Abstract
Gradual inflation of magma chambers often precedes eruptions at highly
active volcanoes. During such eruptions, rapid deflation occurs as
magma flows out and pressure is reduced. Less is known about the
deformation style at moderately active volcanoes, such as Eyjafjallajökull,
Iceland, where an explosive summit eruption of trachyandesite beginning
on 14 April 2010 caused exceptional disruption to air traffic, closing
airspace over much of Europe for days. This eruption was preceded
by an effusive flank eruption of basalt from 20 March to 12 April
2010. The 2010 eruptions are the culmination of 18 years of intermittent
volcanic unrest. Here we show that deformation associated with the
eruptions was unusual because it did not relate to pressure changes
within a single magma chamber. Deformation was rapid before the first
eruption (>5 mm per day after 4 March), but negligible during it.
Lack of distinct co-eruptive deflation indicates that the net volume
of magma drained from shallow depth during this eruption was small;
rather, magma flowed from considerable depth. Before the eruption,
a \~0.05 km3 magmatic intrusion grew over a period of three months,
in a temporally and spatially complex manner, as revealed by GPS
(Global Positioning System) geodetic measurements and interferometric
analysis of satellite radar images. The second eruption occurred
within the ice-capped caldera of the volcano, with explosivity amplified
by magma-ice interaction. Gradual contraction of a source, distinct
from the pre-eruptive inflation sources, is evident from geodetic
data. Eyjafjallajökull's behaviour can be attributed to its off-rift
setting with a 'cold' subsurface structure and limited magma at shallow
depth, as may be typical for moderately active volcanoes. Clear signs
of volcanic unrest signals over years to weeks may indicate reawakening
of such volcanoes, whereas immediate short-term eruption precursors
may be subtle and difficult to detect.
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