G. Larsen

The Hekla eruption 1980–1981

The sixteenth eruption of Hekla since 1104 began on August 17th, 1980, after the shortest repose period on record, only ten years. The eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the Hekla volcanic ridge. The production rate declined rapidly after the first day and the eruption stopped on August 20th. A total of 120 million m3 of lava and about 60 million m3 of airborne tephra were produced during this phase of the activity. In the following seven months steam emissions were observed on the volcano. Activity was renewed on April 9th 1981, and during the following week additional 30 million m3 of lava flowed from a summit crater and crater rows on the north slope.The lavas and tephra are of uniform intermediate chemical composition similar to that of earlier Hekla lavas. Although the repose time was short the eruptions fit well into the behaviour pattern of earlier eruptions. Distance changes in a geodimeter network established after the eruptions are interpreted as due to inflation of magma reservoirs at 7–8 kilometers depth.

Tephrochronology, environmental change and the Norse settlement of Iceland

Abstract The first human impacts on the Icelandic environment came with the Norse colonisation or Landnám of the ninth century AD. The colonisation represents a fundamental environmental change that is both rapid and profound. In this paper we assess geomorphological dimensions of the initial settlement period using a tephrochronology that includes the Landnám Tephra, erupted ca. 870 AD, two tenth century AD tephras KR 920 and E 935, and 11 other well dated tephra layers. We report a new 14C age of 1676 ±12 14C yr BP (cal AD 345 (400) 419) for the tephra SILK-YN which forms a key prehistoric marker horizon that constrains rates of environmental change in the centuries before Norse Settlement. Aeolian sediment accumulation rates show five geomorphological responses to settlement that differ in the rate and trajectory of change. These distinct anthropogenic signals are the result of spatially variable sensitivity to grazing and deforestation, and reflect the extent of local soil erosion. This critical erosion threshold is variable in space and time.

Magma storage beneath Grímsvötn volcano, Iceland, constrained by clinopyroxene-melt thermobarometry and volatiles in melt inclusions and groundmass glass

Basalt eruptions at Grimsvotn volcano, Iceland, are generally of low intensity; however, occasionally, an order of magnitude larger eruptions occur. In order to discuss the reasons for this difference, the degassing budget of S and Cl and crystallization conditions of the eruptive magma were determined from volatile concentration measured in melt inclusion (MI) and groundmass glass and thermobarometry, respectively. Tephra of two of the largest historical eruptions (2011 and 1873) and two much smaller eruptions (2004 and 1823) were investigated. Sulfur and Cl concentrations are higher in groundmass glass of the smaller eruptions due to incomplete outgassing caused by melt quenching in contact with glacial water. Sulfur concentration and degassing budget correlate with erupted magma volumes. Higher volatile concentrations of MI from the larger eruptions re fl ect important recharge of gas-rich magma from depth. The recharge causes a high-magnitude eruption followed by increased eruption frequency over the following decades. Pressure and temperature estimates of crystallization are obtained through equilibrium clinopyroxene-glass pairs, where crystals adjacent to, and in textural equilibrium with, both groundmass glass and that of MI were measured. An average crystallization pressure of 4 ± 1 kbar corresponding to approximately 15 ± 5 km depth was obtained together with a considerable temperature range, 1065 – 1175°C. Similar crystallization depths are obtained for the basalt of the 2014 – 2015 Barðarbunga rifting event and to a low resistivity layer revealed by magnetotelluric surveys. Therefore, an important magma storage depth is inferred at lower crustal depth above the center of the Iceland mantle plume