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Andrew Dugmore and Orri Vésteinsson
3.2. Tephra fall from the 2010 eruption of Eyjafjallajökull covering vegetation at about
600 m above sea level on the northwest flank of the volcano. When this picture
was taken in early August 2010, rainfall was redistributing the tephra layer; the
undisturbed parts of the fallout were having a selective impact on the low mountain
vegetation, smothering some of it but not all. Photo by Andrew Dugmore.
eruption of Hekla, although in AD 1104 the affected area coincided with the
upland margins of settlement, and change may have been driven by unrelated
factors (Thórarinsson 1967, 1979). Centimeter-scale tephra layers that allow
vegetation (particularly grasses) to grow rapidly may have a positive mediumterm
impact. Mosses can be smothered, and the decay of vegetation buried by
the ash can generate a nutrient pulse for the surviving vegetation. Furthermore,
if the tephra is basic (not silicic) in composition, weathering of the ash itself
can generate another fertilizing effect. In addition, dark tephras may warm the
surface because of their lower albedo. This is good for vegetation because cold
is often the key constraint on plant growth in Iceland.
Running counter to these trends of impact duration and type proportional
to depth, a fine dusting of ash associated with adsorbed toxins and dry
weather can result in the deaths of many grazing animals, most notably as a
result of fluorosis poisoning: pollution can persist at high levels when there is
no rain to flush it away. Hekla has erupted twenty-three times since AD 1104
(Thordarson and Larsen 2007), and on almost all occasions for which there are
accurate records livestock has been poisoned by toxic fallout (Gudmundsson
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