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  • 1
    Electronic Resource
    Electronic Resource
    α/β phase transition in quartz monitored using acoustic emissions (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 120 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: It is usually suggested that thermal cracking in a quartz-bearing rock results from the anomalously high volumetric expansion coefficients of quartz (e.g. Simmons & Cooper 1978). It has also been recognized that thermal expansion mismatch and mineral anisotropy contribute to thermal cracking in materials that consist of a polycrystalline aggregate composed of several anisotropic minerals even in the absence of a temperature gradient (Friedrich & Wong 1986). Experiments investigating thermal cracking in rocks commonly involve imaging and quantitative stereology of crack populations induced in rocks treated to various peak temperatures (e.g. Friedrich & Johnson 1978; Homand-Etienne & Troalan 1984; Atkinson, McDonald & Meredith 1984; Meredith & Atkinson 1985). Here we report on acoustic-emission experiments that monitor the process of thermal cracking as it occurs during heating, supported by measurements of crack surface area, pore-fluid permeability, porosity and surface conductivity carried out on rock samples treated to various peak temperatures. The acoustic-emission measurements show a strong peak of microcracking at the phase transition temperature for quartz (˜573°C) superimposed upon a background of microcracking due to thermal expansion. There is also a clear peak of microcracking at higher temperatures (˜800°C) that can be attributed to oxidation-dehydroxylation reactions of hornblende and chlorite. Measurements of fluid permeability, pore surface area, porosity and electrical conductivity, made on samples that have been heat treated to various maximum temperatures, show increases associated with a major episode of cracking in the 500-600°C temperature range, indicating that the new cracks form a well-interconnected network. This has been confirmed by SEM and optical microscopy These results have implications for the electrical conductivity of the continental crust, providing a mechanism enabling the high pore-fluid connectivity needed to explain zones of high electrical conductivity at depth providing that cracks opened in this way remain open at the high pressures existing at depth. It should be recognized, however, that th***se measurements are limited in their direct application since they were obtained under initially dry conditions at laboratory pressures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb01852.x
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  • 2
    Electronic Resource
    Electronic Resource
    Non-universal scaling of fracture length and opening displacement (1994)
    [s.l.] : Nature Publishing Group
    Nature 367 (1994), S. 160-162 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The Krafla fissure swarm (Fig. 1) is one of five volcanotectonic systems in the active rift zone of northeast Iceland, striking (at 10° E) sub-perpendicular to the spreading direction of the Mid-Atlantic Ridge7'8. The swarm is associated with the Krafla central volcano, and takes the form of an ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/367160a0
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  • 3
    Electronic Resource
    Electronic Resource
    Role of pore fluids in the generation of seismic precursors to shear fracture (1992)
    [s.l.] : Nature Publishing Group
    Nature 359 (1992), S. 228-230 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The fracture mechanics model of Main et a/.2 is summarized in Fig. 1. In nature, because crustal deformation is accompanied and accomplished by microcracking and small earthquakes6, sometimes coupled with aseismic deformation, the crust will respond to remotely applied tectonic loading by deforming ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/359228a0
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  • 4
    Electronic Resource
    Electronic Resource
    Modelling the stress-strain behaviour of saturated rocks undergoing triaxial deformation using complex electrical conductivity measurements (1996)
    Springer
    Surveys in geophysics 17 (1996), S. 307-330 
    Details
    ISSN: 1573-0956
    Keywords: Complex electrical conductivity ; Stress-strain modelling ; Cracked solids ; Triaxial deformation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Measurement of complex electrical conductivity as a function of frequency is an extremely sensitive probe for changes in pore and crack volume, crack connectivity, and crack surface topography. Such measurements have been made as a function of pore fluid chemistry, hydrostatic confining pressure, as well as uniaxial and triaxial deformation. This paper will; (1) describe the effects of triaxial deformation on the complex electrical conductivity of saturated porous rocks, (2) use the electrical data to model the mechanical stress-strain behaviour, and (3) compare the modelled behaviour with the stress-strain behaviour measured during the deformation. Experimental conductivity data tracks how the rock undergoes compaction with progressive loss of crack volume, followed by dilatation due to new crack formation, growth of existing cracks, crack interlinkage, and finally failure, as axial strain is increased. We have used the complex electrical data to produce a direction-sensitive (anisotropic) crack damage parameter, and used it to calculate the effective Young's modulus by employing the models of Walsh and Bruner. Comparison of the synthetic stress-strain curves so produced, with the experimentally derived stress-strain curves shows good agreement, particularly for undrained tests. This modelling is an improvement on similar curves produced using isotropic crack damage parameters derived from acoustic emission data. The improvement is likely to be due to the directional sensitivity of the electrical conductivity measurement, and its ability to discriminate between the formation of isolated cracks, and those cracks that contribute to the inter-connected crack space i.e. those cracks upon which transport properties of the rock such as electrical conductivity, and mechanical properties depend most critically during triaxial deformation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01904046
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  • 5
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    Reconstructing magma failure and the degassing network of dome-building eruptions (2013)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2013-03-22
    Description: Volcanic eruptions are regulated by the rheology of magmas and their ability to degas. Both detail the evolution of stresses within ascending subvolcanic magma. But as magma is forced through the ductile-brittle transition, new pathways emerge as cracks nucleate, propagate, and coalesce, constructing a permeable network. Current analyses of magma dynamics center on models of the glass transition, neglecting important aspects such as incremental strain accommodation and (the key monitoring tool of) seismicity. Here, in a combined-methods study, we report the first high-resolution (20 μm) neutron-computed tomography and microseismic monitoring of magma failure under controlled experimental conditions. The data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain-to-failure and importantly, the geometry and efficiency of the permeable fracture network that regulates degassing events. Extrapolation of our findings yields magma ascent via strain localization along conduit margins, thereby providing an explanation for gas-and-ash explosions along arcuate fractures at active lava domes. We conclude that a coupled deformation-seismicity analysis holds a derivation of fracture mechanisms and network, and thus holds potential application in forecasting technologies.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 6
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    Pathways for degassing during the lava dome eruption of Mount St. Helens 2004-2008 (2014)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2014-11-04
    Description: The ability of volatiles to escape rising magma regulates the explosivity of a volcanic system. During silicic lava dome eruptions, strain localization at the conduit margin occurs during magma ascent, creating a damage halo with implications for gas escape. Here we report the first systematic study of permeability network anisotropy across the marginal shear zone of the A.D. 2004–2008 lava dome at Mount St. Helens (Washington State, USA). The results show increasingly large permeability anisotropy of as much as four orders of magnitude (over ~4 m) moving from the interior of the spine through the damage halo. We find the permeability to be essentially isotropic in the spine interior but highly anisotropic in the damage zone and fault core. Our examination of the dome rocks reveals that the permeability anisotropy depends strongly on the presence of vertically oriented shear layers. Here we show that the rate of escape of volatiles will be several orders of magnitude higher vertically through a conduit margin shear zone than horizontally into the conduit wall.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 7
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    Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading (2017)
    Wiley
    Details
    Publication Date: 2017-06-06
    Description: We present a comparative study of crack damage evolution in dry sandstone under both conventional (σ 1  〈 σ 2  = σ 3 ), and true triaxial (σ 1  〈 σ 2  〈 σ 3 ) stress conditions using results from measurements made on cubic samples deformed in three orthogonal directions with independently controlled stress paths. To characterize crack damage we measured the changes in ultrasonic compressional and shear wave velocities in the three principal directions, together with the bulk acoustic emission (AE) output contemporaneously with stress and strain. We use acoustic wave velocities to model comparative crack densities and orientations. In essence, we create two end-member crack distributions; one displaying cylindrical transverse isotropy (conventional triaxial) and the other planar transverse isotropy (true triaxial). Under the stress conditions in our experiments we observed an approximately fivefold decrease in the number of AE events between the conventional and true triaxial cases. When taken together, the AE data, the velocities and the crack density data indicate that the intermediate principal stress suppresses the total number of cracks and restricts their growth to orientations sub-normal to the minimum principal stress. However, the size of individual cracks remains essentially constant, controlled by the material grain size. Crack damage is only generated when the differential stress exceeds some threshold value. Cyclic loading experiments show that further damage commences only when that previous maximum differential stress is exceeded, regardless of the mean stress, whether this is achieved by increasing the maximum principal stress or by decreasing the minimum principal stress.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Stylolites in limestones: Barriers to fluid flow? (2014)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2014-01-01
    Description: Stylolites—products of intergranular pressure-solution—are laterally extensive, planar features. They are a common strain localization feature in sedimentary rocks. Their potential impact on regional fluid flow has interested geoscientists for almost a century. Prevalent views are that they act as permeability barriers, although laboratory studies are extremely rare. Here we report on a systematic laboratory study of the influence of stylolites on permeability in limestone. Our data demonstrate that, contrary to conventional wisdom, the studied stylolites do not act as barriers to fluid flow. In detail, when a stylolite occurs perpendicular to the direction of flow, the permeability simply follows the same power law permeability-porosity trend as the stylolite-free material. We show, using a combination of high-resolution (4 µm) X-ray computed tomography, optical microscopy, and chemical analyses, that the stylolites of this study are not only perforated layers constructed from numerous discontinuous pressure solution seams, but comprise minerals of similar or lower density to the host rock. The stylolites are not continuous high-density layers. Our data affirm that stylolites may not impact regional fluid flow as much as previously anticipated.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 9
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    Effect of temperature on the fracture toughness of anisotropic shale and other rocks (2017)
    Geological Society (of London)
    Details
    Publication Date: 2017-04-05
    Description: Fracture toughness was measured for a range of rock materials as a function of temperature between ambient temperature and 150°C. Measurements were made along all three principal crack orientations for the transversely isotropic Mancos shale and in single orientations for the more isotropic Darley Dale sandstone, Indiana limestone and Lanhelin granite. Fracture toughness was measured using a modified short-rod method with the sample and loading equipment enclosed within an elevated temperature chamber. A slight increase in K Ic was observed in Lanhelin granite with increasing temperatures up to 54°C, before a steady decrease at higher temperatures. For the sandstone and limestone, little change was observed in K Ic over the studied temperature range. In measurements on Mancos shale at elevated temperatures. Fracture toughness was seen to increase slightly with increasing temperature in the arrester orientation over this range, while remaining constant in the other two orientations. These observations can be explained in terms of the development of thermally induced microfractures parallel to the bedding planes in this material. A bimodal distribution of K Ic values in the short-transverse orientation was not observed, as it has been for previously published measurements at ambient conditions.
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
    Published by Geological Society (of London)
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  • 10
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    KG²B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite—Part 2: modelling, microstructures and complementary data (2018)
    Oxford University Press
    Details
    Publication Date: 2018-07-25
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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