Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Experimental techniques on pumice

The aim of the project was to build a broad experimental database on the eruptive behaviour of volcanoes. The concept included an integrative field - laboratory - numerical model feedback approach. All segments were realised and due to the feedback experiments were realised following the results of the models and vice versa. Our results will have important implications for the mechanism of initiation and cessation of volcanic eruptions.

The experimental results include magma properties as rheology a parameter necessary for models on flow and prediction of lava flows. With the characterisation of the fragmentation behaviour the strength of porous magma its fragmentation efficiency as well as the speed of the fragmentation process were analysed. These unique experimental data allowed a review on existing models.

Our threshold curve can be used to predict how much overpressure is required to start explosive fragmentation of magma of known porosity. Current techniques for the geophysical and geochemical monitoring of active volcanoes provide reliable estimates of the pressurization state and depth of magma. This information serve as an input for numerical models able to calculate the porosity and pressure of the magma. In a scenario of expected dome collapse, sector collapse, or even in dyke opening events or vulcanian blasts.

This approach, in combination with field density measurement of large pyroclasts, may help to better understand the local fragmentation dynamics of heterogeneous domes and conduit fillings. Besides the rocks pore volume and the gas pressure differential, the efficiency of degassing, i.e. the rock’s permeability, plays an important role considering the fragmentation behaviour of volcanic rocks. If the permeability is high enough, no overpressure develops; whereas at low magma permeability, gas flow is hindered and overpressure develops that may eventually lead to bubble wall failure and fragmentation. The degassing may occur through cracks or a network of interconnected bubbles. We realised during the experiments, that the permeability (k) is just a characterisation as it is measured in a laminar flow regime. The process of degassing during an eruption or violent degassing event is a forced turbulent flow that behaves different from the numerically used systems.

Experimental data on the density controlled fragmentation speed in combination to textural observations on Montserrat merged to the theory of conduit implosions were the importance of density variations throughout the conduit could be demonstrated. An urgent need for a real 3D-numerical model is revealed were through a similar feedback mechanism as in the present study the physical background is experimentally counterchecked.

Reported by

Ludwig-Maximilians-Universität München
Theresienstr. 41/III
80333 München
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