Journal of Conference Abstracts

Risk Assessment at Soufrière Hills Volcano, Montserrat

P. J. Baxter (pjb21@medschl.cam.ac.uk)

University of Cambridge Clinical School, Addenbrookes Hospital, Cambridge CB2 2QQ.

Small islands are especially vulnerable to natural disasters. In volcanic crises the close proximity of an island's population to an eruptive centre poses a special type of vulnerability. Response to the recent volcanic activity on Montserrat has shown how the risk of death and injury ultimately outweighs the threat of economic or property losses. The development of an alert and warning system to protect the population is therefore of paramount importance, and is essential in closing the gap between technical capacity and warning dissemination and response. The tolerability of risk for a range of health and safety hazards will be discussed, including the potential of the volcanic ash and gas emissions to harm the health of humans and livestock.

STM and LEED Studies of the (001) Surface of Monoclinic Pyrrhotite (4C-type Fe₇S₈)

Udo Becker¹ (becker@ruby.ge.man.ac.uk), Andreas W. Munz², Alistair R. Lennie¹ (Alistair.Lennie@man.ac.uk), Geoff Thornton² & David J. Vaughan¹

¹ Department of Earth Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK.

² Interdisciplinary Research Centre, Surface Science, University of Liverpool, Liverpool L69 3BX, UK.

Monoclinic pyrrhotite (001) surfaces have been examined using Low Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM) following sputtering and annealing the surface at ~300°C.

A phase transition is observed in LEED patterns taken at elevated temperatures. At temperatures above ~300°C, the LEED patterns show only the periodicity of the roughly hexagonally closest packed S atoms, whereas at lower temperatures the ordering of Fe vacancies leads to a 2 x 2 superstructure. This transition is reversible. STM images taken at negative bias voltages exhibit triangular terraces separated by steps. Measured step heights are integer multiples of 2.9 Å, which is one quarter of the 4C Fe₇S₈ unit cell size in the c direction, and corresponds to the distance between two consecutive Fe or S layers. Although the STM images of single terraces appear to have an atomic arrangement corresponding to the ordering within those Fe layers that contain vacancies, bright spots in the images are most likely to represent S atoms, with a vacancy ordering that is induced by the Fe vacancies. This conclusion is supported by experimental STM images, which show a reversed orientation of the surface geometry on successive terraces when separated by steps of 2.9 Å, and by quantum mechanical calculations of STM images that show S 3p-like states at the top of the valence band.

Pyrrhotite (001) surfaces contain triangular etch pits with dimensions ranging from the atomic scale to more than 100 Å across. These are formed by the successive removal of Fe₃S₃ units from the surface. Images taken following exposure of the surface to 6000 L O₂ did not alter the flat terraces but formation of adsorptive structures near steps, especially at corners, was observed.