Holocene major eruptions

During the Holocene, Iceland has experienced more than 20 eruptions per century from about 30 active volcanoes. The magmatic production is completely dominated by basalts. Intermediary and rhyolitic rocks make up less than 10%

Krafla-udbruddene - 1975-1984

Den 20. december 1975 indledtes en udbrudsepisode med sprækkedannelse på ydersiden af vulkanen Krafla i det nordlige Island. Episoden forløb frem til 1984. Vulkanen havde da været inaktiv siden udbruddene mellem 1724 og 1729

Grímsvötn - farer og forebyggelse - nyt udbrud i november 2004

I början av november 2004 hade Grímsvötn, den mest aktiva vulkanen på Island, ytterligare ett utbrott. Av Islands alla vulkaner står Grímsvötn i särklass med sina 60 utbrott under de senaste 1.100 åren

Geodynamic signals detected by geodetic methods in Iceland

The geodynamics laboratory provided by Iceland’s position on an active mid-ocean ridge has been recognized for several decades. Geodetic experiments have been designed and carried out in Iceland since 1938 to verify various global geodynamic theories, such as Wegener’s theory of continental drift, the sea floor spreading hypothesis, plate tectonics, mantle plumes etc. State-of-the-art techniques have been used to obtain data on crustal displacements with ever increasing accuracy to constrain the theories. Triangulation and optical levelling were used in the beginning, later EDM-trilateration. Network GPS surveying began in 1986 and has been used extensively since then to study crustal movements. With the addition of InSAR and continuous GPS in the last decade we have made a significant stride towards the goal of giving a continuous representation of the displacement field in time and space. The largest and most persistent signal is that of the plate movements. Geodetic points in East and West Iceland move with the Eurasia and North America Plates, respectively, and the vectors are consistent with global models of plate movements. The plate boundary zones are a few tens of kilometers wide, within which strain accumulates. This strain is released in rifting events or earthquakes that have a characteristic displacement field associated with them. In the Krafla rifting episode in 1975-1984 a 100 km long section of the plate boundary in North Iceland was affected and divergent movement as large as 8-9 m was measured. The June 2000 earthquakes in the South Iceland Seismic Zone were the most significant seismic events in the last decades. Two magnitude 6.5 earthquakes and several magnitude 5 events were associated with strike-slip faulting on several parallel faults along the transform-type plate boundary. Slow post-rifting and post-seismic displacements were detected in the months and years following these events, caused by coupling of the elastic part of the crust with the visco-elastic substratum. Viscosities in the range 0.3-30 x 1018 Pa s have been estimated from the time-decay of these fields. Similar values are obtained from crustal uplift measured around the Vatnajökull glacier due to the reduced load of the glacier in the last century. Magma movements in the roots of volcanoes are reflected by deformation fields measureable around them. The volcanoes inflate or deflate in response to pressure increase or decrease in magma chambers, and intrusive bodies are revealed by bulging of the crust above them. The most active volcanoes in Iceland, Katla, Hekla, and Grímsvötn, appear to be inflating at the present time, whereas Krafla and Askja are slowly deflating. An intrusion episode was documented near the Hengill volcano in 1994-1998 and two intrusion events occurred in the Eyjafjallajökull volcano in 1994 and 1999, all of which were accompanied by characteristic deformation fields

Geodetic data shed light on ongoing caldera subsidence at Askja, Iceland

Subsidence within the main caldera of Askja volcano in the North of Iceland has been in progress since 1983. Here, we present new ground and satellite based deformation data, which we interpret together with new and existing micro-gravity data, to help understand which processes may be responsible for the unrest. From 2003-2007 we observe a net micro-gravity decrease combined with subsidence and from 2007-2009 we observe a net micro-gravity increase while the subsidence continues. We infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary. Mass movements at active volcanoes can be caused by several processes, including water table/lake level movements, hydrothermal activity and magma movements. We suggest that here, magma movement and/or a steam cap in the geothermal system of Askja at depth, are responsible for the observed microgravity variations. In this respect, we rule out the possibility of a shallow intrusion as an explanation for the observed micro-gravity increase but suggest magma may have flowed into the residing shallow magma chamber at Askja despite continued subsidence. In particular variable compressibility of magma residing in the magma chamber, but also compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation

Integration of micro-gravity and geodetic data to constrain shallow system mass changes at Krafla Volcano, N Iceland

New and previously published micro-gravity data are combined with InSAR data, precise levelling and GPS measurements to produce a model for the processes operating at Krafla volcano, 20 years after its most recent eruption. The data have been divided into two periods: from 1990 to 1995 and from 1996 to 2003 and show that the rate of deflation at Krafla is decaying exponentially. The net micro-gravity change at the centre of the caldera is shown, using the measured Free Air Gradient, to be -85 μGal for the first and -100 μGal for the second period. After consideration of the effects of water extraction by the geothermal power station within the caldera, the net gravity decreases are -73 ± 17 μGal for the first and -65 ± 17 μGal for the second period. These decreases are interpreted in terms of magma drainage. Following a Mogi point source model we calculate the mass decrease to be ~2 x 1010 kg/yr reflecting a drainage rate of ~0.23 m3/s, similar to the ~0.13 m3/s drainage rate previously found at Askja volcano, N-Iceland. Based on the evidence for deeper magma reservoirs and the similarity between the two volcanic systems, we suggest a pressure-link between Askja and Krafla at deeper levels (at the lower crust or the crust-mantle boundary). After the Krafla fires, co-rifting pressure decrease of a deep source at Krafla stimulated the subsequent inflow of magma, eventually affecting conditions along the plate boundary in N-Iceland, as far away as Askja. We anticipate that the pressure of the deeper reservoir at Krafla will reach a critical value and eventually magma will rise from there to the shallow magma chamber, possibly initiating a new rifting episode. We have demonstrated that by examining micro-gravity and geodetic data, our knowledge of active volcanic systems can be significantly improved

The target peneplain of the Lockne impact

The Lockne impact crater in central Sweden, with a diameter of about 7 km, formed in the mid-Ordovician in a marine environment that was deeper than present shelf seas. The present dip of the so-called sub-Cambrian peneplain in the target area of the impact is about 0.85 degrees toward the northwest. The peneplain is cut by a set of northwest-striking, nearly vertical faults with a throw of up to just over 100 m, collectively. The identification of the peneplain and its deformation by faulting allows us to infer that the part of the crater that is exposed to the east of Lake Locknesjön has been lowered by about 100 m relative to parts exposed to the west of the lake and that it has, therefore, been spared from significant erosion. Therefore, the preservation of the whole crater is even better than was assumed in previous work. The peneplain extends to 600-700 m from the rim of the inner crater. Hence, the structural uplift of the rim is quite subdued compared to the craters that formed on land.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

NordVulk 1974-2024

Source at https://geologiskaforeningen.se/geologiskt_forum/.Nordisk Vulkanologisk Institut, senare känt som Nordisk Vulkanologisk Center (NordVulk) på Island fyller 50 år 2024. Tyvärr sammanfaller jubileet med att NordVulk upphör i sin nuvarande form och en lång och framgångsrik era med nordisk vulkanologisk forskning på Island tar slut. Här berättar vi lite om NordVulks historia och bifogar några korta anekdoter från livet där

Krafla-udbruddene - 1975-1984

Den 20. december 1975 indledtes en udbrudsepisode med sprækkedannelse på ydersiden af vulkanen Krafla i det nordlige Island. Episoden forløb frem til 1984. Vulkanen havde da været inaktiv siden udbruddene mellem 1724 og 1729