Jökull - 01.01.2012, Side 35

Deciphering eruption history and magmatic processes from tephra in Iceland
Figure 7. Three liquid-lines-of-descent showing in-
creasing concentrations of K2O with decreasing MgO
in the 2011 Grímsvötn tephra glass. The slope of the
trends increases at lower MgO due to increased pro-
portions of plagioclase in the fractionating mineral as-
semblage. The principal two fractionation trends ap-
pear linked at their lowest K2O values, possibly indi-
cating magma mixing at depth. Glass with composi-
tions falling on all three trends was produced early on
Sunday May 22 when the eruption plume rose up into
the stratosphere (Petersen et al., 2012). The very pres-
ence of the three trends strongly suggests at least three
magma storage zones beneath the volcano that were ac-
tivated during the first eruption day or earlier. Relative
uncertainties in MgO and K2O concentration analyses
are 2 and 3%, respectively.
– Þrjár þróunarlínur kvikubráða, þar sem styrkur K2O eykst með fallandi styrk MgO, varðveittar í gjóskugleri mynduðu
í Grímsvatnagosinu 2011. Halli línanna eykst með lækkandi MgO vegna aukinna áhrifa af kristöllun á plagíóklas á sam-
setningu afgangsbráðar (glersins). Tveir aðalferlarnir tengjast við lágan K2O styrk sem bendir hugsanlega til þess að
kvikublöndun misþróaðra basalta hafi átt sér stað skömmu fyrir gos. Snemma sunnudagsins 22. maí 2011 þegar gos-
mökkurinn náði upp í heiðhvolfið (Petersen et al., 2012) myndaðist gler sem fellur á alla ferlana þrjá. Tilvist ferlanna
þriggja bendir til þess að undir Grímsvötnum séu a.m.k. þrjú kvikuhólf og að þau hafi öll verið virkjuð á fyrsta gosdegi eða
fyrr. Hlutfallsleg skekkja í mældum styrk MgO er 2% og K2O 3%.
tion depends on the environment where the tephra is
deposited and the weather conditions during and af-
ter tephra deposition. A tephra layer that is deposited
on well-vegetated land has much higher preservation
potential than the same tephra layer deposited on un-
vegetated land where it is exposed to wind erosion and
easily washed away by water. When tephra has been
incorporated by vegetation into the soil it is likely to
be preserved for long periods of time. The Hekla
volcano has produced important tephra marker layers
that are used in tephra correlation and as time markers
all over Iceland and in the north Atlantic region. Its
widespread and easily recognisable tephra layers cre-
ate the foundations for Icelandic tephrochronology.
Constructing eruption history for different vol-
canic systems is time consuming. Different chapters
are accessible in soils and sediments all around the
volcanic sources, some easily accessible but others
not. Missing eruptions must somehow be accounted
for and one possibility is to apply the preservation ra-
tio of the tephra layers. This requires intensive and
thorough fieldwork and careful sample handling not to
mention ample patience. Once a successful eruption
history has been established for a given volcano, in-
formation on volcanic activity recorded in the tephra
becomes accessible. Indeed, tephra grains represent
frozen magma droplets that record its physical state
earlier than other products such as lavas, allowing
unequivocal assessments of the magmatic processes
leading to tephra forming eruptions.
Acknowledgements
We are grateful to our reviewers, Andy Dugmore and
Magnús Tumi Guðmundsson, for their constructive
comments which improved the presentation and the
language of this paper. We also thank the editors Ívar
Örn Benediktsson and Bryndís Brandsdóttir for their
patience. Partial support from the Icelandic Research
Fund (Volcano Anatomy) and the French-Icelandic
collaboration program Jules-Verne is acknowledged.
JÖKULL No. 62, 2012 33