Internal structure of Surtsey and surroundings

Constraints from a dense aeromagnetic survey

In this paper Sara Sayyadi et al. analyze the results of an aeromagnetic survey flown in October 2021 over a 60km^2 area covering Surtsey and its surroundings. The purpose of the survey was to explore internal structures of five basaltic vents active at different times over 3.5 years of eruptive activity, from 1963-1967. Researchers used 2D spectral analysis, Euler deconvolution (a semi-automatic interpretation method that is frequently used with magnetic and gravity data), and 3D forward modeling to analyze the results of the survey. These methods of analysis provide evidence for the locations and dimensions of the rock bodies producing magnetic anomalies, as well as the existence of a field of pillow lava on the seafloor. Several magnetic anomalies were noted, with one over the submarine edifice of Surtla, possibly due to the preservation of a dyke intrusion of basalt.

The October 2021 aeromagnetic survey. The schematic sketch at the bottom shows the aircraft’s altitude and clearance.

The aeromagnetic survey utilized line spacing of 200m and a flying altitude of 90m a.s.l. The strongest anomalies, or magnetic variations, detected by the survey were caused by the 30-100m thick sub-aerially erupted lava field located within the southern sector of Surtsey and formed over two eruptive occurrences, one from 1964-1965 and one from 1966-1967. After using 2D spectral analysis and Euler deconvolution the researchers found indications that the causative bodies of anomalies of Surtsey are located within the uppermost 300m of the seafloor. The horizontal dimensions of the causative bodies are similar or smaller than their depth.

Depth estimation relative to the ocean surface for causative bodies according to 3D-Euler deconvolution of the reduced to the pole magnetic map for different structural indexes. (a) sheet (resembling sills or large dykes); SI = 1, (b) vertical pipe (resembling e.g., eruptive conduits); SI = 2, and (c) sphere (bodies of spatial extent small compared to their depth, e.g., knobs of pillow lava or intrusions); SI = 3. d) depth distribution of sources for each SI.

Analysis of the 3D forward modeling of the surveyed area shows an absence of pillow lava and effusive activity in the opening phases at all vents. The modeling also supports the existence of the 10-20m thick pillow lava field apparently fed by channelized intrusive flow of magma through the submarine part of a lava delta. This would have taken place during the earlier eruptive stages (May-July 1964).

The study also found a scarcity of significant magnetic bodies of intrusive basalt within the edifices. This supports magma fragmentation having dominated the submarine eruptions from the beginning of activity. One small magnetic anomaly was observed over the submarine edifice of Surtla, which was built over ten days in 1963-1964. The anomaly is consistent with weak to moderate explosive activity. This activity could have allowed a dyke to be preserved within a submarine tephra mound. In contrast, more violently explosive activity was observed at other vents, which might have destroyed any additional dykes that could have been resolved magnetically. Beneath the southernmost Surtsey vents there are indications of magnetized volcanic rocks. The age of these rocks is unknown but they do predate the Surtsey eruption.

3D forward models of the Surtsey complex. a) Map of the observed residual field with 630 nT subtracted. b) Model map of the survey area with outlines of the model bodies contributing significantly to the anomalies observed for the Surtsey subaerial lavas, J´olnir knob and pillow lava field (south side of the Surtsey)

Citation

Sara Sayyadi, Magnús T. Gudmundsson, James D.L. White, Thorsteinn Jónsson, Maxwell C. Brown, Marie D. Jackson, Internal structure of the volcanic island of Surtsey and surroundings: Constraints from a dense aeromagnetic survey, Journal of Volcanology and Geothermal Research, Volume 451, 2024, 108096, ISSN 0377-0273, https://doi.org/10.1016/j.jvolgeores.2024.108096.

The October 2021 aeromagnetic survey. The schematic sketch at the bottom shows the aircraft’s altitude and clearance.

Depth estimation relative to the ocean surface for causative bodies according to 3D-Euler deconvolution of the reduced to the pole magnetic map for different structural indexes. (a) sheet (resembling sills or large dykes); SI = 1, (b) vertical pipe (resembling e.g., eruptive conduits); SI = 2, and (c) sphere (bodies of spatial extent small compared to their depth, e.g., knobs of pillow lava or intrusions); SI = 3. d) depth distribution of sources for each SI.

3D forward models of the Surtsey complex. a) Map of the observed residual field with 630 nT subtracted. b) Model map of the survey area with outlines of the model bodies contributing significantly to the anomalies observed for the Surtsey subaerial lavas, J´olnir knob and pillow lava field (south side of the Surtsey)