Development, impact and longevity of fractures in magmatic, volcanic and geothermal systems

The migration of fluids in the Earth’s crust embodies the last stage of the internal heat release of our planet. Either spectacularly expressed at the surface through volcanic activity, or more subtly as internal hydrothermal circulation, this phenomenon involves the upwards motion of fluids and magmas that contribute to more efficient heat transfer. On one hand, volcanic eruptions result from the movement of buoyant magmatic liquids towards the surface. On their way up, these magmas cool down, crystallise and upon decompression, build up an internal pressure that dictate the eruptive style: Effusive when the internal pressure is released as it builds up; explosive when the internal pressure accumulates until it is able to fracture the magma. In nature, the shift from effusive to explosive activity is often periodic, reflecting cycles of pressure accumulation and relaxation in the conduit. On the other hand, hydrothermal circulation results from the infiltration of water, of meteoric or magmatic origin, into the rocks making up the crust. Higher temperatures at depth and/ or due to the presence of a magmatic body, trigger the formation of convective cells in which chilled, denser water percolates downwards then heats up, losing density before moving back upwards. Importantly, the circulation of both hydrothermal fluids and magmatic liquids is controlled by the presence of fractures in the crust and the permeability of the surrounding rocks. In this thesis, I first investigate how fractures affect the localisation of fluids in fractured porous rocks through permeability measurements, both at atmospheric pressure and at shallow confining conditions (<30 MPa; ~1 km depth). I demonstrate that the impact of fractures is greater at lower porosities as the permeability is greatly increased. In more porous rocks, higher pore connectivity means that macro-fractures affect the permeability less significantly, as at least some of the fluid flows through the pre-existing connected porous network. I further demonstrate that, during confinement, most of the mechanical closure occurs at shallow conditions (<5 MPa; ~200m depth) for the porosity range tested. In addition, I show that mechanically shutting a fracture does not seal the permeable pathways, and that the fractured system is unable to retrieve the same permeability as the intact system. I then develop an analytical solution for the permeability of variably porous, fractured systems as a function of depth before numerically solving it. I complement this work with two “case studies” in which tensile fractures form and open or heal, and link this to the system permeability evolution through time and the implications for magmatic, volcanic and geothermal systems. In the first scenario, tensile fractures open in a cooling magmatic body to form columnar joints in a basalt. Because the temperature at which cooling joints form remains elusive, causing a lingering scientific debate, I develop a novel type of mechanical testing and show for the first time that, in basaltic systems, these macro-fractures form purely in the elastic regime. This is further supported by the use of the rock’s thermo-mechanical characteristics (namely thermal expansion and tensile strength) to model the tensile stress build-up upon cooling and, once formed, the evolution of the fracture width between two columns. Applying the analytical solution for fractured systems permeability defined earlier, I further model the permeability evolution of a columnar jointing magmatic body, important for the understanding of fluid migration during drilling close to magmatic chambers. Finally, in magmas, the entrapment of exsolving gases, during ascent, force the accumulation of stress in the liquid, building pore pressure and potentially resulting in magmatic fragmentation. Consequently, the accumulated stresses can then be dissipated at times longer than the relaxation timescale of the melt, allowing fractures to heal and the system to recover strength lose permeability. Using synthetic glasses in a newly designed experimental setup, I show that the time required to start the healing process is proportional to the relaxation timescale of the melt. I further demonstrate that the kinetics of fracture healing involve two distinct stages. The first stage sees the fracture walls viscously deforming to dissipate excessive energy along the fracture plane (wetting regime), while the second stage consists of the diffusive exchange of elements across the fracture interface (diffusive regime). I finally surmise that the cyclic activity of persistently active silicic volcanoes could be explained by dynamic permeability and strength variations of material due to repeating fracture and healing cycles. Overall, I show that the development and longevity of fractures have significant impacts on the localisation of fluid flow, highlighting that fractures significantly contribute to the development of anisotropy in magmatic, volcanic and geothermal environments. A better understanding of the longevity of fractures in these systems is of prime importance in the mitigation of hazards associated to volcanic eruptions, but also in the development of cheaper, more efficient geothermal energy

Prueba comparativa de substratos para el cultivo en recipientes

En la óptica de probar la mejor combinación de materiales orgánicos para la constitución de un substrato ideal tendiente a una buena producción hortícola en recipientes, se implementó una prueba que comparase cuatro medios basados en materias orgánicas: estiércol de caballo, en diversas proporciones, y bagazo. Para este estudio, se ha puesto acento en la cantidad de agua utilizada, el rendimiento, la capacidad de producción de un substrato -tomando en cuenta que hubo rotación de cultivos: amaranto, zanahoria- y la rentabilidad por tratamiento. Este experimento nos dio la oportunidad de evaluar nuevamente otros substratos que se habían destacado anteriormente. El mayor aporte de esta experiencia ha sido nuestra construcción sobre el costo del agua y el valor económico de substratos elaborados sobre la base de estiércol de caballo. El proyecto ganaría intensificando la promoción de compost que debe ser probada también en el futuro, porque es el llamado a reemplazar los abonos de granja, excremento de caballo y de vaca, en la constitución del substrato

The permeability of fractured rocks in pressurised volcanic and geothermal systems

AbstractThe connectivity of rocks’ porous structure and the presence of fractures influence the transfer of fluids in the Earth’s crust. Here, we employed laboratory experiments to measure the influence of macro-fractures and effective pressure on the permeability of volcanic rocks with a wide range of initial porosities (1–41 vol. %) comprised of both vesicles and micro-cracks. We used a hand-held permeameter and hydrostatic cell to measure the permeability of intact rock cores at effective pressures up to 30 MPa; we then induced a macro-fracture to each sample using Brazilian tensile tests and measured the permeability of these macro-fractured rocks again. We show that intact rock permeability increases non-linearly with increasing porosity and decreases with increasing effective pressure due to compactional closure of micro-fractures. Imparting a macro-fracture both increases the permeability of rocks and their sensitivity to effective pressure. The magnitude of permeability increase induced by the macro-fracture is more significant for dense rocks. We finally provide a general equation to estimate the permeability of intact and fractured rocks, forming a basis to constrain fluid flow in volcanic and geothermal systems.</jats:p

Sintering of vesiculating pyroclasts

Hot volcanic pyroclasts can sinter, vesiculate, and outgas in concert – a combination of processes which remains poorly constrained. And yet this combination of processes can occur coincidently during deposition from pyroclastic density currents, in conduit-filling pyroclastic debris, and in tuffisites. In many of these settings, it is the sintering-driven evolution of permeability that is key to gas transport through the evolving deposit. Here, we experimentally and theoretically investigate the evolution of the permeable networks during sintering of hot fragmental volcanic systems, which are hydrous and oversaturated at the experimental conditions. Firstly, we find that vesiculation results in shutting of the inter-granular porous network as bubble growth drives expansion of the particles into one another, destroying interconnected pores. Secondly, we observe that degassing by diffusion out of the particle edge results in contraction of the vesicular particles, re-opening pore spaces between them. Therefore, we find that vesiculation, and diffusive outgassing compete to determine both the intra-fragment vesicularity and the permeability during sintering. The development of intra-fragment vesicularity directly impacts the inter-fragment pore space and its connectivity, which decreases during vesiculation and subsequently increases during diffusive outgassing, prompting complex, non-linear permeability evolution.The relative dominance of these processes is fragment size dependent; proportionally, fine fragments lose gas at a higher rate than coarser fragments during diffusive outgassing due to larger surface area to volume ratios. As the systems progress, larger fragments retain a higher proportion of gas and so attain greater vesicularities than finer ones – and therefore, the coarse fragmental pyroclasts experience a greater, yet transient, reduction in connected porosity and permeability. We suggest that where vesiculation is sufficient, it can lead to the complete loss of connected porosity and the sealing of permeable pathways much earlier than in a sintering-only system. Our results suggest that classical sintering models must be modified to account for these vesiculation and diffusive degassing processes, and that only a combined vesiculation, sintering, and diffusive outgassing model can resolve the evolution of permeability in hot clastic volcanic systems

Frictional Behaviour, Wear and Comminution of Synthetic Porous Geomaterials

During shearing in geological environments, frictional processes, including the wear of sliding rock surfaces, control the nature of the slip events. Multiple studies focusing on natural samples have investigated the frictional behaviour of a large suite of geological materials. However, due to the varied and heterogeneous nature of geomaterials, the individual controls of material properties on friction and wear remain unconstrained. Here, we use variably porous synthetic glass samples (8, 19 and 30% porosity) to explore the frictional behaviour and development of wear in geomaterials at low normal stresses ( 641\ua0MPa). We propose that porosity provides an inherent roughness to material which wear and abrasion cannot smooth, allowing material at the pore margins to interact with the slip surface. This results in an increase in measured friction coefficient from <0.4 for 8% porosity, to <0.55 for 19% porosity and 0.6\u20130.8 for 30% porosity for the slip rates evaluated. For a given porosity, wear rate reduces with slip rate due to less asperity interaction time. At higher slip rates, samples also exhibit slip weakening behaviour, either due to evolution of the slipping zone or by the activation of temperature-dependent microphysical processes. However, heating rate and peak temperature may be reduced by rapid wear rates as frictional heating and wear compete. The higher wear rates and reduced heating rates of porous rocks during slip may delay the onset of thermally triggered dynamic weakening mechanisms such as flash heating, frictional melting and thermal pressurisation. Hence porosity, and the resultant friction coefficient, work, heating rate and wear rate, of materials can influence the dynamics of slip during such events as shallow crustal faulting or mass movements

Attitude toward contraception and abortion among Curaçao women. Ineffective contraception due to limited sexual education?

Background In Curaçao is a high incidence of unintended pregnancies and induced abortions. Most of the induced abortions in Curaçao are on request of the woman and performed by general practitioners. In Curaçao, induced abortion is strictly prohibited, but since 1999 there has been a policy of connivance. We present data on the relevance of economic and socio-cultural factors for the high abortion-rates and the ineffective use of contraception. Methods Structured interviews to investigate knowledge and attitudes toward sexuality, contraception and abortion and reasons for ineffective use of contraceptives among women, visiting general practitioners. Results Of 158 women, 146 (92%) participated and 82% reported that their education on sexuality and about contraception was of good quality. However 'knowledge of reliable contraceptive methods' appeared to be - in almost 50% of the cases - false information, misjudgements or erroneous views on the chance of getting pregnant using coitus interruptus and about the reliability and health effects of oral contraceptive pills. Almost half of the interviewed women had incorrect or no knowledge about reliability of condom use and IUD. 42% of the respondents risked by their behavior an unplanned pregnancy. Most respondents considered abortion as an emergency procedure, not as contraception. Almost two third experienced emotional, physical or social problems after the abortion. Conclusions Respondents had a negative attitude toward reliable contraceptives due to socio-cultural determined ideas about health consequences and limited sexual education. Main economic factors were costs of contraceptive methods, because most health insurances in Curaçao do not cover contraceptives. To improve the effective use of reliable contraceptives, more adequate information should be given, targeting the wrong beliefs and false information. The government should encourage health insurance companies to reimburse contraceptives. Furthermore, improvement of counseling during the abortion procedure is important