Saturday, December 25, 2021

"How cities will fossilise"

From the BBC, May 5, 2021:

The grand metropolises of 21st Century civilisation will leave a geological legacy that will last for millennia, but as the author David Farrier writes, some things will endure far longer than others.
It seemed as though the whole world had been buried in concrete. Perhaps it was jetlag, but the view was dizzying: the city, pouring away to the horizon in every direction. Shanghai is one of the world’s largest metropolises. From the viewing platform of the Shanghai Tower, the second tallest building in the world, it looked endless – a wave of skyscrapers rippling outwards, fading to a blue blur of residential blocks in the far distance.

The modern cityscape is as much geological as it is urban. If Shanghai is a concrete desert, New York is the original canyon city, its skyscraper-lined streets forming deep valleys of the kind that, in the past, only great rivers could create over thousands of years. In a late essay, Virginia Woolf pictured herself swooping like a bird over the Hudson estuary, past Staten Island and the Statue of Liberty to the concrete chasms of Manhattan. "The City of New York, over which I am hovering," she wrote in 1938, "looks as if it had been scraped and scrubbed only the night before. It has no houses. It is made of immensely high towers, each pierced by a million holes."

The first cities replicated the environments that once-nomadic people depended on, concentrating shelter and sustenance in one place. The metropolis of the present offers its inhabitants the whole planet in microcosm. As the author Gaia Vince writes, the buildings and infrastructure of the urban landscape simulate "the high view of mountains, the protection of dry caves, the fresh water of lakes and rivers".

If cities have a geological character, it begs the question of what they will leave behind in the stratigraphy of the 21st Century. Fossils are a kind of planetary memory of the shapes the world once wore. Just as the landscapes of the deep past are not forgotten, how will the rock record of the deep future remember Shanghai, New York and other great cities?

You might assume that cities are too ephemeral to leave behind a fossil. "Most buildings are designed to last for 60 years," says Roma Agrawal, structural engineer for the Shard skyscraper in London. "And I always thought, that feels really short, because that’s my lifetime." If you wanted to build something that would stand in tens of thousands of years, "then the forces that you need to contend with become huge", she explains. Most engineers don't look that far ahead.

But while a building might not be designed to stand tall for millennia, that does not mean it will lack a geological legacy. According to Jan Zalasiewicz, emeritus professor of palaeobiology at the University of Leicester, it is "a quite reasonable, even prosaic, geological prediction" that a megacity will leave a fossil. I asked him how he could be so certain. "As a geologist, you’d almost put the question the other way around," he replied. "How can you prevent this?"

It’s a matter, he says, of durability, abundance, and location. The main components of a modern city have their origins in geology and are therefore, in their different ways, highly durable. The majority of the world’s iron ore formed nearly two billion years ago. The sand, gravel, and quartz in concrete are among the most resilient substances on Earth. These hard-wearing materials once existed in natural deposits. But where before it was only water, gravity, or tectonic activity that moved them, now it’s a combination of human initiative and hydrocarbon fuels.

We live in the greatest age of city-building the world has ever seen. Three hundred years ago, there was only one city with a population of one million (Edo, modern-day Tokyo). Today there are more than 500, all of them dwarfed by megacities like Mexico City (population: 21 million), Shanghai (24 million), and Tokyo (now 37 million). As I discovered while researching my book Footprints: In Search of Future Fossils, the quantities of materials involved are staggering. Every 100 years, the mining and construction industries move enough rock around the planet to create a new mountain range, 40km wide, 100km long, and 4km high (25 x 62 x 2.5 miles). Enough concrete has been cast since World War Two to cover the entire planet, land and sea. According to a recent study in the journal Nature, there is now a greater mass of buildings and infrastructure on the planet (1,100 Gigatonnes) than trees and shrubs (900 Gigatonnes).

Location matters in determining the kind of fossil a city will leave. In geological terms, land is never static – either climbing or sinking on a "tectonic elevator". A city like Manchester in the UK, which is situated on ground still rising after the last ice age, will erode entirely over time, washing a trail of brick, concrete, and plastic particles out into the Irish Sea. "But many of the world’s largest cities are deeply anchored in the mouths of deltas and coastal plains," says Zalasiewicz. "And they’re subsiding. Deltas sink, that’s what deltas do." In many cases, human activity is massively hastening this process. Since 1900, Shanghai has sunk by 2.5m (8ft) due to groundwater extraction and the weight of its buildings pressing into marshy ground. Added to which is sea level rise, which could exceed a metre by 2100. "But even without sea level rise," says Zalasiewicz, "it would be inexorable, because the subsidence is steady."

What about a particular structure? Shanghai Tower weighs 850,000 tonnes: a 632m-tall (2,073ft) steel framework with more than 20,000 panes of glass, and 60,000 cubic metres (2.1 million cubic feet) of concrete. How will it fossilise?....