Not natural, not quite unnatural, the strange new rocks of the Anthropocene stretch the boundaries of geology

by John MacDonald  BIO

Workington in Cumbria, England. Photo provided by the author

is senior lecturer in anthropogenic geomaterials, in the School of Geographical and Earth Sciences at the University of Glasgow in Scotland, UK. His research focuses on how solid waste materials from human activities enter the environment and interact with environmental and geological processes.

Edited byRichard Fisher

I’m standing on a beach at Workington, on the western edge of the Lake District in England. Here I find myself contemplating a very unnatural object, while pondering a pretty fundamental question: what, exactly, is a rock? For a geologist like me, this should be easy to answer, but what I’m looking at has made me think otherwise.

At Workington, all seems natural – the sounds of the waves lapping the shore, the call of seabirds, the smell of the ocean, the sight of the stony beach and high cliffs. At first glance, the beach is made largely of a rock platform, which is not a particularly unusual phenomenon – many coastal areas are ‘rock coasts’ made of sandstone, basalt or granite. These rocks are ancient in human years – often millions or even billions of years old – and have been sculpted into their current cliff or platform shapes over hundreds to thousands of years.

Yet among the waves is an object that shouldn’t be there: a wheel and tyre, embedded in the rock that makes up the shore. It’s not stuck in a crevice – the rock has actually formed around it. How can this have happened? The wheel and tyre are of a mid-20th century style, but rocks are ancient, often millions of years old. Aren’t they?

Closer inspection of this hard rock platform shows it is what geologists call conglomerate: a sedimentary rock made of rounded pebbles and cobbles deposited on the Earth surface. Over thousands to millions of years, this material is buried and heated causing minerals to form and fill in the gaps between the pebbles and cobbles, fusing them together into a hard rock mass. At Workington though, this can’t have happened: as well as the tyre, my colleagues and I found several other human-made objects, under 100 years old. The pebbles and cobbles in the conglomerate aren’t natural either: they are all made of slag, a solid by-product of the iron- and steel-making process.

As a geologist, I have studied various types of natural rocks, but recently I have become interested in ‘anthropogenic geomaterials’ – things like industrial slag – and how they become entwined in geological and environmental processes. I came to Workington originally to look at the slag, because I was interested in its potential to scrub-capture carbon dioxide out of the atmosphere. However, when encountering the rock platform with the wheel in it, I was drawn by its incongruity. After studying the geomaterials of Workington more closely with my colleagues Amanda Owen and David Brown, we believe that this little-known section of the English coastline represents a tangible and potentially long-lasting signature of the impact humans are having on the planet.

Unlike many industrial landscapes, nature here has mostly returned, so it would be easy to miss that the beach is composed of human materials. Here a process that normally takes millennia or aeons has happened in a matter of decades. And it’s not the only example: new forms of anthropogenic geology are emerging around the world. These new materials are blurring the borderline between the natural and unnatural. They are also raising a rather fundamental question for geology: what actually is a rock?

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From an early age, we become familiar with the concept of a rock as something hard – maybe due to a grazed knee from a fall as a child. Rock is then one of those things that the vast majority of adults don’t really think about. It’s just there, making up the Earth – as cliffs, pebbles on the beach, maybe carved into the stone that makes up your house. The dictionary definitions of the word rock reflect this: ‘the dry solid part of the Earth’s surface, or any large piece of this that sticks up out of the ground or the sea’ (Cambridge) or ‘rock is the hard substance which the Earth is made of’ (Collins).

The Oxford English Dictionary (OED) offers a slightly more detailed summary – ‘a large rugged mass of hard mineral material or stone forming a cliff, crag, or other natural feature on land or in the sea’. This brings in the fact that rocks are made of minerals: ie, solid substances with well-defined chemical compositions. Minerals can form through purely geophysical processes – such as crystallisation in cooling magma or hydrothermal water – but they can........

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