A new model for the fracture energy budget of phreatomagmatic explosions

The fragmentation of magma and of the hosting country rocks is a major process in explosive eruptions. It is important to quantify the mechanical energy needed for fragmentation in order to assess the physical processes of this volcanic phenomenon. This paper presents a method to calculate the fragmentation energy of country rock using granulometry data of a typical phreatomagmatic Eifel maar volcano explosion. The total fracture area of country rock fragments in one tephra layer was quantified and related to the critical fragmentation energy of these country rocks. The rock parameters critical shear stress and critical fragmentation energy were determined experimentally, whereas the pre-volcanic crack inventory was measured in the field. The paper concludes with the calculation of the energy balance (i.e. partitioning of thermal energy into kinetical energy and mechanical energy of the fragmentation) of one Eifel maar volcanic explosion.