HR: 10:35h
AN: V52B-02    [Abstracts]
TI: Deep crustal storage of large volume of magma prior to catastrophic eruptions: The role of visco-elastic response to magma accumulation
AU: * Sigmundsson, F
EM: fs@hi.is
AF: Nordic Volcanological Center, Institute of Earth Sciences,University of Iceland, Reykjavik, IS-101, Iceland
AB: Volumes of eruption products in individual volcano-tectonic episodes in Iceland span more than three orders of magnitude (less than 0.2 to over 20 km³), suggesting widely different volumes of magma may accumulate in the crust prior to eruptions. Yet, elastic deformation models suggest that only volumes near the low end of this range can accumulate in magma chambers within a medium assumed to behave in an elastic manner, as the uppermost elastic layer of the crust. An overpressure in a magma storage volume within an elastic medium, created e.g. by inflow of new magma, will cause sudden displacement of the magma chamber walls in accordance with Hookes law. No further deformation takes place if the overpressure remains constant. A completely different situation arises if the magma accumulates in a ductile medium, e.g. in a Maxwell material. If overpressure is sustained in magma chamber within a visco-elastic material, e.g. by inflow of new magma, then deformation will by sustained over long times. Crust will flow away from the magma accumulation site, at a speed scaling inversely with the viscosity. For a lower crustal viscosity near magma accumulation site of 1x10¹⁸ Pa s (about five times lower than recently inferred average viscosity in Iceland's lower crust/mantle), a crustal magma accumulation time scale of 1-200 years would reproduce three orders of magnitude range in accumulated magma volumes prior to eruptions, if Maxwell rheology is appropriate. A one-dimensional model of Maxwell material demonstrates this effect. Assume the Earth behaves as a spring and dashpot connected in series. Application of sudden pressure causes immediate elastic strain. If pressure is kept constant over time , t, the strain will grow linearly with time. The total strain can be represented as the elastic strain scaled by a factor of (1+t/τ), where τ is the Maxwell relaxation time, the ratio of viscosity over the elastic rigidity. For viscosity about 1x10¹⁸ Pa s and rigidity of about 16 GPa (rigidity inferred from annual cycle in land elevation in Iceland attributed to snow load on Iceland's ice caps), the Maxwell time is about 2 years. This model suggests that sustained overpressure in a magma storage volume over time span up to about 200 years, can be associated with accumulation of 100 times more magma than in the elastic limit, providing one explanation of how large volumes of magma may accumulate in the mid- and lower crust prior to catastrophic eruptions.
DE: 1211 Non-tectonic deformation
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 8419 Volcano monitoring (7280)
DE: 8425 Effusive volcanism
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting