Jökull - 01.11.1998, Blaðsíða 9

Table 2. Groundmass glass composition' of several marker tephra layers in the study area. Sample locations are shown on Figure 4.
Name Sample no. Si02 Ti02 A1202 FeO MnO MgO CaO Na20 k2o P2Os Sum N
1845 AD Hekla tephra 28-19 60,93 1.13 15,27 8,97 0,24 1,54 5,06 3,98 1,64 0,43 99,21 24
std. 0,71 0,10 0,18 0,54 0,03 0,18 0,29 0,20 0,12 0,05 0,47
1755 AD Katla tephra 28-13 47,01 4,63 12,53 15,28 0,24 4,94 9,68 3,01 0,77 0,64 98,81 35
std. 0,36 0,16 0,12 0,30 0,02 0,17 0,24 0,09 0,06 0,09 0,65
1625 AD Katla tephra 28-14 47,10 4,66 12,47 15,51 0,23 4,84 9,56 3,04 0,78 0,76 99,02 39
std. 0,25 0,13 0,16 0,35 0,03 0,14 0,17 0,08 0,04 0,05 0,36
-1480 AD Veiðivötn tephra 28-15 49,56 2,13 12,90 14,12 0,24 6,09 11,26 2,48 0,26 0,19 99,27 24
std. 0,30 0,10 0,40 0,45 0,03 0,18 0,22 0,07 0,03 0,02 0,21
1104 AD Hekla tephra 28-16 71,98 0,20 13,93 2,89 0,09 0,08 1,91 4,21 2,74 0,02 98,05 26
std. 1,05 0,04 0,52 0,64 0,03 0,03 0,24 0,45 0,23 0,01 1,19
-935 AD Eldgjá tephrá 28-8 to 11 47,36 4,57 12,85 15,30 0,23 5,08 10,07 2,94 0,73 0,73 99,85 36
std. 0,41 0,18 0,19 0,31 0,04 0,31 0,35 0,13 0,08 0,40 0,35
-870 AD Vatnaöldur tephra 28-12 49,64 1,92 13,46 13,43 0,22 6,42 11,34 2,40 0,24 0,17 99,30 48
basalt component std. 0,22 0,09 0,27 0,30 0,03 0,20 0,14 0,09 0,02 0,02 0,36
-870 AD Vatnaöldur tephra 28-12 72,33 0,36 12,47 3,90 0,12 0,07 1,05 4,43 3,95 0,04 98,73 4
rhyolite component std. 0,49 0,12 0,55 0,48 0,04 0,04 0,41 0,50 0,91 0,03 0,87
N, number of analyses; std., standard deviation (2-sigma).
Glass compositions were determined by a Cameca SX50 electron microprobe at Texas A&M University and at University of Hawaii, USA.
Average composition of four Eldgjá fall units at location 2.
tephra is a thin lamina of gray fine ash, which is either
from the 1357 AD eruption at Katla or the 1332 AD
eruption at Grímsvötn. Judging from the stratigraphic
position and color of this tephra, the latter volcano is
considered a more likely source. The following tephra
layer is 2-3.5 cm thick, black, medium ash, containing
lapilli-sized scoria clasts. This layer was previously
thought to be from the year 1311 AD (Larsen, 1978),
but recent studies show that it is the product of the
1262 AD eruption at Katla (Larsen, 1993). The "olive
gray layer" consists of fine grained ash, and is a
widespread time marker in southem Iceland. New ev-
idence indicate that it is from an eruption that oc-
curred within the Hekla-Torfajökull area in the early
13 Century (G. Larsen, unpublished data, 1996). The
thin (~0.2 cm) black ash layer found in several of the
profiles around Leiðólfsfell, about 2 cm below the
olive gray layer, is probably from the Katla eruption
1179 AD. The next tephra layer consists of white 1-2
mm-size pumice clasts dispersed within a 0.5-cm-
thick light colored soil and is a distinct marker layer
m the Síða highlands and the lowlands to the south.
The pumice chemistry indicates that the source is
Hekla (Table 2) and most likely the product of its first
historic eruption in 1104 AD (Larsen, 1979). Well
below the Hekla 1104 tephra layer is a -25 cm thick
black tephra layer consisting of up to 5 identifiable fall
units (Fig. 4). Four of the units are strombolian scoria
lapilli and the lowest unit consist of phreatomagmatic
ash (Fig. 6), all of which have FeTi-basalt composition
identical to the products of the -935 AD Eldgjá erup-
tion (Table 2). The strombolian fall units thicken to-
wards the Central fissures of the Eldgjá fissure system
or the Eldgjá "proper", whereas the dispersal pattern
of the phreatomagmatic unit suggest origin at vents
near or within the eastern part of Mýrdalsjökull (G.
Larsen, unpublished data 1996). It is this tephra layer
that Jónsson (1985) misidentified as the Leiðólfsfell
scoria in the distal areas and led him to conclude that
the Leiðólfsfell cones were the product of an eruption
in the 12 Century (compare Fig. 6 of this paper with
Fig. 4 in Jónsson, 1985). The tephra layer immediately
below the -935 AD Eldgjá tephra is the well known
"Settlement" layer, which was formed in an explosive
fissure eruption on the Vatnaöldur fissure system
around -870 AD (Fig. 5; Table 2).
AGE OF THE CONE GROUP
In the area closest to source (<500 m), where the
Leiðólfsfell scoria is thickest, its surface is either bar-
ren or covered by a thin soil containing the 1918 AD
JÖKULL, No. 46, 1998
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