Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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where vegetation stabilizes the ground surface, combining with layers of
volcanic ash to build Icelandic soils. Based on ages of the different layers of
ash in soil profiles, Icelandic soils accumulated at about half a foot every
thousand years, roughly half an inch per century. The loss of vegetation not
only accelerates erosion, but keeps soil from accumulating once there is
nothing on the surface to trap volcanic ash and windblown silt.
In prehistoric times, relatively loose soil held together by thick native vegetation
slowly built up on top of more cohesive lava and glacial till (an
unstratified mix of clay, sand, and boulders deposited by glaciers). In areas
where the soil sits directly on top of the till, soil accumulated continuously
over ten thousand years. In some areas, exposed layers of soil and ash preserve
evidence for erosion before the Vikings arrived, during periods when
climatic deterioration stressed Iceland’s native vegetation. The combination
of overgrazing and climatic deterioration during the Little Ice Age triggered
the most extensive episode of soil erosion in Iceland’s postglacial history.
During the light-filled Icelandic summer, sheep graze twenty-four hours
a day, roaming over both heath and wetlands. Trampling generates bare
spots up to several feet in diameter. Shorn of a dense root mat, Iceland’s
volcanic soils offer little resistance to wind, rain, or snowmelt. Patches of
bare earth erode rapidly down to hard rock or glacial till, carving little cliffs
ranging in height from one to almost ten feet, depending on the local
depth of the soil. Once started, these miniature escarpments sweep across
the landscape eating away at the remaining pillars of soil and transforming
rich grazing lands into windswept plains of volcanic tephra and rock fragments.
Soil erosion since Norse settlement removed the original soil from
about half the island. Although many factors contribute, overgrazing by
sheep is generally acknowledged as the primary cause. Worms may have
shaped Darwin’s England (once glaciers got through with it), but sheep
shaped Iceland.
Rofabards—the Icelandic name for soil escarpments—erode back half an
inch to a foot and a half per year. On average, rofabard advance amounts
to an annual loss of 0.2 to 0.5 percent of the soil cover from areas across
which rofabards presently occur. At this rate it would take just a few hundred
more years to finish stripping the soil from the whole island. Since
Viking settlement, rofabard erosion has removed the soil from about five
square miles per year. Icelandic scientists fear that many areas of the country
have already passed a threshold that makes further erosion inevitable.
They also know that once stripped of soil the land is pretty much useless.
islands in time