External water and ice effect of wind - empirical findings

Transcription

1 External water and ice effect of wind - empirical findings Magnús T. Gudmundsson, Based on work and contributions from Björn Oddsson, Tobias Dürig, Þórdís Högnadóttir, Thor Thordarson, Ármann Höskuldsson, Ingibjörg Jónsdóttir et al. Institute of Earth Sciences University of Iceland and coworkers from IMO, Würzburg, Bari and Edinburgh MeMoVolc Workshop on Mass Eruption Rates Reykjavík 4 May 2016

2 Katla 1918 Gjálp 1996 Grímsvötn 1998 Grímsvötn 2004

3 Volcanic eruptions in Iceland in the last 100 years Red = Explosive Black = Effusive Blue = Subglacial Year Volcano VEI note style of activity Bárðarbunga-Holuhraun 1 (ice) effusive 2011 Grímsvötn 4 ice explosive 2010 Eyjafjallajökull 3 ice explosive/effusive 2004 Grímsvötn 3 ice explosive 2000 Hekla 3 effusive/explosive 1998 Grímsvötn 3 ice explosive 1996 Gjálp (Grímsvötn) 3 ice subglacial-explosive 1991 Hekla 3 effusive/explosive 1983 Grímsvötn 2 ice explosive Hekla 3 effusive/explosive Krafla fires (9 eruptions) 1 effusive 1973 Heimaey 2 effusive/explosive 1970 Hekla 3 effusive/explosive Surtsey 3 ocean explosive/effusive 1961 Askja 2 effusive Hekla 4 effusive/explosive 1938 Gjálp (Grímsvötn) - ice subglacial 1934 Grímsvötn 3 ice explosive Askja (5-6 eruptions) 2 (lake) effusive/explosive 1922 Grímsvötn 3 ice explosive 1918 Katla 4 ice explosive

4 Magma-water interaction: - Phreatomagmatic eruptions - Process of fragmentation well studied (MFCI) Magma fragmentation (MFCI) local/small scale mass ratio - water/magma = 0.03 highest thermal to kinetic energy conversion decimeter scale Overal water/magma mass-ratio (scale m): - Includes water directly involved in MFCI + all other water interacting with magma and tephra - Theoretical considerations on effects of varying mass ratios (Koyaguchi and Woods, 1996) - Important for plume generation

5 Koyaguchi and Woods (1996): - Basic thermodynamics - estimate avalible thermal energy and mass of water evaporated - Maximum steam/magma mass ratio 35-40% - Plumes enhanced up to about steam/magma ratio of 30% - Steam/magma ratio >35-40% leads to plume collapse No actual data on external steam fraction in a plume of a phreatomagmatic eruption Grímsvötn eruption in 2011

6 Grímsvötn 2004 From Oddsson et al. (2012) See also: Jude-Eton et al. (2012) Relatively small basaltic phreatomagmatic eruption in Grímsvötn: 20 million m 3 of magma 5.6+/-1.0 x kg

7 Grímsvötn 2004 Principal information on plume height: C-band radar in Keflavík 260 km away Aircraft observations Plume at different times fluctuations in activity Oddsson et al. (2012)

8 Grímsvötn 2004 Jude-Eton et al. (2012) Oddsson et al. (2012)

9 Eyjafjallajökull 2010 Plume height, mass discharge rate, ice melting and seismic tremor Plume height equation (a scaled version of the Mastin equation) used for mass discharge corrected with fallout data Seismic tremor to first order inverse relationship between mass discharge rate and strength of seismic tremor Tremor apparently highly variable depending on style of activity (effusive/explosive) tremor data from Icelandic Meteorological Office (IMO)

10 Explosive eruption 17 April phreatomagmatic Plume height (from photo): From sea level: 6.5 km Over vent: 5.0 km 500 m 5 km

11 Explosive eruption 24 April magmatic Plume height: From sea level: 3.6 km Over vent: 2.1 km ~2.1 km 200 m

12 Eyjafjallajökull 2010 Out to sea estimated with exponential method On land Total MER km 3 km 3 Phase Ia: April Phase Ib: April Phase II: 19 April-4 May Phase III: 5-17 May Phase IV: May Sum: km 3 Gudmundsson et al Ash density: 1400 kg/m 3

13 Wind effect Eyjafjallajökull 2010 From Gudmundsson et al. 2012

14 Wind effect Eyjafjallajökull 2010 Ia Ib II III IV From Gudmundsson et al. 2012

15 Ia Ib II III IV Eyjafjallajökull Wind effect Deposit mass erupted during different periods compared with mass predicted from simple plume models based on plume height: - During strong wind plume lower - predicts less erupted mass then observed as fallout on ground

16 Conclusions: Phreatomagmatic eruptions intially cold plumes but latent heat release gives back energy to drive plume effect on height minor Grímsvötn 2004 MER plume sustained - pulsating - fluctuating in size Wind effect observed on plume height fallout mass

17 Eyjafjallajökull 2010: Total fallout in indvidual phases heavily dependent on estimates of fallout at sea Using: - Detailed mapping on land - Extrapolation out to sea constrained by info from Europe - ~70 Million m 3 for April MER: x 10 6 kg/s

18 Plume models with wind effects MER ~ 6-10 x 10 6 kg/s (Bursik et al. 2012, Woodhouse et al. 2013) Need tuning predicted total volume erupted April: ~ 1 km 3 - an order of magnitude more than mapped and estimated at sea

19 Plume models with wind effects MER ~ 6-10 x 10 6 kg/s (Bursik et al. 2012, Woodhouse et al. 2013) Need tuning predicted total volume erupted April: ~ 1 km 3 - an order of magnitude more than mapped and estimated at sea

20 Plume models with wind effects MER ~ 6-10 x 10 6 kg/s (Bursik et al. 2012, Woodhouse et al. 2013) Need tuning predicted total volume erupted April: ~ 1 km 3 - an order of magnitude more than mapped and estimated at sea Thickness in Mainland Europe calculated as: ~ 0.1 mm In reality a few particles found here and there

21 Explosive eruptions in Iceland From Thordarson and Höskuldsson, 2008