Jökull


Jökull - 01.06.2000, Síða 23

Jökull - 01.06.2000, Síða 23
Guðrún Larsen has been that of the accompanying jökulhlaups, be- cause of the short warning time for people and live- stock exposed to the floods and because of the damage inflicted on the environment. This hazard is reduced with the current monitoring of the volcano. The most serious effects of future Katla eruptions could, how- ever, be on power transmission in Southern Iceland as a result of tephra fall and lightning in the eruption cloud. The latter is potentially the greatest hazard to people and livestock in areas within 30-40 km of the eruption site. A modern community with power trans- mission lines, television aerials and electrical fences (rural areas) is more vulnerable to this menace than earlier communities in the vicinity of the volcano. SUMMARY AND CONCLUSIONS Intracaldera basaltic eruptions, the typical hydromag- matic Katla eruptions, occur at intervals of decades. The average frequency during the last 9 centuries is close to two eruptions per century, with the shortest and longest interval of 13 and 80 years, respectively. Intracaldera silicic eruptions occurred at intervals of hundreds of years in the period 6600 - 1700 BP. The average frequency was close to two eruptions per millennium, and the shortest and longest interval were ca. 100 and ca. 1000 years, respectively. Large, predominantly effusive fissure eruptions, involving both the central volcano and fissure swarm occur at intervals of thousands of years. Two such episodes are known in the Holocene. They represent major rifting episodes that may modify the volcanic system, including the subcaldera conditions. Katla eruptions of known calendar dates all began in the spring-fall season. Future eruptions may follow that trend. The vent area in most historical Katla erup- tions was apparently in the eastern part of the caldera. The best documented locations lie within the south- eastern corner of the caldera. Twelve silicic eruptions took place in the ca. 5000 year period between the Hólmsá and Eldgjá fires, dur- ing which time the composition of silicic magma re- mained fairly homogenous. The interval since the last silicic eruption ca. 1700 14C years ago is the longest on record. The reason as to why silicic activity has not recurred may be subcaldera changes brought about by the Eldgjá event. A substantial silicic eruption cannot, however, be excluded in the future. Vent areas in the silicic eruptions have been asso- ciated with the caldera. Accompanying jökulhlaups may have escaped from the caldera through any of the three gaps, depending on pre-Eldgjá topography and the ice thickness in the caldera. The 75 km long Eldgjá fissure is one of the longest known fissures active in a single Holocene eruption. The Eldgjá tephra is the third or fourth largest his- torical tephra layer in Iceland and the lava flows are the among largest in recorded history. A hyaloclastite flow that escaped from the caldera through the Kötlu- jökull gap appears to be part of its products. Since activity on the Katla system was renewed in the 12th century, after an apparent repose of some 200 years, all associated jökulhlaups have escaped through the Kötlujökull pass onto Mýrdalssandur. Possible explanations are changes in ice thickness, subglacial topography of the caldera and/or the loca- tion of the eruption site. The immediate environmental effects of the Eld- gjá event included permanent or temporary destruc- tion of large vegetated areas by tephra fall, lava flows and jökulhlaups. Between 3000 and 4000 km were severely affected, whereof several hundred km were permanently laid waste. The drainage pattern of at least a 2000 km subaerial and subglacial area was permanently altered. Long-term environmental changes brought about by the Eldgjá event are exemplified by the post-Eld- gjá jökulhlaups and include 1) lateral extension and raising of the surface of the Mýrdalssandur plain; 2) extension of the sandur coast line to the south; 3) changes in the course of rivers and 4) continuing de- struction of vegetated areas by jökulhlaups. The immediate and long term changes following the Eldgjá event may be the most extensive environ- mental changes caused by volcanic activity in Iceland since settlement 11 centuries ago. ACKNOWLEDGEMENTS Many people have helped me in various ways in the field and otherwise during my many trips in the Katla-Eldgjá area. Sigurður Þórarinsson introduced 22 JÖKULL No. 49
Síða 1
Síða 2
Síða 3
Síða 4
Síða 5
Síða 6
Síða 7
Síða 8
Síða 9
Síða 10
Síða 11
Síða 12
Síða 13
Síða 14
Síða 15
Síða 16
Síða 17
Síða 18
Síða 19
Síða 20
Síða 21
Síða 22
Síða 23
Síða 24
Síða 25
Síða 26
Síða 27
Síða 28
Síða 29
Síða 30
Síða 31
Síða 32
Síða 33
Síða 34
Síða 35
Síða 36
Síða 37
Síða 38
Síða 39
Síða 40
Síða 41
Síða 42
Síða 43
Síða 44
Síða 45
Síða 46
Síða 47
Síða 48
Síða 49
Síða 50
Síða 51
Síða 52
Síða 53
Síða 54
Síða 55
Síða 56
Síða 57
Síða 58
Síða 59
Síða 60
Síða 61
Síða 62
Síða 63
Síða 64
Síða 65
Síða 66
Síða 67
Síða 68
Síða 69
Síða 70
Síða 71
Síða 72
Síða 73
Síða 74
Síða 75
Síða 76
Síða 77
Síða 78
Síða 79
Síða 80
Síða 81
Síða 82
Síða 83
Síða 84
Síða 85
Síða 86
Síða 87
Síða 88
Síða 89
Síða 90
Síða 91
Síða 92
Síða 93
Síða 94
Síða 95
Síða 96
Síða 97
Síða 98
Síða 99
Síða 100
Síða 101
Síða 102
Síða 103
Síða 104
Síða 105
Síða 106

x

Jökull

Beinleiðis leinki

Hvis du vil linke til denne avis/magasin, skal du bruge disse links:

Link til denne avis/magasin: Jökull
https://timarit.is/publication/1155

Link til dette eksemplar:

Link til denne side:

Link til denne artikel:

Venligst ikke link direkte til billeder eller PDfs på Timarit.is, da sådanne webadresser kan ændres uden advarsel. Brug venligst de angivne webadresser for at linke til sitet.