a marine ecologist researcher, I would prefer to be reporting other types of findings, and not a paper describing this sad new way of plastic pollution," Gestoso told Earther.

"Unfortunately, the magnitude of the problem is so huge that few places are free of plastic pollution." (As studies examining the Mariana Trench, the remote Arctic, and the contents of fish stomachs have shown.)

But how did this particular type of plastic pollution get there? The researchers are unsure but suspect it's to do with the continuous cycle of hydrodynamic-induced crashes of plastic waste against the rocks, i.e. the pattern of the waves and the tide.

There are various possible sources of plastic waste in a maritime community like Madeira, from tourism to the fishing industry. But seeing as most plastic litter is domestic in origin, it is more than likely that the plasticrusts were once single-use packaging materials, like plastic bags.

Referring not just to the newly discovered plasticrusts, but to plastic pollution more generally, the study authors write: "The dimension of the problem is so large that it is possible our current era will generate an anthropogenic marker horizon of plastic in Earth's sedimentary record."

The word Anthropocene literally translates to the geological period (-cene) of the human (anthropo-) and refers to the period in time (geologically speaking) when humans have been the primary driver of climatic and environmental change. It is not just plastic that is to blame, but land use changes and fossil fuel emissions. 

It's a controversial expression, which is in no way universally acknowledged by all experts in the field. There is not even an agreed starting point, with most agreeing it began sometime in the last 100 years but some dating its origins as far back as the birth of agriculture thousands of years ago. 

Still, the study authors say, the "effects of plastic pollution today will likely be traced in sedimentary geological strata", once more highlighting the impact human activity is having on the planet.

The significance of these plasticrusts needs further exploration and Gestoso has said his team is now planning to lead field surveys to find out where the plastics are accumulating. Future studies should look at how plasticrusts affect local wildlife and the food chain, as well as how exactly they form and whether or not they could be a widespread occurrence.