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The effects of magmatic systems maturation on geophysical signals recorded in volcanic areas.

Periodic Reporting for period 1 - MAGMATS (The effects of magmatic systems maturation on geophysical signals recorded in volcanic areas.)

Reporting period: 2018-05-01 to 2020-04-30

Volcanic eruptions are fed from magmatic plumbing system composed of multiple magma reservoirs and conduits. A major challenge in volcanology is determining how much liquid magma is present beneath volcanoes.
Magmas are a mixture of liquid, gas, and crystal. When a magma is transferred from a deep and hot reservoir to a shallower and cooler reservoir it slowly cools down, crystallizes, and releases gases. When a magma contains more crystals than liquid, it becomes locked and cannot erupt.
Magmas have physical properties that differ from solid rocks. For example, seismic waves propagate more slowly in magmas and magmas are better conductor of electricity than solid rocks. Those variations in physical properties can be detected with geophysical techniques. They are used to produce tomographic images of the crust beneath volcano and try to detect magma reservoirs. Unfortunately, it is difficult to distinguish between a magma relatively poor in gas and rich in liquid that is able to feed eruptions from a magma poor in liquid and rich in gas that is locked underground.
Another way to detect a magma reservoir is by measuring ground deformation. When a magma reservoir is pressurized because of the arrival of new magma from depth or because large volumes of gases are released by magma crystallization, the ground surface above the reservoir is uplifted. Here again, it is difficult to distinguish the effect of gas transfer from the effect of magma transfer.

The aim of this project was to model the exsolution of gas from a solidifying magma chamber and to determine how geophysical signals evolve with the maturation of a magmatic system. Our objective was to improve our ability to interpret geophysical images and to evaluate volcanic risks.

We have produced new algorithms to simulate the release of gas during a magma body growth and solidification. Our simulations shows the decoupling between gas and magma and that gas accumulate in the solidified part of the magma body. Sudden gas release explains episodic ground deformation recorded in volcanic areas.
Magma system maturation and ground deformation
In collaboration with J. Biggs from University of Bristol, we explored a new conceptual model to explain the pattern of ground deformation in volcanic areas. According to this model, series of magma pockets are aligned along faults. As the magmatic system matures, the rock between the pockets heats up and soften. In a first stage, when rocks are still cold, the deformation induced by magma pressurisation is localised above the magma pockets. In a second stage, the rocks between magma pockets are hot and becomes viscous and deformations broaden. In a final stage, magma pockets connect, form a large magma chamber, and the deformation covers the whole magmatic area.
We used numerical simulation to calculate that after 10s of thousand years the effect of rock softening affects the ground deformation. The calculations also showed that only magma pockets that are less than 1 km apart do fully converge. Thus, connexion of magma chambers either involves chambers that are separated by a relatively thin screen of country rock or mechanical rupture of the country rock plays a role. These results are published in the Philosophical Transactions of the Royal Society (

Volatiles release
To model the release of water from a growing and solidifying magma chamber, we used data from petrological experiments to produce an algorithm that calculates temperatures, and fractions of liquid, crystals, and water. We implemented in the simulation the transport of the exsolved water. In part of the magmatic system where crystals are in contact and form a solid network, bubbles are rapidly transported through channels, whereas, where the magma is poor in crystals, the bubbles are suspended in the liquid and their velocity is so low that they can be considered immobile. Bubbles ascending through channels are arrested when they encounter liquid magma or solid rock. During solidification, water concentrates at certain levels with both the concentration value and the concentration depth depending on the magma emplacement rate. The most interesting result is the formation of layers of gas in the crystal mush and solidified magma that surrounds the active magma chamber. The pattern of those gas layers depends on the composition of the magma and on its initial water content. A magma that crystallises over a narrow range of temperatures is less able to concentrate gas than a magma that crystallises over a wide range of temperatures. These results have been submitted for publication to Lithos and have been part of a keynote presentation at the Hutton symposia that took place in Nanjin, China, in 2019. We also calculated the flux of water released by the magma chamber. The data indicate that the volume added by gas exsolution does not suffice to cause an overpressure that would rupture the wall of the chamber. However, if gas-rich layers are connected, their buoyancy might cause chamber failure and trigger an eruption. Another paper that explores the role of volatiles in the failure of magma chambers is in preparation.

Enclave deformation
In the context of the collaboration with the volcanology team of my host institute, enclaves deformation was modeled. Granitic magma commonly contains enclave of mafic magmas. Those enclaves are often deformed. The intensity of the deformation was believed to be linked to the contrast in viscosity between the granitic and the mafic magma. Our study that uses computational fluid dynamics with discrete element modelling suggests that it is not the case. According to the simulations, the deformation is linked to the formation of force chains between the crystals and is independent of viscosity. It is an important result because enclave deformation is used to infer the cooling rate of magma, hence the dynamics of magma chambers. These results have been published in Journal of Volcanology and Geothermal Research (
The classical view of a degassing magma chamber was that bubbles rise through the magma and accumulate at the top of the chamber. In contrast, in our simulations, the transport of water takes place in the corona of mush around the chamber and water gets trapped in the solidified part of the magma body. Moreover, we show that the concentration of gas varies dramatically with the magma composition. These results have potential impact on the models of generation of mineral deposit. Minerals like copper are transported by volatiles through veins. Our model suggest that veins initiate in the highly crystalline part of the chamber. Another potential impact of these results regards volcanic hazards. It had been proposed that gas release induces a magma chamber overpressure that could trigger an eruption. Our results do not support this hypothesis; gas exsolution is too slow for the overpressure to be significant. However, there may be circumstances where channels of gas do connect in the mush and their buoyancy fractures the wall rock rock and initiates an eruption. Finally, the simulations indicate that concentration of gas around the magma chamber may lead to an overestimation of magma chambers size if based on geophysical images.
Conceptual model showing the coalescence of thermal aureoles (Biggs and Annen, 2019)
Exsolved water in magma chamber of different compositions (Annen and Burgisser, in review)
Numerical results: temperature evolution around two aligned magma chambers (Biggs and Annen, 2019)