Rachel Murtagh
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PhD thesis: Investigation into the explosiveness of subaqueous basaltic eruptions Supervisor: James White |
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Introduction
Volcanic eruptions during which water and magma come into contact appear to ignite some of the more violent eruptions known on Earth. The problem remains that detailed conditions of this water-magma interaction is not yet fully understood. Ubiquitous global occurrence of this type of eruption necessitates detailed scientific investigations. Through the research of extinct examples of phreatomagmatic eruptions, records can be produced in a safe manner with the view of compiling a comprehensive and novel dataset for the analysis of water’s role in explosive subaqueous eruptions from shallow to deep water environments. As such, fieldwork has been undertaken in the western U. S., Jeju Island, Korea and Iceland. Samples obtained from the field sites allow geochemical analysis, quantification of lapilli vesicularities, vesicle number density and vesicle shape as well as ash grain morphology. These can be used to unravel degassing and ascent histories and fragmentation processes, respectively. This data can lead to better understanding of Surtseyan eruptive styles, their dynamics and intensity, and will allow us ultimately to identify the role water plays in the explosivity of shallow subaqueous eruptions.
The first site, Pahvant Butte is located in southwest Utah. It is a well preserved tuff cone overlying a mound deposit which consists of lapilli- to bomb/block-sized juvenile basaltic clasts with a minor lithic component. It was originally erupted under ~85m of water into Lake Bonneville approximately 15,300 years ago. A range of clast vesicularity is seen, however bubble density numbers are, thus far, consistently high and comparable to ‘dry’ plinian style eruptive products.
Fig1. Pahvant Butte, Utah, USA. |
Fig.2 Various vesicularities from Pahvant Butte. |
The second site, Black Point is situated in eastern California. Another emergent volcano, it was initially erupted into Lake Russell ~13,000 years ago. Similar to Pahvant Butte, its unconsolidated mound consists of glassy ash and lapilli and is topped by indurated, palagonitized tuff deposits. Complex vesicle textures are often seen within a single lapillus, i.e. transitions from the rapidly quenched rim, commonly with breadcrust surface textures, to highly vesicular interior.
Methods outlined in (Houghton and Wilson, 1989) are used to determine clast densities of the lapilli-size fraction. Scanned and BS images of thin sections from high, low and modal densities allow an analysis of vesicle characteristics as seen in Fig.9 and using techniques discussed in Proussevitch et al. (2007a,b).
Fig.3 Black Point mound sequence. |
Fig.4 Black Point, California, U.S. (photo courtesy of Mandie Leonard-Hintz). |
Fig.5 Vesicle textures & breadcrust rim from site BP S6-12-09_1 |
Fig.6 Various vesicularities from Black Point. |
Fig.7 Ash grains from Black Point and Pahvant Butte used to identify surface features indicative of MFCI fragmentation and morphology analysis. |
The next site, Ilchulbong tuff cone is found on the eastern coast of Jeju Island which in turn is located off the south coast of the Korean Peninsula. ‘Jeju Volcanic Island and Lava Tubes’ was designated an UNESCO World Heritage site in 2007, and attracts millions of visitors every year. Geologic interest in Jeju Island was originally in the natural water resources circulating through subsurface aquifers. Consequent drilling revealed a complex stratigraphic sequence including two generations of hydrovolcanism. Ilchulbong is representative of the later phase of hydrovolcanism which saw a Surtseyan-style eruption on the eastern shoreline emerge from a shallow sea.
Fig. 8 Ilchulbong Tuff, Jeju Island, Korea. |
Fig.9 Sequence of images for vesicle size analysis; example from Ilchulbong. |
Fig.10 TAS diagram showing matrix glass chemistry of Ilchulbong samples. |
Finally, the last site of Helgafell is located in southwest Iceland. These deposits form a hyaloclastite ridge or tindar which was formed beneath a Pleistocene ice sheet believed to have been ~500 m thick. Unexpectedly, the hyaloclastite ridge is not underlain by pillow lavas which we would have attributed to the pressures exerted by ~500 m-thick glacier. Instead we see deposits resulting from explosive volcanism. It is believed that Helgafell could be an analogue to the ridge formed during the 1996 Gjálp eruption where 3 km3 of ice were melted and ultimately resulted in a jökulhlaup (sudden flood).
The comparison of results from all three forms of (shallow) water-magma interaction will be an exciting outcome of this project with the overall aim to produce robust data which may lead to a better understanding of water’s role in explosive subaqueous eruptions.
Fig.11 Helgafell tindar, Iceland. |
Fig.12 View down a gully, Helgafell. |
Publications:
Murtagh, R., White, J.D.L., and Sohn, Y.K., (in prep.). The Ilchulbong tuff cone, Jeju Island, South Korea: recent developments and observations.
Abstracts:
Murtagh, R. and White, J.D.L., 2008a. Investigation into the explosivity of shallow-subaqueous basaltic eruptions (introduction to Black Point, CA and Pahvant Butte, UT). Symposium 3-c, no. 387, IAVCEI, Reyjavik.
Murtagh, R. and White, J.D.L., 2008b. Investigating the explosivity of shallow-subaqueous basaltic eruptions (focus on MFCI fragmentation mechanisms). Joint Geological and Geophysical Societies Conference, Wellington.
Murtagh, R. and White, J.D.L., 2009. Investigating the explosivity of shallow-subaqueous basaltic eruptions (focus on Black Point, CA). Session GMPV5, EGU, Vienna.
Murtagh, R., White, J.D.L., and Sohn, Y.K., (2009). The Ilchulbong tuff cone, Jeju Island, South Korea: recent developments and observations. Joint Geological and Geophysical Societies Conference, Oamaru.
References:
Houghton, B.F. and Wilson, C.J.N., 1989. A vesicularity index for pyroclastic deposits, Bulletin of Volcanology, 51, 451-462.
Proussevitch, A.A.,Sahagian, D.L., and Tsentalovich, E.P., 2007a. Statistical analysis of bubble and crystal size distribution: Formulations and procedures, Journal of Volcanology and Geothermal Research, 164, 95-111.
Proussevitch, A.A. ,Sahagian, D.L. , and Carlson, W.D., 2007b. Statistical analysis of bubble and crystal size distributions: Application to Colorado Plateau basalts, Journal of Volcanology and Geothermal Research, 164, 112-126