Gravity anomalies in Iceland's northern volcanic zone

Hughes, Sion (2015). Gravity anomalies in Iceland's northern volcanic zone. PhD thesis The Open University.



Iceland’s northern volcanic zone (NVZ) contains a section of a spreading rift and numerous associated volcanic landforms. In this study, gravity and elevation data are presented for over 700 measurement stations, covering an area of 10,000 km2. The gravity anomalies in the data were used to investigate the location, extent, and distribution of volcanic bodies in the shallow crust, especially at the central volcanoes of Askja and Krafla, and to resolve pathways of dyke swarms and hypothesised plumbing connections between central volcanoes. Two different modelling approaches were used: GRAVMAG, to model the sub-surface in 2.5-D, along profiles across specific parts of the NVZ, and GROWTH 2.0, to invert the data and produce 3-D models. Where both methods were in broad agreement, weight was lent to the credibility of the model.

The results confirmed the presence of the regional gravity field, yet more than doubled its previously measured range (Eysteinsson and Gunnarsson, 1995) due to both improved sampling density and the availability of more accurate digital terrain models. Large positive anomalies were seen beneath Askja and Krafla, modelled as smaller, shallower bodies (~ 80-100 km3) beneath sites of recent eruptive activity, with larger, deeper bodies (~450-1750 km3) below them. Small gravity anomalies across the NVZ generally modelled less reliably due to low resolution in GROWTH 2.0 models and possible signal artefacts in GRAVMAG. Where possible, surface geomorphology guided and reinforced the GRAVMAG modelling, reducing this problem.

Gravity profile data in the Myvatn area showed a number of short-wavelength gravity anomalies, which in conjunction with clues from surface geomorphology, translated well into GRAVMAG models of sub-surface dykes, which could continue to be traced after surface features were no longer visible. Plumbing links between central volcanoes could not be resolved, both for model resolution limitations and lack of access to critical parts of the study area.

Bodies suggestive of elongated magma chambers were modelled in both the west of the Krafla area, and at Herdubreiðartogl, features normally associated with high spreading rates. If genuine, and not artefacts, then these previously unknown bodies suggest that spreading rates here, in the past, have been higher than previously supposed, which may affect the development of central volcanoes. As the GROWTH 2.0 results for these features at the Krafla area were ambiguous, future research incorporating an enhanced survey network in that area is the suggested solution.

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