Jökull - 01.12.1977, Page 13

determined age, chemical characteristics, and
grain size distribution.
Grain size distribntion
The volcanoes listed above fall into tliree
broad groups according to composition of the
products, and mechanism and environment of
eruption (e.g. Walker 1973): (1) Öræfajökull
and Hekla are Plinian in character, and give
rise to silicic and intermediate tephras; (2)
Katla, Grímsvötn and Kverkfjöll(?) are sub-
glacial, and their tephras should be character-
istic of aqueous cliilling — “Surtseyan” in the
terminology of Walker and Croasdale (1972).
The same may apply to some of the ntinor
fissure eruptions north of Dyngjujökull, and to
the NE end of the Lakagígar 1783 fissure which
extended beneath the glacier; (3) The sub-aereal
fountain eruptions outside the Vatnajökull ice
cap, like the Askja 1961 eruption, form the
third kind, Strombolian/Hawaiian (Walker
1973).
mm , 1, 1 1 1 1 .
50 'I 1 GKv 'Áö 'k'k 1 I
25 o # Bórdarbunga _
° °* * ? Kv1477
8 < . * H-3
10 - \ \*N ° O K-1360 \ "\s • other Kafla layers \ kK-1485
5 o N \ * I V \ _
o
* * XX
2.5 %° * *. \ k l(N *\ t. k \
1.0 _ t*k *#■- \ _
* * a, . “ \
.5 *N\ _
▼ * * ^3
.25 -
0.1 1 ii II I
10 25 50 100 250 500 1000 km
Fig. 5. Maximum diameter of grains (ordinate)
vs. distance from the source (abscissa) plotted
logarithmically:
(1) Line having the slope — i/2, as predicted by
the equation of Stokes for the fall of par-
ticles in viscous medium.
In spite of considerable work in recent years
on the grain morphology and size distribution
of various kinds of tephras in Iceland and else-
where (Walker 1973, Walker & Croasdale 1972,
Sheridan 1971, Persson 1966, Larsen 1975,
Betijamínsson 1975, Egilson 1974) this kind of
information has scarcely been secured yet for
the eruptions pertinent to this study. However,
in order to get a handle on the variation of
some tephra characteristics with distance from
the source, a compilation has been made of the
diameter of the largest grain (mm) in a sample,
vs. distance from the source, for a few tephras.
Fig. 5 shows the result.
In order to interpret these data, consider the
factors governing the distance a grain of ash is
carried: In the simplest model the grain reaches
its maximum height in tlie volcanic cloud,
whence it falls down again under the influence
of gravity, and is carried at the same time along
witli the wind. It can easily be shown that
particles of tephra size reach terminal velocity
(2) Line having the slope — 1, as predicted by
Newton’s equation for the fall of particles
in turbulent viscous medium. The line
envelopes data points for the supposed
Kverkfjöll 1477 tephra.
(3) Line enveloping data points for H3 and
K-1485.
The letters at the top denote approximate
distances from the drill site at Bárdarbunga to
Grímsvötn (G), Kverkfjöll (Kv), Askja (A), Ör-
æfajökull (Ö), Hekla (H), and Katla (Iv). The
cogwheel-asterisks denote samples from the Bárd-
arbunga core.
Mynd 5. Stœrsta pvermál korna (y-ás) móti
fjarlagð frá eldstöð (x-ás), dregin lógarithmískt
fyrir nokkur þekkt öskulög. Stafirnir efst tákna
fjarlagð eldstöðva frá Bárðarbunguholunni, G
— Grimsvötn, Kv — Kverkfjöll, A — Askja, Ö —
Örœfajökull, H — Hekla, K — Katla. Lína merlit
1 hefur hallann — l/2, skv. lögmáli Stokes; lina
2 hefur hallann — 1, skv. lögmáli Newtons um
fall korna í seigu og iðandi (turbulent) efni.
Linan snertir jafnframt gagnapunkta 1477-goss-
ins (sem talið er vera) úr Kverkfjöllum. Lina 3
afmarkar gagnapunkta frá H-3 og K-1485 gjósli-
unni. Tannhjólsmerkin eru gagnapunklar frá
Bárðarbungukjarnanum.
JÖKULL 27. ÁR 1 1