Jökull - 01.12.1980, Page 24

termed Skeiðará jökulhlaups and the present
paper refers to these only.
Records of 30—40 jökulhlaups are avail-
able, the earliest dating as far back as the
fourteenth century. Many of the jökulhlaups
have occurred simultaneously with volcanic
eruptions in Grímsvötn. During the
nineteenth century and the first three to four
decades of the twentieth century jökulhlaups
occurred approximately once each decade
whereas in the last four decades jökulhlaups
have taken place at 3 — 7 year intervals. For
historica! records on the jökulhlaups the
reader is referred to Thorarinsson (1974) but for
the cause and process to Björnsson (1974) and
Tórnasson (1974).
The jökulhlaups in Skeiðará are
characterized by a great increase in the
quantity of dissolved solids, rising from about
50—90 mg/1 to about 300—400 mg/I. The
petrographical composition of the suspended
load also changes as is clearly demonstrated
by the ratio dark glass/light glass (dark glass:
refractive index > 1.55; light glass: refr. ind.
^ 1.55). This relationship has been studied
since 1964 (Fig. 2). In between jökulhlaups the
ratio has been in the range of 10—20 whereas
in the jökulhlaups, especially during the peak
in the sediment load in 1972, the ratio is many
times higher. During the 1972 jökulhlaup a
certain development occurred in the
petrographical composition of the suspended
load. The coarse silt fraction became almost
90% glass of the same refractive index as the
Grímsvötn volcanic ash (Tómasson 1972). Such
a development was not observed in the jökul-
hlaups of 1965 and 1976. In this respect the
jökulhlaup of 1972 was exceptional, cf. Fig. 2.
Fig. 2. The variation of the
ratio dark glass/light glass in the
suspended' sediment of the grain
size fraction 0.06—0.02 mm in
Skeiðará. — Mynd 2. Breytingar á
hlutfalli dökks og Ijóss glers í svifaur
0.02—0.06 mm í Skeiðará.
22 JÖKULL 30. ÁR