Jökull - 01.01.2012, Page 28

B. A. Óladóttir et al.
Figure 2. Map showing the position of Holocene
volcanic systems in Iceland. Modified from
Jóhnnesson and Sæmundsson (1998). Colour
coded systems have produced important tephra
layers for Icelandic tephrochronology. Hekla (H,
red), Katla (K, violet), Bárdarbunga-Veiðivötn
(B, green), Öræfajökull (Ö, orange), Askja (A,
pink), Snæfellsjökull (S, light blue) and Torfa-
jökull (T). – Eldstöðvakerfi sem hafa verið virk
á nútíma (að mestu eftir korti Hauks Jóhannes-
sonar og Kristjáns Sæmundssonar, 1998). Eld-
stöðvakerfi þar sem gjóska er ríkjandi eða mikil-
vægur hluti gosefna, t.d. vegna leiðarlaga, eru
sýnd í lit. Hekla (H, rauður), Katla (K, fjólublár),
Bárðarbunga-Veiðivötn (B, grænn), Öræfajökull
(Ö, appelsínugulur), Askja (A, bleikur), Snæfells-
jökull (S, ljósblár) og Torfajökull (T).
from the chemical composition alone. Additionally,
the compositional variability within a given volcanic
system may overlap with that of another system, as
in the case of the Grímsvötn and Kverkfjöll volcanic
systems. This emphasises the importance of using
all available characteristics for secure correlation be-
tween tephra layers that also helps to eliminate sec-
ondary tephra layers (e.g. Boygle, 1999; Westgate and
Gordon, 1981). With the aid of stratigraphy, chemical
analyses and tephra correlation, secondary tephra lay-
ers, with or without blown-in grains originating from
different volcanoes, can be deleted from the construc-
tion of eruption history of the volcano being studied
(e.g. Óladóttir et al., 2011a).
Tephra preservation
Tephra preservation largely controls the completeness
to which an eruption history can be constructed. Sev-
eral factors influence where and how tephra is pre-
served. The tephra dispersal is mainly determined by:
(1) the eruption intensity controlling the height of the
eruption plume, (2) the eruption duration, (3) the frag-
mentation level of magma, as stronger fragmentation
creates smaller particles that remain in the atmosphere
for a longer time and are therefore transported farther
from source, and (4) the prevailing wind direction at
the time of eruption (e.g. Pyle, 2000; Francis and Op-
penheimer, 2004).
The environment where the tephra is deposited
controls the preservation potential (e.g. Larsen and
Eiríksson, 2008b; Ayris and Demelle, 2012). Here the
focus is on terrestrial tephra preservation. The high-
est survival probabilities for tephra deposited on land
is in well-vegetated areas where the vegetation cover
provides shelter from eroding winds just after depo-
sition. As wet layers are less susceptible to deflation
the best preservation conditions for the finest tephra
layers are probably in wet vegetated areas.
Unless tephra thickness is excessive the plants
and their root system can grow up through the tephra
(Blong, 1984), stabilizing the deposit. Before such
stabilisation, the tephra layer may have been partly
eroded and/or compacted but its lower part is likely
to maintain primary structures. Rainwater seeping
through the layer may have carried the finest material
downwards and into the soil below (Figure 3). After
that, the tephra will be preserved in the soil for long
periods of time, given that no soil erosion takes place.
Weather conditions at the time of tephra deposi-
tion play a role in tephra preservation. Tephra de-
posited on ice and snow is easily washed away dur-
ing later thawing unless it gets covered by snow, e.g.
in accumulation areas of glaciers, that acts as a shield
for the primary tephra increasing its preservation po-
tential. Wind erodes dry tephra but as soon as rain
has dampened the tephra the wind erosion is at least
temporarily halted.
26 JÖKULL No. 62, 2012