Ground deformation in volcanic regions can be caused by a variety of processes. Below we briefly explain some of the main causes that commonly show up in our database.
Magmatic
Volcano deformation may occur through the migration, accumulation, degassing, cooling and crystallisation of magma. Measuring resultant surface displacement changes can yield information about the size and geometry of magma reservoirs, as well as their temporal variability.
When magma is intruded into the shallow crust the surrounding rock deforms in order to accommodate the new material, and often results in ground deformation. The inflation of a magma reservoir is a common precursor to an eruption, which may be followed by rapid deflation as magma is erupted. This transport of magma typically occurs vertically (i.e., magma moves towards the Earth’s surface), but can also progress laterally along dykes (e.g., Dabbahu; Wright et al., 2006). More often, rising magma may not reach the Earth’s surface and stall in the crust and form a sill (e.g., Eyjafjallajökull; Pedersen & Sigmundsson, 2006).
Cooling of a magma body can result in volume loss due to crystallisation, consequently leading to ground subsidence. This process typically produces long-term slow subsidence rates, like at Medicine Lake Volcano, USA, where rates have been measured at roughly -10 mm/yr since 1954 (Parker et al., 2014).
Magmatic degassing may cause ground deformation through two main processes: 1) volatiles in magmas, such as water, carbon dioxide and sulphur dioxide, will exsolve as magma rises and decompresses, and 2) as a magma body cools and crystallises, volatiles are continuously partitioned into the melt until saturation is reached and bubbles are formed. Intense periods of magmatic degassing are thought to explain many bradyseismic events at Campi Flegrei (Chiodini et al., 2003).
The amount of ground deformation observed due to magmatic process is highly variable. For example, an injection of magma into a reservoir may be partly accommodated by the compressibility of the pre-existing magma. The injection depth and volume will also have strong controls.
Hydrothermal
Hydrothermal volcano deformation results from the flow of fluids in a hydrothermal reservoir, surface processes associated with hydrothermal features (e.g., hot springs and fumaroles), or from the extraction of geothermal fluids.
Volcanoes are often associated with hydrothermal systems due to high heat transfer from magma storage regions. The flow of fluids within these hydrothermal reservoirs can lead to ground deformation. This has been identified and studied at many volcanoes worldwide, including White Island, New Zealand (Fournier & Chardot, 2012), Campi Flegrei, Italy (Hurwitz et. al., 2007), and Yellowstone, USA. For the same reasons, many geothermal reservoirs are located in volcanic regions and ground subsidence associated with the extraction of geothermal fluids is common.
To complicate matters, hydrothermal systems have the potential to both enhance and suppress magmatic signs of volcanic deformation. Therefore some volcanoes display ‘hybrid-type’ deformation, with both magmatic and hydrothermal components (Gottsmann et al., 2006).
Surface deposits
Volcanoes are built of poorly consolidated rock at much faster rates than they erode, making them inherently unstable. Consequently, shallow processes acting on a volcanic edifice can result in measurable ground deformation. This may include the cooling and contraction of eruptive products and the collapse/movement of a flank or entire edifice.
The most commonly observed deformation associated with surface deposits is the cooling, solidification and compaction of fresh eruptive products, which may result in ground subsidence. This has been observed at Mt. Etna, Italy (Stevens et. al., 2001). Spreading of the edifice, typically under gravity, can also result in deformation. For example, volcano deformation resulting from the spreading of eruptive deposits along shallow or basal sliding planes has been identified at Arenal volcano, Costa Rica (Ebmeier et. al., 2010) and Mt. Etna, Italy (Lundgren et. al., 2001).
Faulting / tectonics
The re-activation of tectonic faults or fractures during volcanic unrest may lead to observed volcano deformation. Features such as caldera ring-faults, eruptive fissures and intra-caldera faults can all become active. This has been recorded at Campi Flegrei, Italy, Tendürek, Turkey (Bathke et. al., 2013) and Sierra Negra, Galápagos (Jónsson et. al., 2005). Like hydrothermal deformation, the underlying cause of faulting may be magmatic in origin.
References:
Bathke, H., Sudhaus, H., Holohan, E. P., Walter, T. R., & Shirzaei, M. (2013). An active ring fault detected at Tendürek volcano by using InSAR. Journal of Geophysical Research: Solid Earth, 118 (8), 4488-4502, doi:10.1002/jgrb.50305.
Chiodini, G., Todesco, M., Caliro, S., Del Gaudio, C., Macedonio, G., & Russo, M. (2003). Magma degassing as a trigger of bradyseismic events: The case of Phlegrean Fields (Italy). Geophysical Research Letters, 30 (8), doi: 10.1029/2002GL016790.
Dzurisin, D. (2006). Volcano deformation: new geodetic monitoring techniques. Springer Science & Business Media, ISBN: 978-3-540-49302-0.
Ebmeier, S. K., Biggs, J., Mather, T., Wadge, G., & Amelung, F. (2010). Steady downslope movement on the western flank of Arenal volcano, Costa Rica. Geochemistry, Geophysics, Geosystems, (11), Q12004, doi:10.1029/2010GC003263.
Fournier, N., & Chardot, L. (2012). Understanding volcano hydrothermal unrest from geodetic observations: Insights from numerical modeling and application to White Island volcano, New Zealand. Journal of Geophysical Research: Solid Earth, 117 (B11), doi: 10.1029/2012JB009469.
Gottsmann, J., Rymer, H., & Berrino, G. (2006). Unrest at the Campi Flegrei caldera (Italy): A critical evaluation of source parameters from geodetic data inversion. Journal of Volcanology and Geothermal Research, 150 (1-3), 132–145, doi: 10.1016/j.jvolgeores.2005.07.002.
Hurwitz, S., Christiansen, L. B., & Hsieh, P. A. (2007). Hydrothermal fluid flow and deformation in large caldera: Inferences from numerical simulations. Journal of Geophysical Research: Solid Earth, 112 (B2), doi: 10.1029/2006JB004689.
Jónsson, S., Zebker, H., & Amelung, F. (2005). On trapdoor faulting at Sierra Negra volcano, Galapagos. Journal of volcanology and geothermal research, 144 (1), 59-71, doi: 10.1016/j.jvolgeores.2004.11.029.
Lundgren, P., Casu, F., Manzo, M., Pepe, A., Berardino, P., Sansosti, E., & Lanari, R. (2004). Gravity and magma induced spreading of Mount Etna volcano revealed by satellite radar interferometry. Geophysical Research Letters, 31, L04602, doi: 10.1029/2003GL018736.
Parker, A. L., Biggs, J., & Lu, Z. (2014). Investigating long-term subsidence at Medicine Lake Volcano, CA, using multitemporal InSAR. Geophysical Journal International, 199 (2), 844-859, doi:10.1093/gji/ggu304.
Pedersen, R., & Sigmundsson, F. (2006). Temporal development of the 1999 intrusive episode in the Eyjafjallajökull volcano, Iceland, derived from InSAR images. Bulletin of Volcanology, 68 (4), 377-393, doi:10.1007/s00445-005-0020-y.
Stevens, N. F., Wadge, G., Williams, C. A., Morley, J. G., Muller, J., Murray, J. B., & Upton, M. (2001). Surface movements of emplaced lava flows measured by synthetic aperture radar interferometry. Journal of Geophysical Research, 106 (B6), 11293–11313, doi: 10.1029/2000JB900425.
Wright, T. J., Ebinger, C., Biggs, J., Ayele, A., Yirgu, G., Keir, D., & Stork, A. (2006). Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode. Nature, 442 (7100), 291-294, doi:10.1038/nature04978.
