Jökull - 01.01.2012, Side 34

B. A. Óladóttir et al.
Figure 6. Calculated mixing proportions of the basaltic
end-member in the benmoreite and trachyte tephra from
the Eyjafjallajökull summit caldera. The first tephra
were produced by approximately half-and-half mixture
of incoming evolved basalt composition and residual al-
kaline rhyolite whereas during the second half of April
2010, the mixing proportions of basalts decrease regu-
larily until early May when a deeper and more primitive
basalt was injected into the magma system. After this
renewed activity the basalt proportion diminish again
until the production of trachyte during the last week of
the eruption, when only 10% basaltic end-member is
observed. The eruption ended when these basaltic in-
jections became too small for driving the mixed magma
to the surface (see detail of the calculation scheme and
further discussion in Sigmarsson et al., 2011).
– Þáttur basalts (í þunga %) í kvikublöndun fyrir gos á benmoreít og trakít gjósku úr toppgíg Eyjafjallajökuls.
Fyrsta gjóskan sem myndaðist var helmings blanda af þróaðri basaltkviku og leifum af alkalísku rýólíti frá
fyrri tíma. Blandhlutföllin breyttust reglulega það sem eftir lifði aprílmánaðar en í byrjun maí kom innspýting
af frumstæðu basalti inn i kvikukerfið sem jók virkni gossins. Eftir þessa innspýtingu minnkaði þáttur basalts
í kvikublönduninni reglulega þar til trakít fór að myndast í síðustu viku gossins en þá var basalt þátturinn
kominn niður í u.þ.b. 10%. Gosinu lauk þegar basalt innskotin voru orðin of lítil til að knýja kvikublöndurnar
til yfirborðs (sjá frekari umfjöllun í Sigmarsson o.fl., 2011).
tying of a silicic magma reservoir (Sigmarsson et al.,
2011). Therefore, the next eruption at this volcano
is likely to extrude silicic magma with corresponding
tephra production.
Grímsvötn volcano 2011, early phase grain vari-
ability and magma storage
Detailed analyses of major-element composition in
tephra from the first day of the Grímsvötn 2011
eruption reveal considerable variability (Figure 7).
Three trends of increasing K2O concentration during
magma evolution (indicated by decreasing MgO con-
centrations) are observed with possible magma mix-
ing between two of the trends. The largest variabil-
ity is observed in a single grain with abundant plagio-
clase, clinopyroxene, olivine and iron-titanium oxide
(Figure 1c). Calculated pressure from clinopyroxene-
liquid equilibria (Putirka, 2008) reveals at least two
levels of magma storage at depths of 15 and 6 km and
temperature falling from 1150◦C to approximately
1100◦C. Polybaric magma differentiation thus char-
acterized the basaltic magma produced during the last
Grímsvötn eruption and is likely to have occurred over
considerable depth range beneath the volcano (Sig-
marsson, 2012) shortly before its violent explosion.
SUMMARY AND CONCLUSIONS
Ever since the pioneering days of tephrochronology,
this field of study has become increasingly impor-
tant as a valuable tool in different disciplines of
Earth sciences. Sigurður Thorarinsson set the cor-
nerstones for Icelandic tephrohronology by mapping
the widespread Hekla tephra layers and thereby open-
ing some chapters of the eruption history of Iceland.
His main tools were written documents and distinct
tephra layers in the soil (Thorarinsson, 1967). Reach-
ing further back in time, the only evidence left to the
tephrochronologists are the tephra layers preserved in
different environmental settings.
The preservation potential of tephra increases as
the tephra is more widely distributed. The preserva-
32 JÖKULL No. 62, 2012