Diliff via Wikimedia Commons // CC BY-SA 3.0
Buddhist nun Pema Chödron may have said it best: “Everything—every tree, every blade of grass, all the animals, insects, human beings, buildings, the animate and the inanimate—is always changing, moment to moment.” Though they often operate on a timescale that can be beyond our human perception, geological features are not exempt from the flow of time. In some places, that change happens fast. Geologists say chalk cliffs on England’s southern shore are eroding 10 times faster than they once did. The researchers published their findings in the Proceedings of the National Academy of Sciences.
The picturesque chalk cliffs known as the Seven Sisters swoop gracefully along the British coast, attracting tourists and photographers. Yet for their serene appearance, the cliffs are not exactly safe.
Chalk is one of the softest minerals and easily broken—especially when it’s being constantly pounded by the sea. The site saw major landslides in 1999 and 2001, and a massive cliff-fall in May 2016 sent tons of rock into the water below. (“While we would encourage people to enjoy the beautiful coastline of East Sussex,” reads the Seven Sisters Country Park website, “we would remind visitors that you do have a duty of care and responsibility for your own safety.”)
Understanding coastal erosion has become a big issue in a world facing rising sea levels. The tricky part is studying something that, by definition, is no longer there. Even tons of fallen rock will break down and be scattered by the sea.
But the ghosts of the old coastline still haunt the rock that remains. To find them, geologists used a technique called cosmogenic nuclide dating, which measures the extent of cosmic radiation in rock to determine its age and how long it’s been exposed. This, in turn, can paint a picture of how that rock has moved or been changed over time.
The white cliffs are studded with pieces of hard, chemically inert flint—a rock that makes a far more reliable historical witness than soft chalk. Working perpendicular to the cliffs themselves, the researchers pulled chunks of flint from exposed rock in a line beginning at the cliff and ending near the water’s edge.
They crushed the flint into microscopic pieces, then put them through a cosmogenic nuclide array to determine their age and history.
Next, the researchers fed that data into a mathematical model of the coastline, which allowed them to estimate the cliffs’ rate of erosion going back thousands of years.
They found that the coast is indeed crumbling fast—but they also learned that this pace is a relatively recent development. For most of the cliffs’ history, the authors write in their paper, the rate of erosion held steady at between 2 and 6 centimeters per year. But that rate has accelerated mightily in the last few centuries, now cruising along at 22 to 32 centimeters a year.
What changed (or changed more)? The authors can’t say for sure. Natural climate change is one possibility; wave action did become more violent during the so-called Little Ice Age, which took place from the 14th to 19th centuries. The cliffs have also become more vulnerable over the last few centuries, as ocean currents and human engineers picked away at the band of sediment protecting the coast from the ocean’s full force.
November 7, 2016 – 3:01pm