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What Are All The Ways That Land Can Disappear Beneath Your Feet?

From sinkholes to liquefaction, we look at how solid earth can shrink and elude our grasp

Coastal regions and islands are vanishing due to a lethal combination of erosion, sea rise and subsidence, or the slow sinking of land over time. The network of 1200 coral islands and atolls that makes up the Maldives in the Indian Ocean is ground zero. (imageBROKER / Alamy )
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Once, the tiny island of Tangier, Virginia in the Chesapeake Bay was best known for its long crabbing tradition and the unique, old-English-meets-southern-twang dialect spoken by its 500 residents. Today, Tangier is better known for the fact that it’s disappearing—rapidly.

Records show that the island—which clocks in today at just 1.3 square miles—has lost two-thirds of its land to the ocean since the 1850s. It’s currently losing another 15 feet of coastline every year, according to the Army Corps of Engineers. If nothing is done, scientists predict that Tangiers will be completely swallowed up in as few as 25 years, forcing all of its long-time residents to flee to the mainland.

If this story sounds familiar, it should. Everywhere you look, land is losing out to the rising sea. In the past decade we’ve seen the coral islands and atolls that make up the Maldives start to sink into the Indian Ocean, and several reefs of the Solomon Islands vanish into the South Pacific. In the next 50 years, researchers project that this trend will only accelerate as the effects of human-influenced climate change take their toll.

In light of these disappearing acts, we talked to three earth science experts to explain some of the most common—and most dramatic—ways that the ground we walk on can dematerialize.

Let’s start with Tangier. As with most islands, the main problems here are coastal erosion and sea level rise, two forces that work together to slowly wear away the edges of land masses. “That’s a double whammy,” says Simon Engelhart, a geoscientist at the University of Rhode Island whose research focuses on how sea rise and erosion affects coastlines. “All of those add up in the worst possible direction.”

Neither of these is going away anytime soon. In the Chesapeake, sea level is going up 3 to 4 millimeters per year; worldwide, the National Oceanic and Atmospheric Administration predicts that global sea level rise could be as high as 6.6 feet by the year 2100. As a result, some estimates predict over 13 million climate refugees fleeing the coasts for higher ground before the next century. 

And with every inch of sea, water encroaches further and further inland and enhances erosion. “You don’t have to change the strength of storms or the size of the waves they’re bringing in to allow them to eat further and further into the shoreline,” says Engelhart

But land loss can also take place in far more momentous ways. “You can have large holes that open underneath your feet—they’re very drastic,” says Lindsay Ivey Burden, a geotechnical engineer and professor of civil and environmental engineering at the University of Virginia. Ivey Burden is describing an extreme case of land disappearing: sinkholes.

She describes the recipe for this phenomenon: First, the soil must be enriched with carbonate minerals like limestone or sandstone. Then, add in enough flowing groundwater to the mix, and those minerals start to dissolve. Dissolve enough and they collapse, swallowing whatever was above them into the earth. 

Thankfully, she says, occurrences of sudden, gaping chasms are relatively uncommon, as sinkholes are more likely to form gradually. (Unless you’re in Florida, where most of the peninsula is perched atop porous, carbonate rock that has notorious habit of falling out from under you.) Luckily, in these sinkhole-prone areas, efforts to monitor soil chemistry and use satellites to scope out at-risk zones by Ivey Burden and other engineers can help predict future sinkage.

Then there’s liquefaction, a phenomenon that’s every bit as terrifying as it sounds. Here’s how it works: When wet, sandy soil is rattled suddenly by a strong earthquake, water pressure increases and individual grains lose contact with one another. “The soil basically liquefies,” Ivey Burden says. “It becomes like water, and things sink into it.” As soon as the shaking stops, the soil solidifies again, trapping whatever sank—you, your dog, your car.

Worst of all, because earthquakes are hard to predict, liquefaction is almost impossible to prepare for. But fortunately, because liquefaction requires such specific conditions, it's rare and tends to occur only after quakes in tremblor-prone places like California and New Zealand.  

Of course, most of the land loss we see today is by more ubiquitous, less flashy mechanisms. One of the most subtle—but also most substantial over long stretches of time and space—is called subsidence, the slow and steady sinking of land over thousands of years.

To explain why, we have to first transport ourselves 20,000 years ago, to a time when the Laurentide Ice Sheet extended from the North Pole, covering Canada and much what would become present day New England. This ice sheet was, unsurprisingly, quite heavy. It compressed the land it covered, causing the viscous crust to squish slowly out the other end and elevate the land along the coastal Mid-Atlantic in an attempt to reach equilibrium. After the Laurentide melted, that once-elevated land began slowly sinking again, like a seesaw over geological timescales. Today, that land is still sinking by about a millimeter per year.

This process of subsidence is why part of the Mid-Atlantic is being lost to the ocean. Of course, on coasts, subsidence is exacerbated by other factors like erosion and sea level rise. This effect is best illustrated by the hundreds of islands along the Mid-Atlantic coast of the United States. These islands are just like Tangier—except they’re already underwater. The brackish waters of Chesapeake Bay hide an Atlantis of early American settlements that have slowly succumbed to the sea since early Americans colonized them in the mid-1600s.

Humans have also thrown a wrench in the natural cycles that protect coastal land in ways that have little to do with climate change. For thousands of years, Engelhart says, the U.S. coasts were protected from washing away by biological barriers made by salt marshes and mangrove trees. Sand and mud from upstream would be swept for miles through rivers, and then deposited onto these coasts. These roots created a physical barrier that trapped sediment and created a natural seawall that kept up with sea level rise and erosion.

Yet agriculture runoff, development and loss of sediments from dams have shrunk these habitats over time. Unfortunately, dams—while integral to protecting low-lying towns from floods and generating electricity—also halt the transfer of these key sediments. “Dams effectively cut off all the sediment that’s moving through the watersheds,” says Patrick Barnard, a coastal geologist with the U.S. Geological Survey. “You’ve got a severe shortage of sediment that would normally supply these estuarine shorelines,” he says.

Concrete seawalls, like those used in places like Florida, Japan and the Netherlands, can provide an artificial barrier to the forces of the sea. But they’re a double edged sword, because they also hasten the demise of natural ecosystem barriers. “They protect infrastructure, but they’re very detrimental,” Barnard says. Engelhart agrees. “The minute you put hard engineering in, you just lose all those natural protective environments,” he says. “It quickly becomes just a solid, concrete wall protecting you.”

Other human engineered efforts to rebuild lost land have also gone poorly. Just ask the tenants in San Francisco’s Millennium TowerConstructed in 2005, this gleaming, 58-story luxury tower is anchored into a landfill and sits on what’s referred to as "reclaimed land," which is just what it sounds like: sediment that’s been dredged in from offshore and built upon, but often lacks structural integrity. By 2010, the tower had begun to sink and tilt enough to raise alarm; today it leans precipitously

With the highest population densities, coastal communities remain some of the most desirable places to live. But glamorous waterfront homes are also most likely to disappear, get thrashed by a hurricane or swept away in a tsunami. “These are ground zero areas for climate change impacts,” Barnard says. While some may disagree about what processes are behind these disappearing acts, for island residents like those who live on Tangier, land loss is a daily reality that can't be ignored.

“Even if the ocean isn’t rising, if you’re sinking, then it’s all the same to you,” says Barnard. “We can’t wish it away.”

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About Kyle Frischkorn

Kyle Frischkorn is a graduate student in oceanography at Columbia University, and a 2017 AAAS Mass Media fellow at Smithsonian Magazine. In between being in the lab, on a boat, or in a lab on a boat, he’s written about science for GQ, Lucky Peach, Eater, Scientific American and Atlas Obscura.

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