Volcanic Geodesy Flashcards
What is geodesy?
Geodesy is the study of ground deformation
Why does ground deformation occur at volcanoes?
In response to volume and pressure changes in the crustal plumbing system e.g. due to magmatic intrusion, cooling, crystallisation
Do all volcanoes erupting experience ground deformation?
NO! Just as with seismicity volcanic eruption can happen without any deformation being detected, whilst on the other hand, ground deformation can occur within an eruption occurring
What can ground deformation be measured by?
A range of instruments
E.g. tilt and strain meters, GPS, radars
Outline on the ground geodesy (tiltmeters)
Using tiltmeters - which have spirit/liquid levels to see how the ground is changing - these should ideally be buried to remove noise
Tilt cycles relate to inflation and deflation phases - inflation is conduit pressurisation, where deflation occurs when there is extrusion into a dome
Cyclicity in the case of Soufriere Hills was so regular that volcanologists could travel into the field and sample gas since they were so confident that it would be safe (Voight et al., 1999)
Outline on the ground geodesy (strainmeters)
Oil-filled rings that are sensitive to dilation or compression around them
Borehole strainmeters can detected changes up to 10 ^ -12 and can be deployed at a greater distance than other instruments
E.g. Hekla - in the 1991 eruption, 5 strainmeters were located 15-45km away from the volcano and recorded marked strain over a 30-minute period during the propagation of the dyke to the surface (Linde et al., 1993) - the 2000 Hekla eruption was then forecast using this information - warnings announced the eruption would begin in 15 minutes, and it began in 17 (Sparks, 2003)
Outline the use of GPS
Global Position Systems
GPS tracks the motion of ground stations relative to a known satellite location
You can track the increasing distance between given GPS sites e.g. an increase between two GPS sites occurred due to a mantle surge at Kilauea (Poland et al., 2012)
What is InSAR?
Interferometric Synthetic Aperture Radar
Illuminates the Earth’s surface with microwave radiation and captures the returning signals to provide an image of terrain (this is SAR) - used to create digital elevation model - InSAR is then created by cross-analysing multiple DEMS in order to represent movement of the ground
How does InSAR allow the study of volcanic processes?
Using assumptions about the physical nature of the deforming source and the surrounding country rock it is possible to calculate source volume and depth of given surface deformation
What are the positive of InSAR? (4)
Sparks et al., 2012
1) Very useful for remote volcanoes where detailed topographic information may be detailed
2) Captures images regardless of weather
3) Very large spatial coverage (helpful for inaccessible areas, and ranges of volcanoes) - if a specific volcano shows unrest a ground team can then be sent out
4) Do not need a ground component (as opposed to GPS)
What are the limitations of InSAR?
Sparks et al., 2012
1) Various processes can destroy coherence e.g. erosion, deposition, changes in vegetation, soil moisture, snow cover
2) Must convert raw deformational data into useful volcanological insights - computational difficulty
3) Models do not yet capture the full complexity of crustal responses to magma pressurisation - e.g. rely on the Mogi model of a pressure source in an elastic half-space: but the crust becomes ductile at shallow depths (5-10km) and therefore purely elastic models are not adequate
Ji et al., 2018
Use InSAR to monitor the Kamchatka Peninsula, one of the most active volcanic arcs in the Pacific Rim
Due to logistical difficulty, previous instrumentation was difficult
Study deformation from 2000-2010 and recognise patterns of magma intrusion and reservoir formation
Rivera et al., 2017
Use InSAR to study Cotopaxi volcanic eruption in 2015 (Ecuador)
A volcano threatening 300,000 people
Find deformation in months leading up to eruption
Key indicator for predicting future eruptions perhaps
Name three other measurement types
Infrasound, thermal emissions, water emissions
