Lemaire, Thomas (2025) Lava flow emplacement and lava tube morphologies and mechanisms of formation at Mt. Vesuvius, Italy. [Tesi di dottorato]

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Abstract

Volcanic eruptions have an important impact on the environment, on urbanised or rural areas. Large eruptions can have a worldwide impact. Effusive eruptions are usually harmless to human lives but can cause great damage to infrastructures, cultivated lands and houses as lava flows burn and bury anything they encounter. The distance travelled by lava flows is controlled by many parameters, such as eruptive rate and rheology of the lava. The formation and development of lava tubes, highly efficient lava transport structures, within a flow can greatly increase its distance of emplacement. Somma-Vesuvius is famous for its explosive activity, but the last eruptive cycle from 1631 to 1944 was a long period of open-conduit conditions with effusive to mildly explosive eruptions. Historical documentation describes the presence of lava tubes at Vesuvius, opening the possibility of finding lava tubes on the slopes of the volcano. Vesuvius and its surroundings are highly inhabited and present a high risk of destruction by effusive eruptions. Past events are reminders of such risk. It is thus crucial to better understand the emplacement of lava flows and the formation and development of lava tubes, especially at volcanoes producing silica-undersaturated potassium-rich lavas due to the absence of studies in such context. The work carried out during this Ph.D. aimed at understanding lava flow emplacement, and lava tube formation and development in the lava flow fields present at Vesuvius. A detailed morphometry analysis of the largest lava tube formed in the 1858 flow field was conducted using a combination of Terrestrial Laser Scanner and airborne imagery. Lavas forming the tube were characterised by petrological, geochemical, and textural analyses performed by Scanning Electron Microscope, Electron Microprobe Analysis and synchrotron x-ray microtomography. The stability of the lava tube structure was assessed by Acoustic Emissions tests with a novel signal processing method combining Empirical Mode Decomposition (EMD) and Hilbert-Huang Transform (HHT). The analyses show that the 1858 lava flow field reached its maximum extent quickly and then expanded both laterally and vertically. The size of the flow field appears to be influenced by the topography and the amount of crystals in the lava. Our findings suggest that the lava tube studied was formed by inflation of a lava flow breakout. The tube remained active for a long period and experienced at least twelve drainage and refill cycles. The thermo-redox conditions inside the tube were unstable and did not display a consistent trend during the activity of the tube. Microlitic crystallisation within the lava tube was enhanced at the sides of the conduit due to thermal conditions and shear rates compared to the centre of the tube. The structure of the lava tube is fractured and contains loosened masses in some areas, but the overall structure remains stable. Moreover, a geoitinerary and virtual tour along the 1858 lava flow field was designed to showcase five peculiar volcanic landforms, three lava tubes, an ephemeral vent and a tumulus, that allows people to evidence effusive activity and discover volcanic features formed during lava flow emplacement at Somma-Vesuvius. A website was developed to host the virtual tour, which comprises 360° images and an explicative panel for each site. The website aims to sensibilize people either not used to hiking or with physical impediment about volcanic landforms and their scientific relevance. The pertinence of the geoitinerary has been assessed by a Strengths, Weaknesses, Opportunities and Threats (SWOT) analysis. The geoitinerary was conceptualized for visitors to discover volcanoes and volcanic activity and may contribute to divert tourist traffic away from the volcanic cone, while raising awareness on volcanic processes and volcanic hazards.

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