Jökull


Jökull - 01.12.1980, Page 24

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

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Jökull

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