Erik Sturkell

Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption

Gradual inflation of magma chambers often precedes eruptions at highly active volcanoes. During such eruptions, rapid deflation occurs as magma flows out and pressure is reduced. Less is known about the deformation style at moderately active volcanoes, such as Eyjafjallajökull, Iceland, where an explosive summit eruption of trachyandesite beginning on 14 April 2010 caused exceptional disruption to air traffic, closing airspace over much of Europe for days. This eruption was preceded by an effusive flank eruption of basalt from 20 March to 12 April 2010. The 2010 eruptions are the culmination of 18 years of intermittent volcanic unrest. Here we show that deformation associated with the eruptions was unusual because it did not relate to pressure changes within a single magma chamber. Deformation was rapid before the first eruption (>5 mm per day after 4 March), but negligible during it. Lack of distinct co-eruptive deflation indicates that the net volume of magma drained from shallow depth during this eruption was small; rather, magma flowed from considerable depth. Before the eruption, a ∼0.05 km3 magmatic intrusion grew over a period of three months, in a temporally and spatially complex manner, as revealed by GPS (Global Positioning System) geodetic measurements and interferometric analysis of satellite radar images. The second eruption occurred within the ice-capped caldera of the volcano, with explosivity amplified by magma–ice interaction. Gradual contraction of a source, distinct from the pre-eruptive inflation sources, is evident from geodetic data. Eyjafjallajökull’s behaviour can be attributed to its off-rift setting with a ‘cold’ subsurface structure and limited magma at shallow depth, as may be typical for moderately active volcanoes. Clear signs of volcanic unrest signals over years to weeks may indicate reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect.

Rift‐transform kinematics in south Iceland: Deformation from Global Positioning System measurements, 1986 to 1992

Crustal deformation in the plate boundary regions in south Iceland is estimated from repeated Global Positioning System (GPS) geodetic measurements in the period 1986–1992. We compare coordinate solutions for the 1986 and 1989 surveys with the results from the most recent survey in 1992. Horizontal position uncertainty is about 2 cm for the 1986 and 1989 coordinates and about 4 mm for the 1992 coordinates. Little internal deformation is observed in the area west of the western volcanic zone (within the North American plate) and at the southern tip of the eastern volcanic zone (within the Eurasian plate). The observed relative velocity of these two areas is 2.1±0.4 cm/yr in direction N117±11°E (1σ uncertainties), compatible with the 1.94 cm/yr widening in direction N 104°E across south Iceland predicted from the NUVEL-1 global plate motion model. Left-lateral shear strain is developing across the intervening transform zone, the E-W trending south Iceland seismic zone (SISZ). Strain is concentrated within a 20- to 30-km-wide zone that correlates with seismic activity in the SISZ. About 85±15% of the relative plate motion is accommodated by this zone in such a way that the area south of the SISZ is moving toward the east with the Eurasian plate and the area north of the SISZ is moving toward the west with the North American plate. Accordingly, north of the SISZ the western rift zone can accommodate a maximum of 15±15% of the relative plate motion. Within the SISZ the shear strain results in an anticlockwise rotation of lineaments oriented north-south, at a rate of 0.5–1 μrad/yr. The shear strain accumulation can be accommodated by “bookshelf faulting” on mapped recent north-south faults in the SISZ. If the deformation is accommodated by an array of N-S faults spaced 1–5 km apart, an average slip rate of about 0.5–5 mm/yr is required on each fault. The rate of geometric moment release due to earthquakes averaged over centuries, m˙0, expected from such an array is about the same as expected from a simple transform fault, 2vAD, where 2v is the relative plate motion, A is the total length of the seismic zone, and D is the thickness of the brittle crust. If 2v = (0.85±0.15) × 1.94 cm/yr, D = 10–15 km, ana A = 75–85 km, then m˙0 = 1.0–2.5 × 107 m3/yr, comparable to the rate of geometric moment release in large historical earthquakes in the SISZ for the last several centuries. We conclude that historical seismicity in the SISZ can be attributed to left-lateral shear accumulation across the seismic zone at a similar rate throughout historical times as during the years 1986–1992.

The marine impact crater at Lockne, central Sweden

Abstract The Lockne crater (63°00′20″N, 14°49′30″E), dated as Middle Ordovician, Caradoc, chitinozoan Zone of Lagenochitinal dalbyen‐sis, conodont Subzone of Baltoniodus gerdae, consists of a 7.5 km wide inner crater that is surrounded by a 3 km wide outer crater. The inner crater has been identified by drilling on both sides of Lake Lockne. Its floor is strongly crushed crystalline basement with a variable thickness of monomictic breccia. It is filled with more than 200 m of resurge deposits and nearly 100 m of secular, post‐impact Dalby Limestone, parts of which are covered by a thin Caledonian nappe outlier. The outer crater formed at the top surface of the crystalline basement that it exposes in a state of strong crushing and with a patchy cover of monomictic breccia. The presence and structure of the outer crater suggest that Lockne may be a nested crater with the basement acting as a strong, lower layer, and the sediment cover and the water acting together as a weak upper layer, the thickness of whi...

Continuous deflation of the Askja caldera, Iceland, during the 1983–1998 noneruptive period

The Askja volcano at the spreading plate boundary in north Iceland consists of nested calderas, the latest formed in an eruption in 1875. Several eruptions have occurred since in Askja, the most recent in 1961. Precise leveling has been conducted yearly at Askja since 1983. In 1993, a dense GPS network was measured in and around the Askja caldera consisting of more than 20 points, and we remeasured this complete network for the first time in 1998. Askja subsided during the period from 1983 to 1998. Observed deformation fits broadly with a “Mogi” point source model with best fitting location near the center of the main Askja caldera (65.0448°N, 16.7805°W) at a depth of 2.8 km. From 1983 to 1991 the yearly subsidence rate between the end points of a leveling profile decayed gradually from ∼10 mm/yr to an average of ∼7 mm/yr in the 1991–1998 period. Total subsidence at the Askja center in the 1983–1998 period is at least 75 cm, and the integrated volume of surface subsidence is ∼0.037 km3. This period has been a “quiet” period at Askja with no eruptions, large earthquakes or known dike injections. Such a high rate of “background” deformation has not been observed at other volcanoes in Iceland. The eruption of a neighboring volcano in 1996 has not resulted in a modified deformation pattern at Askja, indicating no pressure connection at depth between the two systems. Solidification may account for part of the observed contraction and subsidence in Askja.

Glacio‐isostatic deformation around the Vatnajökull ice cap, Iceland, induced by recent climate warming: GPS observations and finite element modeling

[1] Glaciers in Iceland began retreating around 1890, and since then the Vatnajokull ice cap has lost over 400 km 3 of ice. The associated unloading of the crust induces a glacio-isostatic respo ...

Climate effects on volcanism: influence on magmatic systems of loading and unloading from ice mass variations, with examples from Iceland

Pressure influences both magma production and the failure of magma chambers. Changes in pressure interact with the local tectonic settings and can affect magmatic activity. Present-day reduction in ice load on subglacial volcanoes due to global warming is modifying pressure conditions in magmatic systems. The large pulse in volcanic production at the end of the last glaciation in Iceland suggests a link between unloading and volcanism, and models of that process can help to evaluate future scenarios. A viscoelastic model of glacio-isostatic adjustment that considers melt generation demonstrates how surface unloading may lead to a pulse in magmatic activity. Iceland’s ice caps have been thinning since 1890 and glacial rebound at rates exceeding 20 mm yr−1 is ongoing. Modelling predicts a significant amount of ‘additional’ magma generation under Iceland due to ice retreat. The unloading also influences stress conditions in shallow magma chambers, modifying their failure conditions in a manner that depends critically on ice retreat, the shape and depth of magma chambers as well as the compressibility of the magma. An annual cycle of land elevation in Iceland, due to seasonal variation of ice mass, indicates an annual modulation of failure conditions in subglacial magma chambers.

Geology of the Early Palaeozoic Lockne impact structure, Central Sweden

Abstract The diameter of the crater at Lockne as seen on topographic maps is 7–8 km. The Tandsbyn Breccia, the principal evidence of impact, consists of strongly crushed local basement. Its distribution follows the crater margin. On the north margin, basement granite rests on an inverted sub-Cambrian erosion surface on the lowermost Middle Cambrian. Tangential faults occur at the periphery of the structure. After the impact occurred under the sea during the middle Ordovician, there was a resurgence of ejecta-loaded sea water which deposited a chaotic breccia (Lockne Breccia), which has the appearance of a debris flow. The Lockne Breccia, together with a less coarsely grained turbidite (“Loftarsten”), which lies immediately above it, predominantly consist of clasts of lower to middle Ordovician limestone and, mainly subordinate, inclusions of Tandsbyn Breccia and isolated clasts of crystalline basement. Fragments of impact melt are important components of the turbidite. A protective cover of sediments formed after the impact, is, in turn, overlain by an outlier of a Caledonian overthrust nappe, which occupies the central and topographically deepest part of the crater. Shocked quartz has been identified in the Loftarsten.

2 Katla and Eyjafjallajökull Volcanoes

Abstract The Katla volcano is covered by the Mýrdalsjokull ice cap and is currently one of the most active volcanoes in Iceland. It has erupted twenty times the past 1,100 years. The neighbouring volcano Eyjafjallajokull has erupted twice, simultaneously with Katla. As glaciers cover both volcanoes, their eruptions are phreato-magmatic by nature. The volcanoes are located directly south of where surface expressions of the rift cease. Seismically, Katla is one of the most active volcanoes in Iceland, showing an annual cycle in activity, observed from at least 1960 and less pronounced since 2004. From 1999 to late 2004, GPS measurements revealed steady inflation of the volcano, showing uplift and outward horizontal displacement. Until 1990s, Eyjafjallajokull had been seismically quiet for several decades. Seismic activity there was high in 1994 and again in 1999, related to the emplacement of two intrusions.

Volcano deformation at active plate boundaries: Deep magma accumulation at Hekla volcano and plate boundary deformation in south Iceland

Volcano deformation at active plate boundaries: Deep magma accumulation at Hekla volcano and plate boundary deformation in south Iceland

A two‐magma chamber model as a source of deformation at Grímsvötn Volcano, Iceland

Grimsvotn Volcano is the most active volcano in Iceland, and its last three eruptions were in 1998, 2004, and 2011. Here we analyze the displacement around Grimsvotn during these last three eruptive cycles using 10 GPS stations. The observed displacements in this region generally contain a linear component of tectonic and glacio-isostatic origin, in agreement with the previously estimated values of plate motions and vertical rebound. Larger amplitude deformation observed close to Grimsvotn at the GFUM continuous GPS station clearly reflects a major volcanic contribution superimposed on a tectonic component. We estimate and subtract the tectonic trend at this station using regional observed displacement. The direction and pattern of the residual volcanic displacement (for coeruptive and intereruptive periods) are consistent for all three of these eruptive cycles. The posteruptive inflation is characterized by an exponential trend, followed by a linear trend. In this study, we explain this temporal behavior using a new analytic model that has two connected magma chambers surrounded by an elastic medium and fed by a constant basal magma inflow. During the early posteruptive phase, pressure readjustment occurs between the two reservoirs, with replenishment of the shallow chamber from the deep chamber. Afterward, due to the constant inflow of magma into the deep reservoir, the pressurization of the system produces linear uplift. A large deep reservoir favors magma storage rather than surface emission. Based on displacement measured at GFUM station, we estimate an upper limit for the radius of the deep reservoir of ∼10 km.