Green Lake Landslide and other giant and very large postglacial landslides in Fiordland,New Zealand

Green Lake Landslide is an ancient giant rock slide in gneiss and granodiorite located in the deeply glaciated Fiordland region of New Zealand. The landslide covers an area of 45 km² and has a volume of about 27 km³. It is believed to be New Zealand's largest landslide, and possibly the largest landslide of its type on Earth. It is one of 39 known very large (10⁶–10⁷ m³) and giant (≥10⁸ m³) postglacial landslides in Fiordland discussed in the paper. Green Lake Landslide resulted in the collapse of a 9 km segment of the southern Hunter Mountains. Slide debris moved up to 2.5 km laterally and 700 m vertically, and formed a landslide dam about 800 m high, impounding a lake about 11 km long that was eventually infilled with sediments. Geomorphic evidence supported by radiocarbon dating indicates that Green Lake Landslide probably occurred 12 000–13 000 years ago, near the end of the last (Otira) glaciation. The landslide is described, and its geomorphic significance, age, failure mechanism, cause, and relevance in the region are discussed, in relation to other large landslides and recent earthquake-induced landslides in Fiordland. The slope failure occurred on a low-angle fault zone undercut by glacial erosion, and was probably triggered by strong shaking (MM IX–X) associated with a large (≥ M 7.5–8) earthquake, on the Alpine Fault c. 80 km to the northwest. Geology was a major factor that controlled the style and size of Green Lake landslide, and in that respect it is significantly different from most other gigantic landslides. Future large earthquakes on the Alpine Fault in Fiordland are likely to trigger more very large and giant landslides across the region, causing ground damage and devastation on a scale that has not occurred during the last 160 years, with potentially disastrous effects on towns, tourist centres, roads, and infrastructure. The probability of such an event occurring within the next 50 years may be as high as 45%.     

Frequency distributions and correlations of solar X-ray flare parameters

We have determined frequency distributions of flare parameters from over 12000 solar flares recorded with the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) satellite. These parameters include the flare duration, the peak counting rate, the peak hard X-ray flux, the total energy in electrons, and the peak energy flux in electrons (the latter two computed assuming a thick-target flare model). The energies were computed above a threshold energy between 25 and 50 keV. All of the distributions can be represented by power laws above the HXRBS sensitivity threshold. Correlations among these parameters are determined from linear regression fits as well as from the slopes of the frequency distributions. Variations of the frequency distributions were investigated with respect to the solar activity cycle.Theoretical models for the frequency distribution of flare parameters depend on the probability of flaring and the temporal evolution of the flare energy build-up. Our results are consistent with stochastic flaring and exponential energy build-up, with an average build-up time constant that is 0.5 times the mean time between flares. The measured distributions of flares are also consistent with predicted distributions of flares from computer simulations of avalanche models that are governed by the principle of self-organized criticality.     

Integral field spectroscopy with the GEMINI multiobject spectrographs

We describe the integral field unit (IFU) which converts the Gemini Multiobject Spectrograph (GMOS) installed on the Gemini-North telescope to an integral field spectrograph,which produces spectra over a contiguous field of view of 7 × 5 arcsec with spatial sampling of 0.2 arcsecover the wavelength range 0.4-1.0 μm.GMOS is converted to this mode by the remote insertion of the IFU into thebeam in place of the masks used for the multiobject mode. A separate fieldof half the area of the main field, but otherwise identical, is alsoprovided to improve background subtraction. The IFU contains 1500lenslet-coupled fibres and was the first facility of any type for integralfield spectroscopy employed on an 8/10 m telescope.We describe the design, construction and testing of the GMOS IFU and present measurements of the throughput both in the laboratory and at the telescope. We compare these with a theoretical prediction made before construction started. All are in good agreement with each other, with the on-telescope throughput exceeding 60% (averaged over wavelength). Finallywe show an example of data obtained during commissioning to illustrate the power of the device.     

The second order characteristic condition for plastic flow

In this paper the second order characteristic (discontinuous bifurcation) condition is derived for the granular flow (fully plastic) equations. This second order bifurcation equation is shown to be formally identical to the first order localization requirement during steady elastoplastic deformation provided the elastic compliance tensor is substituted for the product of the plastic multiplier with the flow Hessian. For isotropic yield and flow functions the invariant form of the characteristic condition is given in detail, as well as an alternative expression in adapted co‐ordinates. The characteristic condition can be regarded as defining a hardening function which is maximized to identify the critical angles. When the method is applied to 3D Coulomb flow, Mohr's 3D fracture plane conditions are obtained uniquely. Copyright © 2003 John Wiley & Sons, Ltd.     

Initial phase of chromospheric evaporation in a solar flare

In this paper we discuss the initial phase of chromospheric evaporation during a solar flare observed with instruments on the Solar Maximum Mission on May 21, 1980 at 20:53 UT. Images of the flaring region taken with the Hard X-Ray Imaging Spectrometer in the energy bands from 3.5 to 8 keV and from 16 to 30 keV show that early in the event both the soft and hard X-ray emissions are localized near the footpoints, while they are weaker from the rest of the flaring loop system. This implies that there is no evidence for heating taking place at the top of the loops, but energy is deposited mainly at their base. The spectral analysis of the soft X-ray emission detected with the Bent Crystal Spectrometer evidences an initial phase of the flare, before the impulsive increase in hard X-ray emission, during which most of the thermal plasma at 10⁷ K was moving toward the observer with a mean velocity of about 80 km s^-1. At this time the plasma was highly turbulent. In a second phase, in coincidence with the impulsive rise in hard X-ray emission during the major burst, high-velocity (370 km s^-1) upward motions were observed. At this time, soft X-rays were still predominantly emitted near the loop footpoints. The energy deposition in the chromosphere by electrons accelerated in the flare region to energies above 25 keV, at the onset of the high-velocity upflows, was of the order of 4 × 10¹⁰ erg s^-1 cm^-2. These observations provide further support for interpreting the plasma upflows as the mechanism responsible for the formation of the soft X-ray flare, identified with chromospheric evaporation. Early in the flare soft X-rays are mainly from evaporating material close to the footpoints, while the magnetically confined coronal region is at lower density. The site where upflows originate is identified with the base of the loop system. Moreover, we can conclude that evaporation occurred in two regimes: an initial slow evaporation, observed as a motion of most of the thermal plasma, followed by a high-speed evaporation lasting as long as the soft X-ray emission of the flare was increasing, that is as long as plasma accumulation was observed in corona.     

Post-solidification cooling and the age of Io's lava flows

The modeling of thermal emission from active lava flows must account for the cooling of the lava after solidification. Models of lava cooling applied to data collected by the Galileo spacecraft have, until now, not taken this into consideration. This is a flaw as lava flows on Io are thought to be relatively thin with a range in thickness from ∼1 to 13 m. Once a flow is completely solidified (a rapid process on a geological time scale), the surface cools faster than the surface of a partially molten flow. Cooling via the base of the lava flow is also important and accelerates the solidification of the flow compared to the rate for the ‘semi-infinite’ approximation (which is only valid for very deep lava bodies). We introduce a new model which incorporates the solidification and basal cooling features. This model gives a superior reproduction of the cooling of the 1997 Pillan lava flows on Io observed by the Galileo spacecraft. We also use the new model to determine what observations are necessary to constrain lava emplacement style at Loki Patera. Flows exhibit different cooling profiles from that expected from a lava lake. We model cooling with a finite-element code and make quantitative predictions for the behavior of lava flows and other lava bodies that can be tested against observations both on Io and Earth. For example, a 10-m-thick ultramafic flow, like those emplaced at Pillan Patera in 1997, solidifies in ∼450 days (at which point the surface temperature has cooled to ∼280 K) and takes another 390 days to cool to 249 K. Observations over a sufficient period of time reveal divergent cooling trends for different lava bodies [examples: lava flows and lava lakes have different cooling trends after the flow has solidified (flows cool faster)]. Thin flows solidify and cool faster than flows of greater thickness. The model can therefore be used as a diagnostic tool for constraining possible emplacement mechanisms and compositions of bodies of lava in remote-sensing data.     

Ion microprobe analysis of oxygen isotope ratios in granulite facies magnetites: diffusive exchange as a guide to cooling history

Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2–8 m and precision in the range of 1 (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of ¹⁸O at this scale. High precision micro-analyses of this sort will permit wide-ranging new applications in stable isotope geochemistry. The analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite. Magnetites are near to Fe₃O₄ in composition, and optically and chemically homogeneous. A combination of ion probe plus conventional BrF₅ analysis shows that individual grains are homogeneous with ¹⁸O=8.9±1 SMOW from the core to near the rim of 0.1–1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that rims are 9 depleted in ¹⁸O. These low ¹⁸O values increase in smooth gradients across the outer 10 m of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150–350 m diameter magnetites which average 1.2 lower in ¹⁸O than coarse magnetites due to low ¹⁸O rims. Conventional analysis of coexisting calcite yields °¹⁸O=18.19, suggesting that bulk ¹⁸O (Cc-Mt)=9.3 and yielding an apparent equilibration temperature of 525° C, over 200° C below the temperature of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for oxygen in calcite under hydrothermal conditions at high P(H₂O) indicates that diffusion is faster in magnetite and modelling of the low ¹⁸O rims on magnetite would suggest that the Adirondacks experienced slow cooling after Grenville metamorphism, followed by a brief period of rapid cooling, possibly related to uplift. Conversely, the data for calcite at low P(H₂O) show slower oxygen diffusion than in magnetite. Modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals, suggesting that the low ¹⁸O rims form by exchange with late, low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets. In either case, the ion microprobe results indicate that two distinct events are recorded in the post-metamorphic exchange history of these magnetites. Recognition of these events is only possible through microanalysis and has important implications for geothermometry.     

Solar hard X-ray bursts

The major results from SMM are presented as they relate to our understanding of the energy release and particle transportation processes that lead to the high-energy X-ray aspects of solar flares. Evidence is reviewed for a 152–158 day periodicity in various aspects of solar activity including the rate of occurrence of hard X-ray and gamma-ray flares. The statistical properties of over 7000 hard X-ray flares detected with the Hard X-Ray Burst Spectrometer are presented including the spectrum of peak rates and the distribution of the photon number spectrum. A flare classification scheme introduced by Tanaka is used to divide flares into three different types. Type A flares have purely thermal, compact sources with very steep hard X-ray spectra. Type B flares are impulsive bursts which show double footpoints in hard X-rays, and soft-hard-soft spectral evolution. Type C flares have gradually varying hard X-ray and microwave fluxes from high altitudes and show hardening of the X-ray spectrum through the peak and on the decay. SMM data are presented for examples of type B and type C events. New results are presented showing coincident hard X-rays, O v, and UV continuum observations in type B events with a time resolution of 128 ms. The subsecond variations in the hard X-ray flux during 10% of the stronger events are discussed and the fastest observed variation in a time of 20 ms is presented. The properties of type C flares are presented as determined primarily from the non-imaged hard X-ray and microwave spectral data. A model based on the association of type C flares and coronal mass ejections is presented to explain many of the characteristics of these gradual flares.