Jökull - 01.12.1975, Blaðsíða 4

Hw water layer thickness
1 length scale
p water pressure in glacier water passages
P = pwg (zc — zb), hydrostatic subglacial water
pressure which would exist if there were a
hydraulic connection along the glacier bed
between the levels z0 and zb
pb virtual water pressure at the glacier bed-
rock
Pl = p(g (zs — z), ice overburden pressure inside
the glacier
Pj = pig (zs — zw), total glacier overburden pres-
sure
s coordinate for distance measured down-
glacier parallel to the bed-rock
t time
x horizontal coordinate for distance measur-
ed down-glacier
cp (x, z, t) potential for water pressure in the
(x, z)-plane
(pb potential for water pressure at the bed-rock
Pi density of ice
pw density of water
INTRODUCTION
A jökulhlaup is a sudden burst of water from
a glacier dammed marginal lake or from a water
reservoir inside a glacier. The present work con-
siders only the jökulhlaups which originate at
water reservoirs within glaciers.
Jökulhlaups from the interior of glaciers have
been frequent in Iceland. Some of the jökul-
hlaups originate at geothermal areas. Thirty to
forty jökulhlaups have been traced to the geo-
thermal area at Grímsvötn (Thorarinsson 1975a,
Björnsson 1974). Other jökulhlaups are caused
by volcanic eruptions. Several active central
volcanoes are at present covered by ice (see
Fig. 1). Seventeen volcanic eruptions have been
traced to Ivatla in Mýrdalsjökull, all causing
catastrophic jökulhlaups (Thorarinsson 1975b).
The eruptions in Öræfajökull in the years 1362
and 1727 ancl in Eyjafjallajökull in 1821 — 1823
were all accompanied by jökulhlaups. Seven
great jökulhlaups which occurred in the river
Jökulsá á Fjöllum in the period from 1655 to
1729 were caused by volcanic eruptions, pre-
sumably in the Kverkfjöll area (Thorarinsson
1950). A few eruptions have occurred at
Thórdarhyrna, the last one in 1903. Volcanic
eruptions have been frequent at Grímsvötn.
The jökulhlaups from Grímsvötn are usually
not caused by the eruptions (Thorarinsson 1953,
Björnsson 1974). Further studies may reveal
signs of jökulhlaups at other central volcanoes,
such as Snæfellsjökull, Tindfjallajökull, Torfa-
jökull, Kerlingarfjöll, and Tungnafellsjökull.
Fissure eruptions may occur anywhere be-
neath glaciers in the active volcanic zones.
About nine eruptions, presumably on subglacial
fissures, have been reported in the western part
of Vatnajökull during the last four centuries
(Thorarinsson 1975a). The locations are un-
certain but some of the eruptions have caused
jökulhlaups in the river Skaftá, Hverfisfljót,
and Djúpá (the last large one in 1753), and in
Thjórsá (1783). — Tablemountains and hyalo-
clastic ridges bear witness to subglacial volcanic
activity throughout the volcanic zones.
The present paper examines the cause of
jökulhlaups from the interior of glaciers. A
general review is given of the theory of water
flow in temperate glaciers. Formation of water
reservoirs at the bed of glaciers is considered.
Conditions are defined for which jökulhlaups
occur from the reservoirs. The conditions may
be fulfilled at a geothermal area, and can also
be present if a volcanic eruption occurs at the
bed of the glacier. The origin of possible jökul-
hlaups from the interior of glaciers in Iceland
is discussed on the basis of this study.
WATERFLOW IN GLACIERS,
RESERVOIRS, AND JÖKULHLAUPS
The conditions for the formation of a water
reservoir beneath a temperate glacier will be
examined. The conditions for which jökul-
hlaups occur from the reservoirs will be con-
sidered later.
Formation of water reservoirs
Consider the flow of water in a temperate
glacier (see also Weertman 1972, Shreve 1972,
and Nye in press). Fig. 2 shows a profile of a
glacier of an arbitrary shape. The slopes in-
volved are small and only slowly varying. A
layer of water is present at an impermeable
bed-rock. The effective negative pressure gradi-
ent which drives water along the bed is
— d(pb/ds; s is a coordinate measured down-
2 JÖKULL 25. ÁR