During and after volcanism has ended, volcanic ocean islands are progressively reduced in elevation by subsidence, giant landslides, coastal erosion, fluvial erosion and hillslope landsliding, ultimately creating shallow banks and guyots. Darwin (1842) originally suggested that coral atolls originated from the fringing reefs of volcanic islands when their underlying volcanic edifices subside, a process that is now known to be common (Ramalho et al., 2013). Geophysical data have revealed the importance of giant landslides (Mitchell et al., 2002). Time-lapse coastline measurements together with modelling of the shapes of the rock platforms left by erosion have shown the importance of coastline retreat (Quartau et al., 2010; Zhao et al., 2019). Although the ridge and valley morphologies of volcanic islands suggest that fluvial erosion and hillslope landsliding are also important (Macdonald et al., 1983), we lack a general quantitative model of the rates of consequential landscape denudation. Widely available remote-sensing data now make such modelling possible.
In this project, you will develop a general model of landscape denudation for volcanic ocean islands, interpret it with climate data and use it to explain variations in seabed geomorphology revealed by new multibeam echo-sounder datasets collected around many of the Atlantic islands. For example, volcanic islands within trade wind zones commonly have asymmetric geometries with wider shelves and steep hillslopes on their windward sides (Menard, 1986; Mitchell et al., 2003b). While greater wave erosion clearly plays a role in creating these asymmetries, it is unclear how much of their evolution occurs because of enhanced rainfall and hillslope landsliding.
Remote-sensing data will be used to develop time-series of landslide occurrences from their appearance in images from stripping surface vegetation (Goswami et al., 2011). The intention will be to develop a large database of landslide incidences across as many of the temperate and tropical Atlantic islands as possible. Estimates of sediment fluxes to the coasts from this method will be compared directly to adjacent submarine geomorphology from multibeam sonar data (e.g., Goswami et al., 2016). The excavation of deep valleys commonly leaves behind flat intact areas that are original parts of the volcanoes before erosion - "planezes" (Ollier, 1984; Ollier and Terry, 1999). You will be able to use the new global topography datasets derived from the SRTM and ASTER to work out how much volume has been eroded between planezes.
Working with Portuguese external supervisors Ricardo Ramalho and Rui Quartau you will have access to extensive multibeam and other geophysical data around the Azores, Madeira and Cape Verdes islands. Further new multibeam sonar datasets collected for biological habitat mapping around Ascension, St. Helena and Tristan da Cunha islands have also been made available for this project by researchers from the British Antarctic Survey.
Through carrying out this project, you will gain some knowledge of data manipulation for applied as well as academic research. The project should be suitable for students wishing to develop or extend their capabilities in data science, computing, remote-sensing, marine environmental assessment, marine geophysical surveying and research. It has the potential to lead to careers in a range of academic and commercial work in geophysical and hydrographic survey, and environmental consultancy.
Darwin, C., 1842. The structure and distribution of coral reefs. Smith, Elder and Co., London.Menard, H.W., 1983. Insular erosion, isostasy, and subsidence. Science 220, 913-918.
Goswami, R., Mitchell, N.C., Brocklehurst, S.H., 2011. Distribution and causes of landslides in the eastern Peloritani of NE Sicily and western Aspromonte of SW Calabria, Italy. Geomorph. 132, 111-122.
Goswami, R., Mitchell, N.C., Brocklehurst, S.H., Argnani, A., 2016. Linking subaerial erosion with submarine geomorphology in the western Ionian Sea (south of the Messina Strait), Italy. Basin Res. 29, 641-658.
Macdonald, G.A., Abbott, A.T., Peterson, F.L., 1983. Volcanoes in the Sea. The geology of Hawaii. 2nd. Ed. University of Hawaii Press, Hololulu, Hawaii.
Menard, H.W., 1986. Islands. Scientific American Books, New York.
Mitchell, N.C., Masson, D.G., Watts, A.B., Gee, M.J.R., Urgeles, R., 2002. The morphology of the flanks of volcanic ocean islands: A comparative study of the Canary and Hawaiian hotspot islands. J. Volcanol. Geotherm. Res. 115, 83-107.
Mitchell, N.C., 2003a. Susceptibility of mid-ocean ridge volcanic islands and seamounts to large-scale landsliding. J. Geophys. Res. 108, doi: 10.1029/2002JB001997.
Mitchell, N.C., Dade, W.B., Masson, D.G., 2003b. Erosion of the submarine flanks of the Canary Islands. J. Geophys. Res. 108, DOI: 10.1029/2002JF000003.
Mitchell, N.C., Lofi, J., 2008. Submarine and subaerial erosion of volcanic landscapes: comparing Pacific Ocean seamounts with Valencia Seamount, exposed during the Messinian Salinity Crisis. Basin Res. 20, 489-502.
Mitchell, N.C., Quartau, R., Madeira, J., 2012. Assessing landslide movements in volcanic islands using near-shore marine geophysical data: south Pico island, Azores. Bull. Volcanol. 74, 483-496.
Ollier, C.D., 1984. Geomorphology of South Atlantic volcanic islands Part I: The Tristan da Cunha group. Zeischrift fur Geomorphologie 28, 367-382.
Ollier, C.D., Terry, J.P., 1999. Volcanic geomorphology of northern Viti Levu, Fiji. Austral. J. Earth Sc. 46, 515-522.
Quartau, R., Trenhaile, A.S., Mitchell, N.C., Tempera, F., 2010. Development of volcanic insular shelves: Insights from observations and modelling of Faial Island in the Azores Archipelago. Marine Geology 275, 66-83.
Ramalho, R.S., R. Quartau, A.S. Trenhaile, N.C. Mitchell, C.D. Woodroffe, S.P. Avila, 2013, Coastal evolution on volcanic oceanic islands: A complex interplay between volcanism, erosion, sedimentation, sea-level change and biogenic production. Earth-Science Rev. 127, 140-170.
Zhao, Z., N.C. Mitchell, R. Quartau, F. Tempera, L. Bricheno, 2019. Submarine platform development by erosion of a Surtseyan cone at Capelinhos, Faial Island, Azores. Earth Surface Processes and Landforms.