Jökull - 01.12.1977, Side 22

Vatnajökull
Five layers in the core may be attributed to
recorded “eruptions in Vatnajökull”, i.e. corre-
sponding to the years 1887, 1823, 1766, 1753, and,
conceivably, 1706. On chemical grounds these
layers appear to be of heterogeneous origins —
V 1887 is a basaltic andesite (SiO^ = 54%), V
1823 has a “caldera composition”, similar to
that o£ Grímsvötn, whereas V 1766 and 1706
have “primitive” chemistry. No analysis exists
of V 1753, but the refractive index of the glass
indicates basaltic composition.
V 1706: Three annals state for the year 1706:
“Fires in the autumn in Grímsvötn” (Thorar-
insson 1974, p. 54). As stated the chemical com-
position of the 1706 tephra does not fit that of
other Gríntsvötn products, which constitutes a
powerful argument against an origin there.
However, it has been shown by Thorarinsson
and others (e.g. Thorarinsson 1950) that the
Grímsvötn watershed reaches far outside the
caldera itself, and that eruptions all the way
from Þórdarhyrna in SW Vatnajökull to the
southern reaches of Dyngjujökull in the NW
part, may cause jökulhlaups in Skeidará (or
Jökulsá for that matter). The fissure swarms and
central volcanoes in that area exhibit consider-
able range in chemical composition, as seen
from the analyses presented here. It is therefore
suggestecl that the 1706-eruption took place
within the Skeidará watershed, but outside the
Grímsvötn central volcano itself.
V 1753: The eruption in 1753 was described in
contemporary sources as having taken place in
Sídujökull; — Thorarinsson (1974, pp. 57—58)
suggests a location NE of Pálsfjall, where de-
pressions in the glacier still attest. to thermal
activity beneath (cf. lljörnsson 1977). In the
eruption, rivers draining SW Vatnajöktdl,
Djúpá and Skaftá, flooded catastrophically, stich
that the former rose by some 200 feet. The land
around was ruined by the debris of the jökul-
hlaup: rocks, gravel and ice. The billowing
smoke and fires were seen from mountains
bordering the lowlands, and wind from the NE
brought the ash down to the adjoining farm-
lands and pastures, causing death to the live-
stock (Thorarinsson 1974, pp. 57—58).
20 JÖKULL 27. ÁR
V 1766: In 1766 Tlijórsá flooded repeatedly,
and contemporary writers describe haze in the
air, sulphurous smells, and fine asli issuing from
the direction of Vatnajökull. The floods, stemm-
ing from Tungnaá, a tributary to Tlijórsá, and
the meteoric phenomena, may be attributed to
an eruption in western Vatnajökull (Thorarins-
son 1974, pp. 58—59).
V 1823: The 1823-tephra has “caldera chem-
istry”; the grain size parameters are unknown.
In 1823 a volcanic cloud was observed over
Vatnajökull in the beginning of February, and
ash fell in Skaftártunga, SW of Vatnajökull.
Sveinn Pálsson, a highly reliable contemporary
naturalist, does not mention a jökulhlaup,
which seems to exclude Grímsvötn as a possible
source (Thorarinsson 1974, pp. 69—70). The
last previous hlaup took place in 1816, 7 years
earlier, so that the caldera should liave been
full enougli by 1823 for an eruption to trigger
a new hlaup. The chemical composition, liow-
ever, suggests an origin in a “mature” volcano,
such as the Grímsvötn-Lakagígar system.
V 1887: For the years 1885—1889 there are
occasional references to earthquakes and light
ashfall around Vatnajökull, and in 1887 a jökul-
hlaup took place in Súla on western Skeidarár-
sandur, which is cliaracteristic for the purely
glacial jökulhlaups from the ice-dammed lake
Grænalón. Thorarinsson (1974, pp. 96—97),
however, suggests that a small eruption took
place in 1887 near Thórclarhyrna. The tephra
is fairly coarse, similar in that respect to many
of the Grímsvötn tephras, but of basalt-andesitic
composition. Its position in the core, however,
is more in keeping with thc years 1888 or 1889.
In view of all this it appears that volcanic
activity of feeble lmt long-winded nature was
abroad near Tliórdarhyrna in the years 1887—
89. (See the last chapter for evidence that this
tephra is front 1889.)
Eight unknown tephra layers.
The profile contains eight unknown tephra
layers in addition to the 1854 one, that was ten-
tatively allotted to Grímsvötn. Of these, three
have olivine tholeiite composition (1697, 1720,
1707), and the fourth (1768) has two kinds of