Fakultät für Geowissenschaften - Digitale Hochschulschriften der LMU

Summary

The dynamics of magma fragmentation is a controlling factor in the behavior of explosive volcanic eruptions. Fragmentation changes the eruption dynamics from a system of bubbly flow to one of gas-particle flow. To date, the influence of the fragmentation process itself on the eruption dynamics has been largely neglected in eruption models. This is understandable, as the explosive expansion of mixtures of pressurized gases and pyroclasts in volcanic eruptions is a complex process that cannot be studied directly. The dynamics of the gas-particle mixture resulting after magma fragmentation in volcanic eruptions was experimentally investigated in a shock-tube apparatus. We performed fragmentation experiments with natural volcanic samples with diverse porosities (10 – 67 vol. %), different applied pressures (4-20 MPa) and distinct temperatures (room temperature and 850°C). Two different types of experiments were performed. In the first type, we measured the ejection velocity of a plate placed loosely on top of a volcanic sample in order to account for the ejection of the caprock in Vulcanian eruptions. In the second type we simultaneously measured the fragmentation speed and the ejection velocity of the gas-particle mixture in the absence of a plate. In both cases the results are in good agreement with a general model for Vulcanian eruptions based on 1-D shock-tube theory, including magma fragmentation, and considering the specific conditions of each experiment.

Our results show that the fragmentation process plays an important role in the dynamics of the gas-particle mixture. The reasons include the following: 1) the energy consumed by fragmentation reduces the energy available to accelerate the gas-particle mixture; 2) the fragmentation speed controls the pressure available for the ejection of the gas-particle mixture which in turn determines the velocity, density and mass discharge rate; 3) the grain-size distribution produced during fragmentation controls the mechanical and thermal coupling between the gas phase and the particles; 4) the fragmentation process may produce heterogeneities in the concentration of particles. In volcanic eruptions all these factors can affect the eruption dynamics significantly.

The model presented herein is consistent with the experimental results and is capable of describing the dynamics of brittle fragmentation in Vulcanian eruptions and yielding more realistic initial pressures at the onset of fragmentation than previous models. We applied this model to recent Vulcanian eruptions of Popocatépetl and Colima volcanoes (Mexico) and estimated the initial gas pressure required to disrupt the caprock, fragment the underlying magma and eject ballistic projectiles to the observed distances. Further, the model is used in concert with a ballistic model to relate initial pressure and gas content with ballistic range. This coupled model was calibrated and validated with field and video observations of ballistics ejected during different Vulcanian eruptions at Popocatépetl Volcano. The model relates the zones which could be affected by the impact of ballistic projectiles to the initial pressure that can be estimated from seismic and geophysical monitoring, providing valuable information for more refined short-term hazard assessment at active explosive volcanoes.

Finally, a general methodology to delimit the zones that can be affected by ballistic projectiles is presented and applied to Popocatépetl Volcano. Three explosive scenarios with different intensities have been defined according to the past activity of the volcano and parameterized considering the maximum kinetic energy associated with ballistic projectiles ejected during previous eruptions. For each explosive scenario, the ballistic model is used to calculate the maximum range of the projectiles considering the optimum launch conditions. Our results are presented in a ballistic hazard map with topographic profiles that depict the likely

This thesis deals with adsorption, self-assembly, and surface reactions of organic molecules on solid substrates, with the aim to fabricate higher hierarchical twodimensional

(2D) structures. It is of genuine interest in materials science to develop strategies and methods for reproducible growth of extended molecular assemblies with specific and desired chemical, physical and functional properties. The experimental technique used was Scanning Tunneling Microscopy (STM) - an outstanding method to gain real space information of the atomic-scale realm of adsorbates on crystalline surfaces.

The investigated systems are characterized by a complex interplay between adsorbateadsorbate interactions and adsorbate-substrate interactions. In one series of experiments this could be illustrated through self-assembly of hydrogen bonded heteromeric molecular networks on a chemically relatively inert graphite substrate. In this case, van-der-Waals forces between adsorbate and substrate have to be balanced with intermolecular hydrogen bonds in concert with weaker van-der-Waals forces. Since the magnitude of van-der-Waals forces between adsorbates and substrates correlates with the contact area, this type of interaction becomes more dominant for larger molecules.

By stronger interactions which do not depend on molecule size, it was also possible to grow isotopological molecular networks, i.e. networks following a similar building plan. By varying for instance the length of aliphatic spacers, supramolecular structures with tuneable lattice parameter could be formed. Studies of organic molecules on chemically more active metal substrates show that more complex processes can be involved. In particular the concept of reactivity and surface-catalyzed reactions are discussed and illustrated by an intuitive example. It is demonstrated that strong molecule-substrate interaction can induce unimolecular reactions such as deprotonation of molecules or more generally dissociation of intramolecular bonds. This interaction strength, thus substrate reactivity is highly influenced by a variety of factors which include material, crystallographic surface orientation, and

temperature. Further more the importance of so-called active sites on crystal surfaces, i.e. special sites with significantly increased interaction strength, is taken into account and exemplified with experimental results. Exploiting these fundamental principles, C-Br bond scission of brominated aromatic compounds was demonstrated upon adsorption on reactive substrates and followed by successful incorporation in covalently bonded networks. However, irreversibility of covalent bonds prevents similar control and error correction mechanisms over the system as compared to hydrogen bonded networks. A high defect density and a low degree of ordering is the consequence for the resulting 2D structures. In a final set of experiments aromatic thiol molecules could be assembled into highly or

dered structures via metal-coordination bonds. The 2D gas of freely diffusing adatoms of a copper surface was thermally excited to finally transform a trithiolate precursor structure into metal-coordination networks via Cu-S metal coordination bonds. Two different coordination geometries were observed giving rise to the formation of two

morphologically distinct phases. These studies revealed the impact of the adatom gas for surface reactivity and chemistry of metals.

After introducing the Discontinuous Galerkin (DG) method a detailed misfit analysis on its numerical approximation is performed. We investigate the accuracy of the scheme, the element type (tetrahedrons and hexahedrons), the spatial sampling of the computational domain and the number of propagated wavelengths. As the error norm we chose a time-frequency representation, which illustrates the

time evolution of the spectral content. The results of this analysis are confirmed by a multi-institutional code validation project.

In order to improve efficiency, we expand the computer code to non-conforming, hybrid meshes. In 2 dimensions, riangulars and quadrilaterals can be combined within one computational domain. Several convergence tests are carried out and the newly invented scheme is applied to different test cases including thin layers and variable material.

Furthermore, as absorbing boundaries suffer from spurious reflections at artificial boundaries of the computational domain, we introduce a convolutional perfectly matched layer (CPML) to the scheme. Due to the loss of definite stability, we accomplish several test cases in order to examine the scheme’s behavior. A switchoff criterion for the CPML is suggested.

Considering topographic effects on seismic waves, we perform a systematic study of different parameterizations involving the wave type and frequency of the input signal, the dataset resolution and various amplification factors of real topography in the region of Grenoble, France. Special events are simulated at Mt. Hochstaufen, Southern Bavaria, and compared to real recordings.