Using laboratory experiments to illuminate volcanic processes Jackie E.Kendrick

 31/10/2019

Using laboratory experiments to illuminate volcanic processes Jackie E.Kendrick

Lava dome in the crater of the Cleveland volcano, Iceland

People have been thriving alongside volcanoes with the rich soils and hydrothermal related them. But the volcanoes are also a threat to those comunities. Effusive volcanoes, with more basaltic lava, are less dangeous as the eruptions usually happen in the same predictable areas, are slover and cover a smaller area. The more dangerous eruptions are from explosive volcanoes with silica rich magmas, rich in volatiles. In these cases, the very viscous magma can form a lava dome blocking the main eruption conduit. In order to predict the next eruption, seismics are widely used to look at the rock fracturingas the new magma is injected. I the talk from Jackie E.Kendric, she pointed out the need to better understant the failure of volcanic rocks in order to predict the failure of lava domes or the presence of new pressure derived faults acting as new conduits during eruptions. Jackie E.Kendrick is a experimentalist and field geologist, researching in the domains of volcanology and geology at the University of Liverpool.

 Jackie pointed out how volcanoes are used in her field as Earth research labs. When the rocks in a volcano, especially near the top or in the lava dome, the addded stress leads to rupture which decreases pressure, allowing the volatiles to form bubbles and drive the magma to an explosive eruption. The aim is to understand the behaviour and cyclicity of this type of eruption. This leads to the study of what processes and properties controle the rupture of materials.

First, the parameters need to be defined as to what environment is considered. Lava domes are highly explosive and, for now, unpredictable. The main concern is that the system is hererogeneous, more data is needed to make accurate models. But for the overall volcano, if assuming an near homogenous material, the properties that controle the rupture of materials are well known.

Unconfined rock strengh test, reaching failure

Experiments were set up in order to understand the elasticic behaviour of the rocks. It is known that the porosity increases the closer the rocks are to failure and that the tensial strength is very low in rocks. The experiment was set up un order to have cyclic loading (load/unload cycles without failure) to probe the elasticity of hte material. This causes damage to the rock over time, which finally leads to the brittle destruction of the rock when it crumbles. This is different to simple tome dependent pressure, which causes creep. In the case of this experiment, the assumprion of the rock being homogeneous holds when looking at the lava dome as it formed in a singular event. The cyclic loading represents the damage done by low magnitude earthquakes (0.1 scale), frequly caused by the injection of new magma.

Strain rates are also important as slow strain rates causes the rocks to flow wereas hig strain rates drive the rocks to failure. As viscous magma tends to keep more fluid under strain, this leads to more fracturing, which in turn can lead to a trigger to explosions.

From in situ tomography analysis, under strain, the magma can flow rapidly as a fracture form. The crystal mush content seems to behave similar to clays at this pressure. This pressure dependance leads to the problem of strain localization. Usially, the colder, more viscous magma is on top, blocked by the lava dome. This can lead to a more plastic behaviour of the crystalizing magma at the base of the lava dome, thus affecting the fractures in the area.

Some questions are still unanswered and research is still going on. One of them is the fact the the friction caused by the earthquakes should provide heating locally. Does it promotes or prevent eruptions? Does it remelts some of the magma? would it make it easier or harder to slip ? The current hypothesis is that it would promote more friction, but it would be highly composition dependant (basaltic composition would flow, but Mt St Helene like lava composition is too viscous).
Additional parametes that could be accounted for in the future are the permeability of the system, the fracture morphology and the fracture healing mechanisms.