Hvað leynist undir Kötlu?

Hvað leynist undir Kötlu?
Bergrún Arna Óladóttir, Olgeir Sigmarsson, Guðrún Larsen og Þorvaldur Þórðarson
Bls: 115–122 2.–4. hefti 75. árg. 2007

SAMANTEKT
Gossaga Kötlukerfisins hefur verið lesin úr gjóskulögum sem varðveitt eru í jarðvegi á Álftaversafrétti, austan Mýrdalsjökuls. Gjóskan er basaltgjóska úr millibergröðinni sem sýnir litla, en greinilega, breytingu með tíma. Gossögunni má skipta í átta tímabil út frá breytingum í aðalefnasamsetningu gjóskunnar. Samskonar hegðunarmynstur birtist tvisvar á þeim ~8400 árum sem rannsökuð eru, sem skýra má á þann hátt að einfalt aðfærslukerfi þróist yfir í kerfi ganga og sillna sem síðar myndar grunnstætt kvikuhólf. Breytingar á aðfærslukerfinu hafa áhrif á gostíðni eldstöðvarinnar með þeim hætti að hún eykst þegar kerfi ganga og sillna tekur við af einföldu aðfærslukerfi en fellur á ný þegar kvikuhólf hefur myndast. Samkvæmt þessu líkani er Katla nú á tímabili einfalds aðfærslukerfis og gostíðni hennar er þar af leiðandi lág.

SUMMARY
A study of the tephra stratigraphy east of the Katla volcano in South-Iceland has been undertaken in a composite soil section. The section records ~8400 years of explosive activity from Katla volcano and includes 208 tephra layers of which 126 samples were analysed for major-element composition. The age of individual Katla layers was calculated using soil accumulation rates (SAR) derived from soil thicknesses between 14C-dated marker tephra layers. Temporal variations in major-element compositions may be used to divide the ~8400 year record into eight intervals of 510–1750 year durations. The change in concentration of incompatible elements (i.e., K2O) in individual intervals is characterized as steady, irregular and steadily increasing. The observed pattern of changes in tephra composition within individual time intervals suggests different conditions in the plumbing system beneath Katla volcano. Two cycles are observed throughout the Holocene, each involving three stages of plumbing system evolution, which may be explained in this way: A cycle begins with a simple plumbing system, indicated by a steady state in major element compositions. This is followed by a sill and dyke system, a period characterized by irregular variations in major-element composition with time, and eventually leads to the formation of a magma chamber, represented by a uniform increase of incompatible element concentration with time. The eruption frequency within each cycle increases from the stage of a simple plumbing system to the sill and dyke complex stage and then drops again during magma chamber stage. In accordance with this model, the Katla volcano is at present in a stage of a simple plumbing system characterized by steady state magma composition and relatively low eruption frequency.