174 research outputs found
Peering into the Crystal Fabric of Rocks
No full textThis Oceanus article describes the work of Greg Hirth, a geologist studying rock deformation and crystallography. It discusses how rocks deform as a function of pressure and heat (called rheology) and includes a short section about the possibility of microbial life in the heat and pressure-induced cracks between crystals. Hyperlinks to related articles are contained within the text of the article. The article also features a list of related links and color images of rocks and minerals under a microscope
Data for "Fault interactions enhance high-frequency earthquake radiation"
No full textThis dataset includes all results (in pandas dataframe pickle format) from the data from the paper, "Fault interactions enhance high-frequency earthquake radiation" by S. X. Chu, V. C. Tsai, D. T. Trugman, and G. Hirth. It also contains a python notebook showing how to read the dataset, as well as sample functions to calculate misalignment and density ratios as described in that paper
Reply [to “Comment on ‘The effects of pressure and porosity on the micromechanics of the brittle‐ductile transition in quartzite’ by Greg Hirth and Jan Tullis”]
No full textOlivine friction at the base of oceanic seismogenic zones
No full textAuthor Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): B01205, doi:10.1029/2006JB004301.We investigate the strength and frictional behavior of olivine aggregates
at temperatures and effective confining pressures similar to those at the base
of the seismogenic zone on a typical ridge transform fault. Triaxial compression
tests were conducted on dry olivine powder (grain size ≤ 60 μm) at effective
confining pressures between 50 and 300 MPa (using Argon as a pore
fluid), temperatures between 600°C and 1000°C, and axial displacement rates
from 0.06 to 60 μm/s (axial strain rates from 3 × 10−6 to 3 × 10−3 s−1).
Yielding shows a negative pressure dependence, consistent with predictions
for shear enhanced compaction and with the observation that samples exhibit
compaction during the initial stages of the experiments. A combination
of mechanical data and microstructural observations demonstrate that
deformation was accommodated by frictional processes. Sample strengths were
pressure-dependent and nearly independent of temperature. Localized shear
zones formed in initially homogeneous aggregates early in the experiments.
The frictional response to changes in loading rate is well described by rate
and state constitutive laws, with a transition from velocity-weakening to velocitystrengthening
at 1000°C. Microstructural observations and physical models
indicate that plastic yielding of asperities at high temperatures and low axial
strain rates stabilizes frictional sliding. Extrapolation of our experimental
data to geologic strain rates indicates that a transition from velocity weakening
to velocity strengthening occurs at approximately 600°C, consistent
with the focal depths of earthquakes in the oceanic lithosphere.This research was supported
by the Deep Ocean Exploration Institute at WHOI and NSF grants to Greg Hirth and
Brian Evans
Recommended from our members
Mechanisms and longevity of strain localization during dynamic recrystallization of olivine
No full textThe mechanisms that lead to localized deformation within Earth’s lithosphere are critical to the formation and longevity of tectonic plate boundaries. Large-scale geodynamic models that incorporate parameters for rheological ‘damage’ and ‘healing’ are most successful at reproducing self-consistent plate-like behavior on Earth and other terrestrial planets, but the microphysical processes that maintain localization are not well understood. While the preservation of fine grain sizes in crustal and mantle shear zones suggests that grain size reduction (damage) by dynamic recrystallization is one of the most important mechanisms of strain localization, subsequent grain growth (healing) may inhibit localization. In this dissertation, I describe three projects focused on evaluating these processes and their contribution to the persistence of plate boundaries. In Chapter 2, I test the consistency of existing stress-grain size piezometers that are used to estimate the strength of the lithosphere. Using quartz–feldspar- and olivine–orthopyroxene-bearing mylonites, which are common mineral pairs in crustal and mantle rocks, I quantify new piezometers for feldspar and orthopyroxene that are consistent with observations from natural rocks, and can be used to estimate stress magnitudes in rocks such as granulites and pyroxenites, for which the quartz and olivine piezometers are unsuitable. In Chapter 3, I quantify the rate of syn- and post-deformation grain growth in experimentally deformed, wet natural olivine aggregates, and compare it to that predicted by the existing olivine grain growth law to highlight the influence of microstructurally stored strain energy on grain growth. I derive a new olivine grain growth law that predicts significantly slower growth than previously reported for wet olivine, suggesting that grain size reduction by dynamic recrystallization plays an important role in the longevity of strain localization. In Chapter 4, I compare the mechanical and microstructural evolution of dry olivine aggregates that I experimentally deformed under both constant strain rate and constant stress conditions, and use detailed microstructural analysis to identify the mechanisms that accommodate strain weakening during and after grain size reduction. The results suggest that strain localization in monophase olivine does not occur at constant strain rate conditions but is common under constant stress boundary conditions.Earth and Planetary Science
Newtonian versus non-Newtonian upper mantle viscosity : implications for subduction initiation
No full textAuthor Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 32 (2005): L19304, doi:10.1029/2005GL023457.The effect of rheology on the evolution of the slab-tip during subduction initiation is analyzed using 2-D numerical flow models. Experimentally determined flow laws have both strong temperature- and stress-dependence, which leads to large local variations in viscosity with direct consequences for subduction initiation. We find that models with Newtonian viscosity lead to flat or coupled subduction due to hydrodynamic stresses that pull the slab-tip up towards the overriding plate. Non-Newtonian rheology reduces these hydrodynamic stresses by decreasing the wedge viscosity and the slab coupling to wedge-corner flow, rendering the small negative-slab buoyancy of the slab-tip sufficient to maintain its dip during the early stages of subduction
Rheologic controls on slab dynamics
No full textAuthor Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 8 (2007): Q08012, doi:10.1029/2007GC001597.Several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age. However, studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones. Numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics. Employing 2-D numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine, we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity, subducting plate age, and subduction duration can be reproduced without trench motion (i.e., slab roll-back) for locations away from slab edges. Successful models include a stiff slab that is 100–1000 times more viscous than previous estimates from models of plate bending, the geoid, and global plate motions. We find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling, with a small dependence on subducting plate age. Once the slab sinks into the lower mantle the primary processes controlling slab evolution are (1) the ability of the stiff slab to transmit stresses up dip, (2) resistance to slab descent into the higher-viscosity lower mantle, and (3) subduction-induced flow in the mantle-wedge corner.This research was partially supported by NSF award
EAR0125919
¿Química o color?: comparación entre el uso de fluorescencia de rayos-X portátil y las técnicas visuales de clasificación de obsidiana de Tepeticpac. 50. Arqueología
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Filtered EBSD data set of listvenites from Oman Drilling Project Hole BT1B
No full textThe dataset contains electron backscattered diffraction (EBSD) filtered data acquired on samples collected on cores from Oman Drilling Project Hole BT1B (International Continental Drilling Project Expedition 5057‐4B). A suite of 7 samples in foliated listvenites was analyzed through 11 EBSD maps of variable area width at specific sites of interest. The scanning resolution varies from 0.5 µm to 3 µm. Raw pixel data was filtered with the Channel 5 analysis suite from HKL Technology (Oxford Instruments) and consists in noise reduction followed by a wild spikes extrapolation (level 6; in few cases level 5). Datafiles are provided as channel text files (CTF), which can be processed and viewed with Channel 5 or MTEX
Museo de Sitio de Xochicalco: Morelos
No full textLa información de esta guía está basada en los datos de los investigadores Alfredo López Austin, Kenneth Hirth, Esther Pasztory, Mario Córdova, Giselle Canto y Laura LedesmaLa colección del Museo cuenta con piezas obtenidas en las diferentes temporadas de excavación efectuadas en la zona arqueológica de Xochicalco; contiene desde los más sencillos objetos de alfarería, hasta piezas scultóricas de enorme complejidad iconográfica</p
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