Greenland and Iceland have been periodically reshaped by megafloods over thousands of years, a new paper in the journal Earth-Science Reviews has revealed.
British research duo Jonathan Carrivick and Fiona Tweed have provided the first evidence of gargantuan Greenlandic floods and extensively reviewed the record of comparable events in Iceland. The researchers set out to better understand what constituted a megaflood and find traces of them recorded in the landscapes of these icy islands.
In media stories and even within the scientific literature the authors found that terms like “catastrophic flood,” “cataclysmic flood,” and “super flood” have been used indiscriminately and interchangeably. There are, however, strict definitions associated with each. A “catastrophic flood,” for instance, occurs when peak discharge exceeds 100,000 cubic meters per second — more than 18 times greater than the flow over Niagara Falls. Multiply that by ten (i.e. 1,000,000 cubic meters per second) and you get a sense of what constitutes a true megaflood.
Despite expressly seeking records of megafloods in the landscape and literature, Carrivick and Tweed found that a more practical approach was to identify events with “megaflood attributes.” Scientists have recorded very few true megafloods since those that cascaded off the Laurentide Ice Sheet, which once mantled much of North America in the aftermath of the Last Glacial Maximum. While there have been few recent floods that exceed one million cubic meters per second, there have been several with comparable erosive power and lasting landscape impacts.
Shaped by water
In Greenland, Carrivick and Tweed found 14 sites where huge floods had rampaged down fjords and across expansive “sandur,” or outwash plains. These have typically been outbursts from ice-dammed lakes, which have periodically unleashed inconceivably vast volumes. The glacial lake Iluliallup Tasersua empties every five to seven years and has a capacity of more than six cubic kilometers of water. At its peak, that flow would drown New York City’s Central Park in a column of water deeper than four Empire States Buildings.
Iceland, too, has experienced its fair share of monstrous floods. Many of them have were triggered by volcanic eruptions. Due to the unique setting of Iceland, where the active fire-breathing mountains of the Mid-Atlantic island are blanketed with ice caps and glaciers, erupting magma invariably explodes into the underside of a quenching ice mass. This interaction, more often than not, results in an outburst flood known locally as a “jökulhlaup,” which produces tremendous amounts of power that is capable of reshaping and inundating the island’s plains.
The region surrounding Öræfajökull, one of the most active volcanoes in Iceland, is infamous for having suffered from devastation wrought by both fire and ice.
“After it erupted in 1362, the whole area was renamed as ‘Öræfi,’ which means ‘The Wasteland,” Tweed told GlacierHub. “They renamed the area because it had been inundated by a grey sludge, hyper-concentrated flow deposits and volcanic ash which had eradicated the farmland and rendered it unusable.”
The eruption was the largest in Europe since Vesuvius immortalised the communities of Pompeii and Herculaneum in AD79. The floodwaters rushed out at over 100,000 cubic meters per second — qualifying as a “catastrophic flood.” The torrent was so powerful that it was able to transport rocks weighing 500 metric tons, each equivalent to four and a half blue whales. Despite not strictly meeting the definition of a megaflood, the event certainly bore many of the hallmarks of one.
But the impacts of such deluges are not limited to their power to remold centuries-old landforms, toss about house-sized chunks of ice, or transport a beach-worth of sediment in a matter of hours.
Outbursts in Greenland can release as much as six billion metric tons of water within a matter of 7-10 days. This rapid draining of a glacier-lake basin radically changes the pressure atop the ice sheets, causing isostatic rebound, which can result in fractured shorelines, as localized sections of coast re-expand.
Water from an outburst flood often passes through a highly pressurized network of conduits within, beneath, and alongside ice. This can trigger a “seismic tremor.” So-called “glacier-derived seismicity” has been measured via seismometers since the early 2000s and experienced by eye-witnesses in the vicinity of Grænalón, one of the most famous jökulhlaup systems in Iceland. The authors note that while these events can be detected and felt, there is negligible impact from them.
Consequences for communities and corporations
Glacier floods also impact the communities living in the shadow of ice. Carrivick and Tweed’s previous work revealed that Iceland has experienced at least 270 glacier outburst floods across 32 sites, killing at least seven people. This makes Iceland among “the most susceptible regions to glacier floods” — and the economic costs that often result.
Icelanders are well acquainted with the natural dangers. Volcanic eruptions, floods, and other geohazards are signature characteristics of their homeland.
Looking to the future, Tweed said: “We can expect to have jökulhlaups for another 200 years, until the ice is gone.”
Such dire flood predictions are unlikely to rattle the stoic Icelanders, who are more liable to fear the prospect of an Iceland bereft of its namesake.
In even less populous Greenland, with people rarely situating themselves in known flood paths, the impacts appear to be less disastrous. That said, Carrivick noted: “When these big outburst floods go into the fjords, and move out of the fjords and up and down the coasts, you get these visible sediment plumes.”
The influx of sediment and freshwater changes the temperature, salinity, and turbidity of the water in a fjord and the nearby ocean, which can drive fish out the region. “It basically shuts down the fishing industry for a couple of days at least,” Carrivick said.
This has potentially massive economic consequences, as 95 percent of Greenland’s exports are fish and fishery products, not to mention that the fishery industry provides employment to approximately 12 percent of the population and puts 87 kilograms of fish on every Greenlander’s table each year.
Yet longstanding industries are not the only ones exposed to the fickleness of Greenland’s glacier outbursts. As the ice sheet melts, a number of resources are being eyed by extractive industries. Carrivick recounted meeting teams of Swiss experts who had been commissioned by Australian mining companies to set up rigs and conduct mineralogical investigations in deglaciating regions.
He also remarked on the prospects of the hydropower industry, which has taken advantage of booms in other nations, like Nepal. “It might be an exaggeration, but I think it’s goldrush time,” he said. Regulators, he added, might struggle to keep up with monitoring and mitigating environmental impacts.
Whatever the future holds for Iceland and Greenland, Carrivick and Tweed’s research advances significantly scientific knowledge of the history of flooding on these two islands and makes a strong case for remaining attentive to the changes occurring on their diminishing ice masses.
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