Subglacial eruptions

This study performs a reanalysis of the seismicity recorded during the 1996 Gjálp eruption that occurred at NW Vatnajökull, Iceland. The seismicity was recorded by the temporary HOTSPOT network consisting of 30 three-component broadband stations. In total 301 events were identified between 29 September and 12 October and their phases were manually picked. A velocity model was estimated from P-phase travel times by using VELEST. Events were first located using the algorithm NON-LINLOC in order to obtain absolute locations. Precise relative locations were obtained with HYPODD by utilizing catalog and cross-correlation differential travel times. Results show that events clustered first along the SW rim of the Bárðarbunga caldera and later along the Gjálp fissure, with most hypocentral depths located between 3 and 8 km. Waveforms of the 10 largest events that followed the Bárðarbunga earthquake were inverted in order to obtain moment tensors. For all events we found that the deviatoric moment tensor fits the data better than pure double-couple or full moment tensor solutions. Events along the Bárðarbunga caldera exhibited reverse focal mechanisms, while those at the Gjálp fissure exhibited mostly strike-slip faulting. Seismic velocity variations calculated using ambient noise interferometry, point to the possibility that a small subglacial eruption occurred at Bárðarbunga before the main earthquake. This removed melt from the magma chamber causing its roof to collapse, and also resulted in the lateral migration of magma towards the Gjálp fissure. The 2014-2015 Bárðarbunga-Holuhraun eruption shares common characteristics with the 1996 Gjálp eruption, although the size of the latter was much smaller.

Lava lobes are small volcanic features (~ 10 m high) which have been observed at various locations in Iceland and at Nevados de Chillán, Chile. However, the mechanism for the creation of lava lobes remains controversial. It is plausible that lava was emplaced within pre-formed ice cavities which were themselves generated by volcanic fumaroles. This study investigates the feasibility of this theory using an analogue experimental approach. The generation of ice cavities by fumarolic activity was investigated on a laboratory scale and an experiment was designed to observe and quantify the physical processes which took place. All significant heat transfer was constrained, and a model of the melting process constructed, based on experimental observations. The laboratory generated cavities were found to most closely resemble a truncated prolate spheroid in morphology. Lava lobes are commonly conical; various processes are suggested to explain the difference between the field observations and those observed in the laboratory. A cavity growth rate of 1.8 x 10^-4 m^3/s was estimated from experimental data. This equates to a period of 6 x 10^5 s to generate an ice cavity (4 m in height), which is realistic when compared to the ascent time of rhyolitic magma. This study discusses the major contributing factors which affect cavity growth and morphology including: meltwater drainage; debris and impurities within the ice; fumarole dynamics, and ice deformation. Experimental data obtained during this study could be used to calibrate more sophisticated analogue or numerical models of the physical processes of fumarolic ice melting.

Tipe Erupsi Surtseyan adalah sejenis erupsi hidromagmatik, di mana magma berinteraksi dengan air hingga menimbulkan sebuah reaksi eksplosif. Tipe Erupsi Subglacial merupakan tipe erupsi yang terjadi akibat interaksi antara lava dengan es atau glacial. Tipe Erupsi Submarine merupakan tipe erupsi yang terjadi di bawah air. Hampir 75 persen erupsi ini ditemukan di daerah pemekaran Samudera atau mid-oceanic ridges.