The work in this thesis concentrates on Katla volcano in southern Iceland. This is one of Europe’s most active volcanoes and its history tells us that it poses many threats to society, both locally (Iceland) and on a broader scale (Europe). Its geological setting is complex, where the effects of a melting anomaly in the mantle and a changing rift geometry, perturb the classical setting of volcanism in a rifting setting. The work has focused on two aspects. The first is the varying distribution of physical properties in the subsurface around the volcano. The second is the distribution of microearthquakes around the volcano. The physical properties that we study are the speeds of seismic waves that reflect variations of temperature, composition and fracturing of the rocks. These can, therefore, help us learn about long-term processes in the volcano. The seismicity gives shorter-term information about deformation associated with current processes. I have applied two tomographic techniques to study Katla’s subsurface to a depth of 5-10 km, namely local-earthquake and ambient-noise tomography. The former makes use of the timing of waves generated by local earthquakes to constrain the earthquakes’ locations and the distribution of wave speed. Here I have concentrated on compressional waves or P waves with a typical frequency content around 10 Hz. With the latter, surface waves are extracted from microseismic noise that is generated far away at sea and their timing is measured to constrain their wave-speed distribution, which then is used to map shear-wave velocity variations. This is done at a typical frequency of 0.3 Hz. I find that the volcano contains rocks of higher velocity than its surroundings, that Katla’s caldera is underlain by low velocities at shallow depth that may be explained by hot or partially molten rocks and that beneath the caldera lies a volume of particularly high velocities that may constitute differentiated cumulates. But, I also find that it is not simple to compare results from such different wave types and discuss a number of complications in that regard. In addition to the well-known microearthquake distribution in the caldera region of Katla and to its west, we have discovered two additional areas of microearthquake activity on the volcano’s flanks, south and east of the caldera. These point to current activity and are, therefore, of interest from a hazard point of view. However, it is difficult to pinpoint their underlying process. Speculation about possible interpretation leads me to hydrothermal processes or small pockets of melt ascending due to their buoyancy or locally enhancing fluid pressure, thereby lowering the effective stress.

Katla volcano, South Iceland, is a bimodal volcanic system hosting an unusual high number of silicic rocks within a basaltic rift setting. The petrogenesis of silica-rich rocks at Katla is controversial and two end-member hypotheses have been suggested. One model involves closed-system fractional crystallisation from a basaltic parental magma and the other emphasises partial melting of hydrothermally altered basaltic crust. To contribute to this debate, this study provides new δ18O data from a suite of high-silica eruptive rocks with complementary basaltic eruptive material from the Katla caldera complex. Petrographically, the rocks of this study display evidence of partial melting indicated by symplectites in xenoliths as well as variously extensive resorption features. The new δ18O data display a range of the basaltic suite between +4.3 and +8.5‰ (n=15), whereas the few intermediate samples range between +4.1 and +5.9‰ (n=3). The silicic xenoliths available in the study range between -4.9 and -2.3‰ (n=4) while the silicic eruptive material ranges between +2.7 and +6.4‰ (n=30). The dominant part of the high-silica eruptive material (97%) is considered as low-δ18O (≤5.0‰) and plots below typical MORB-type magmas (5.7‰ ±0.3). The wide range of δ18O values of the Katla silicic suite is not possible to explain by closed-system fractional crystallisation and to determine the origin of the high-silica Katla rocks, the new δ18O ratios were utilised in fractional crystallisation (FC) and assimilation and fractional crystallisation (AFC) modelling. The results indicate that a two-stage magma evolution process can explain the origin of the high-silica low-δ18O Katla rocks. The early-stage involves differentiation at deep- to mid-crustal levels by fractionation and assimilation processes involving both high and low δ18O crustal materials, allowing production of intermediate to felsic magmatic compositions. Following this deep differentiation, evolved magmas experience δ18O modification at shallow crustal levels by assimilation of low-δ18O hydrothermally altered crustal material or interaction with low-δ18O waters. Such a two-stage magma evolution process is consistent with geophysical and geobarometric studies of a two-tiered magma plumbing system beneath Katla that supports a mid- to deep-crustal basaltic magma storage system and simultaneous shallow crustal silicic magma storage. / Vulkanen Katla på södra Island är ett bimodalt vulkaniskt system med ovanligt mycket kiselrika bergarter i en basaltisk riftmiljö. De kiselrika bergarterna från Katla har ett kontroversiellt ursprung och två huvudhypoteser har blivit föreslagna. En modell innefattar fraktionerad kristallisation av en basaltisk ursprungsmagma och den andra modellen betonar rollen av partiell smältning av hydrotermiskt altererad basaltisk skorpa. Som en del i denna debatt bidrar denna studie med ny δ18O-data från en serie kiselrikt eruptivt material med komplimenterande basaltiskt eruptivt material från Katlas kalderakomplex. Det studerade materialet i denna studie påvisar tecken på partiell smältning vilket indikeras av symplektiter i xenoliter samt resorptionstecken av olika omfattning. De nya δ18O-värdena för den basaltiska serien varierar mellan +4,3 och +8,5‰ (n=15), medan de få intermediära proverna varierar mellan +4,1 och +5,9‰ (n=3). De kiselrika xenoliterna i denna studie varierar mellan 4,9 och -2,3‰ (n=4) medan det kiselrika eruptiva materialet varierar mellan +2,7 och +6,4‰ (n=30). Den största delen av det kiselrika eruptiva materialet (97%) har låga δ18O-värden (≤5,0‰) som är lägre än typisk basalt från mittoceanryggar (5,7‰ ±0,3). Det breda intervallet av δ18O-värden hos det kiselrika eruptiva materialet är inte möjligt att förklara genom fraktionerad kristallisation och för att undersöka ursprunget användes de nya δ18O-värdena i modeller för fraktionerad kristallisation (FC) samt assimilering och fraktionerad kristallisation (AFC). Resultatet indikerar att en utvecklingsprocess för magman i två steg kan förklara ursprunget av det kiselrika eruptiva materialet med låga δ18O-värden från Katla. Det tidiga skedet involverar differentiering på medeldjupa till djupa nivåer i skorpan genom fraktionerings- och assimilationsprocesser som involverar material från skorpan med både höga och låga δ18O-värden, vilket bidrar till produktion av intermediära till kiselrika magmakompositioner. Efter denna djupa differentieringsprocess genomgår utvecklade magmor en förändring i δ18O-värden högt upp i skorpan genom assimilation av hydrotermiskt altererat material med låga δ18O-värden eller genom interaktion med vatten med låga δ18O-värden. Denna tvåstegsprocess stöds av geofysiska och geobarometriska studier av ett tvådelat magmasystem under Katla vilka talar för ett medeldjupt till djupt basaltiskt magmalagringsutrymme och ett samtida grunt kiselrikt magmalagringsutrymme.

Relative seismic velocity variations in the Earth’s crust can be estimated by using ambient seismic noise records from a pair of stations. Velocity variations can be caused by stress perturbations in the subsurface. Therefore, information on stress changes in the crust can possibly be retrieved from measured velocity variations in the medium. The measurement is done by comparing the coda part of two cross-correlation functions (CCFs) obtained from ambient noise recordings at two seismic stations; a current CCF that is considered to represent the actual state in the study medium at a specific time and a reference CCF that is considered to represent its average state. Here, the method is applied to the area around Katla volcano in southern Iceland. Katla is an active subglacial volcano and therefore frequent stress changes can be expected to take place there. Long-term changes (of the order of 1-2 months) in relative seismic velocity were estimated over a period of 7 months in 2011. These changes were of the order of about 0.1% for a frequency range of 0.2-1.0 Hz. For this frequency range, surface waves around Katla have been estimated to be most sensitive to velocity changes taking place at depths of about 1-5 km but the sensitivity kernels also have a peak at the surface. The scattering volume (in this case area since we are working with surface waves) depends on both the inter-station distance and how far into the coda the measurements are made. The inter-station distances vary between 5.8 and 23.4 km. Measurements are made 30 s into the coda. This results in scattering areas on the order of 100 km2. The velocity variations have a negative trend over July and over a two month period from the end of August until early November, and a positive trend in August and from early November until the end of the study period in late December. These variations are possibly the results of a combination of changes in the ground water level beneath the glacier, surface load changes and possibly hydrothermal and magmatic pressurization changes. No significant velocity change was estimated in the area associated with the tremor event that took place in early July in 2011. / Seismiska vågor är vibrationer i jordytan som genereras av jordbävningar, explosioner eller andra processer som skakar jorden. Seismiska vågor färdas genom jordens lager och innehåller därför information om jordens inre struktur. Dessa vibrationer kan hämtas med ett känsligt instrument som kallas seismometer. Seismiska vågor färdas med en viss hastighet som beror på hur hård och tung berggrunden är. Förändringar av dessa egenskaper kan därför resultera i förändringar av hastigheten. Dessa förändringar kan orsakas av spänningsförändringar under marken, till exempel trycket i porer eller variationer i vikten ovanför marken, exempelvis från en glaciär. I denna uppsats studeras förändringar av seismiska vågors hastighet kring vulkanen Katla på Island under 7 månader, 2011. Katla är en av Islands mest aktiva vulkaner och är belägen under en glaciär, Mýrdalsjökull. Detta görs genom att använda omgivande seismiskt brus, som består av seismiska vågor. Bruset genereras av tryckvariationer i samband med havsvågor. Bruset analyseras med en korrelationsanalys som bland annat isolerar spridda vågor från detaljer i strukturen och variationer av dessa med tid kan användas til mätningar av hastighets förändringar. Resultaten tyder på förändringar i relativ seismik hastighet avstorleken 0.1% som varar i en till två månader. Hastigheten minskar i juli och över en tvåmånadersperiod från slutet av augusti till början av november, men ökar i augusti och från början av november till slutet av december. Dessa variationer kan ha orsakats av en kombination av förändringar i grundvattennivån under glaciären, förändringar i glaciärens vikt och magmatiska processer. Inga tydliga förändringar i samband med sekvenser av små jordbävningar som ägde rum i början av juli 2011 kunde observeras frånförändringar i relativ seismisk hastighet runt Katla.

This thesis attempts to develop our understanding of volcanic magma plumbing systems and the magmatic processes that operate within them, such as fractional crystallisation, crustal partial melting, assimilation, and magma mixing. I utilise petrology, rock and mineral geochemistry, and isotope systematics to seek to improve our ability to forecast the eruptive frequency and style of active volcanoes, an aspect often lacking in current volcano monitoring efforts. In particular, magma reservoir dynamics are investigated from a mineral scale at Katla volcano in Iceland, to a sub-mineral scale at Merapi, Kelud, and Toba volcanoes in Indonesia. The magma plumbing architecture of Katla volcano on Iceland is explored in the first part of this thesis. Crystalline components within tephra and volcanic rock preserve a record of the physical and chemical evolution of a magma, and are analysed through oxygen isotopic and thermobarometric techniques to temporally constrain changes in reservoir depth and decode the petrogenesis of the lavas. We find both prolonged upper crustal magma storage and shallow level assimilation to be occurring at Katla. The results generated from combining these analytical strands reveal the potential for unpredictable explosive volcanism at this lively Icelandic volcano. The second part of this thesis examines the magma plumbing systems of Merapi, Kelud and Toba volcanoes of the Sunda arc in Indonesia at higher temporal and petrological resolution than possible for Katla (e.g., due to the crystal poor character of the rocks). For this part of the thesis, minerals were analysed in-situ to take advantage of sub-crystal scale isotopic variations in order to investigate processes of shallow-level assimilation in the build-up to particular eruptions. We find that intra-crystal analyses reveal an otherwise hidden differentiation history at these volcanoes, and establish a better understanding as to how they may have rapidly achieved a critical explosive state. The outcomes of this thesis therefore deepen our knowledge of evolutionary trends in magma plumbing system dynamics, and highlight the importance of understanding the geochemical processes that can prime a volcano for eruption. Lastly, I emphasise the vital contribution petrology can make in current volcano monitoring efforts.