| Summary: | The tephra making up the tuff cones in Surtsey, formed in 1963-1964 in the phreatomagmatic phase of the Surtsey eruption and showed the first signs of palagonitization at the surface in 1969, a year after the onset of the geothermal system was detected. The early work on Surtsey established that at 100 °C, it takes one to two years for the tephra to convert into dense palagonitized tuff while the rate of palagonitization is considerably slower at lower temperatures. The present study compiles all published as well as unpublished data on the surface manifestations of geothermal activity and measurements in the drill hole completed in 1979, to give a comprehensive account of the evolution of the thermal area at Surtsey during the period of 1968 - 2018. Most of this work was done by the late Sveinn P. Jakobsson. Overall, the time series demonstrate a slow but clear trend of cooling of Surtsey with time: the thermal activity within the lava rapidly cooled from recorded emission temperatures in fumaroles of up to 460°C in 1970, to ambient temperatures within 30 – 40 years after emplacement. In contrast, the thermal area within the tephra/tuff exhibits a gradual onset of geothermal activity. The onset on Surtur was detected in 1968 and high temperatures still prevail at the surface where temperatures have only declined from 100 to 80 – 90 °C in 50 years. The onset on Surtungur was detected in 1974 and the maximum temperatures recorded have remained within the 90 – 100 °C range since 1979. The intermediate area between Surtur and Surtungur has exhibited activity broadly in the same way as Surtur and maximum temperatures that remained within the 90 – 100 °C range from 1979 – 2000, are now clearly declining. Maximum temperatures in the 1979 drillhole were 141 °C in 1980 but they have been steadily declining, reaching 123 °C in 2018. Icelandic Research Fund – RANNÍS
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