Converting NAD83 GPS heights into NAVD88 elevations with LVGEOID, a hybrid geoid height model for the Long Valley volcanic region, California

A GPS survey of leveling benchmarks done in Long Valley Caldera in 1999 showed that the application of the National Geodetic Survey (NGS) geoid model GEOID99 to tie GPS heights to historical leveling measurements would significantly underestimate the caldera ground deformation (known from other geodetic measurements). The NGS geoid model was able to correctly reproduce the shape of the deformation, but required a local adjustment to give a realistic estimate of the magnitude of the uplift. In summer 2006, the U.S. Geological Survey conducted a new leveling survey along two major routes crossing the Long Valley region from north to south (Hwy 395) and from east to west (Hwy 203 – Benton Crossing). At the same time, 25 leveling bench marks were occupied with dual frequency GPS receivers to provide a measurement of the ellipsoid heights. Using the heights from these two surveys, we were able to compute a precise geoid height model (LVGEOID) for the Long Valley volcanic region. Our results show that although the LVGEOID and the latest NGS GEOID03 model practically coincide in areas outside the caldera, there is a difference of up to 0.2 m between the two models within the caldera. Accounting for this difference is critical when using the geoid height model to estimate the ground deformation due to magmatic or tectonic activity in the calder

Expendable bubble tiltmeter for geophysical monitoring

An unusually rugged highly sensitive and inexpensive bubble tiltmeter has been designed, tested, and built in quantity. These tiltmeters are presently used on two volcanoes and an Alaskan glacier, where they continuously monitor surface tilts of geological interest. This paper discusses the mechanical, thermal, and electric details of the meter, and illustrates its performance characteristics in both large ( > 10^(-4) radian) and small ( < 10^(-6) radian) tilt environments. The meter's ultimate sensitivity is better than 2 X 10^(-8) radians rms for short periods (hours), and its useful dynamic range is greater than 10^4. Included is a short description of field use of the instrument for volcano monitoring

Landform degradation on Mercury, the Moon, and Mars: Evidence from crater depth/diameter relationships

Morphologic classification of craters and quantitative measurements of crater depth as a function of diameter are used to investigate the relative degradational histories of Mercury, the moon, and Mars. Martian craters exhibit considerable depth variation and are generally shallower than their lunar or mercurian counterparts. On Mercury and the moon, visually fresh and degraded craters on smooth plains show no significant depth degradation except that attributed to lava flooding or local inundation by ejecta from large impacts. More heavily cratered regions on both planets display a large range of both visual and depth degradation, suggesting that most landform modification occurred before the final phase of formation of the oldest smooth plains on both planets. Depth/diameter data presented here are discussed as they relate to two early history scenarios. One scenario based on cratering and the ballistic transport of material has been suggested for Mercury, the moon, and Mars by several authors. Owing to discrepancies between this ballistic scenario and observations of crater densities and morphologies, we suggest that landforms on all these bodies also record nonballistic degradation associated with the formation of intercrater plains. Whichever scenario is applied, early, intense, bombardment-associated degradation appears to be a common element in the histories of the terrestrial planets

Topography of the polar layered deposits of Mars

Synthesis of polar topographic data derived from the Mariner 9 radio occultation, ultraviolet spectrometer, and television imaging experiments provides new information on the behavior of polar volatiles and the topographic configuration of the martian polar layered deposits. Gentle slopes in the vicinity of the south pole may serve to shift the point of minimum annual solar insolation from the pole to a site within the perimeter of the offset residual frost cap. Localized defrosting which gives rise to the dark-banded appearance of both residual caps correlates with a series of outward-facing slopes descending from central topographic highs. Stability of the volatile involved apparently is largely insolation controlled. The south polar residual cap lies entirely higher (at lower pressure) than the northern cap, implying that the south residual cap is an unlikely site for any permanent surface deposit of solid carbon dioxide. Photogrammetric models of both residual caps reveal a series of regularly spaced topographic undulations descending from central topographic highs within the underlying layered deposits. Scarplike to troughlike in cross section, these features slope 1°–5° and are 100–1000 m in local relief. The south polar layered deposits lie almost entirely at higher elevations than those in the north. Total thickness of the deposits is inferred to be 1–2 km in the south and 4–6 km in the north

Mount St. Helens Retrospective: Lessons Learned Since 1980 and Remaining Challenges

Since awakening from a 123-year repose in 1980, Mount St. Helens has provided an opportunity to study changes in crustal magma storage at an active arc volcano—a process of fundamental importance to eruption forecasting and hazards mitigation. There has been considerable progress, but important questions remain unanswered. Was the 1980 eruption triggered by an injection of magma into an upper crustal reservoir? If so, when? How did magma rise into the edifice without producing detectable seismicity deeper than ∼2.5 km or measurable surface deformation beyond the volcano’s north flank? Would precursory activity have been recognized earlier if current monitoring techniques had been available? Despite substantial improvements in monitoring capability, similar questions remain after the dome-forming eruption of 2004–2008. Did additional magma accumulate in the reservoir between the end of the 1980–1986 eruption and the start of the 2004–2008 eruption? If so, when? What is the significance of a relative lull in seismicity and surface deformation for several years prior to the 2004–2008 eruption onset? How did magma reach the surface without producing seismicity deeper than ∼2 km or measurable deformation more than a few hundred meters from the vent? Has the reservoir been replenished since the eruption ended, and is it now primed for the next eruption? What additional precursors, if any, should be expected? This paper addresses these questions, explores possible answers, and identifies unresolved issues in need of additional study. The 1980–1986 and 2004–2008 eruptions could have resulted from second boiling during crystallization of magma long-resident in an upper crustal reservoir, rather than from injection of fresh magma from below. If reservoir pressurization and magma ascent were slow enough, resulting strain might have been accommodated by viscoelastic deformation, without appreciable seismicity or surface deformation, until rising magma entered a brittle regime within 2–2.5 km of the surface. Given the remarkably gas-poor nature of the 2004–2008 dome lava, future eruptive activity might require a relatively long period of quiescence and reservoir pressurization or a large injection of fresh magma—an event that arguably has not occurred since the Kalama eruptive period (C.E. 1479–1720)

1. Scarps, Ridges, Troughs, and Other Lineaments on Mercury. 2. Geologic Significance of Photometric Variations on Mercury

Volcanic and tectonic implications of the surface morphology of Mercury are addressed in two separate sections. In Part 1, mercurian scarps, ridges, troughs, and other lineaments are described and classified as planimetrically linear, arcuate, lobate, or irregular. A global pattern of lineaments is interpreted to reflect modification of linear crustal joints formed in response to stresses induced by tidal spindown. Large arcuate scarps on Mercury most likely record a period of compressional tectonism near the end of heavy bombardment. Shrinkage owing to planetary cooling is the mechanism preferred for their production. Two planimetrically lobate escarpments probably formed by uplift along intersecting elements of the global mercurian lineament pattern. One may subsequently have been modified by extrusive igneous activity along its trace. Most irregular scarps inside craters are interpreted to be tectonic features formed in response to local stresses, perhaps induced by subsurface magma movements. Large linear ridges on Mercury may record a period of volcanism responsible, at least in part, for intercrater plains formation. Linear ridge production is speculatively attributed to accumulation of extruded material along linear vents, and to differential erosion around relatively resistant dikes intruded into near-surface materials. Linear, open-ended troughs are well-developed in a distinct terrain unit on Mercury characterized by intense modification of pre-existing landforms. Regional trends defined by these troughs are consistent with those of the global mercurian lineament pattern. Combined with their regional setting, this suggests that the troughs formed by differential erosion along linear crustal fractures. A few are radial from nearby large craters, and may be highly modified chains of secondary impact craters. Scarps, ridges, and troughs in and around Caloris Basin define trends radial from the basin center and concentric with its rim. A radial system of linear ridges outside Caloris probably reflects the combined effects of ejecta deposition and erosion during the basin-forming event. Planimetrically irregular ridges developed in smooth plains inside Caloris may owe their origin to regional subsidence, perhaps in response to magma withdrawal from below to form smooth plains outside the basin rim. Gravitational readjustment owing to loading by plains material may be responsible for scarp and ridge formation outside Caloris. Finally, isostatic readjustment to basin excavation may have caused regional uplift inside the basin to form a system of planimetrically irregular troughs. In Part 2, measurements of local normal albedo are combined with computer-generated photometric maps of Mercury to provide constraints on the nature of mercurian surface materials and processes. If the mercurian surface obeys the average lunar photometric function, its normal albedo at 554 nm is .16±.03. This is roughly 40% higher than the corresponding lunar value, but the difference may be largely attributable to differences in the photometric function s of the two bodies, and to unmodelled effects such as multiple scattering. The existence of relatively bright smooth plains confined to crater floors is most easily reconciled with a volcanic origin for some mercurian smooth plains. Lack of photometric contrast across most large escarpments on Mercury is consistent with the tectonic origin for these features inferred from morphologic studies. Local photometric and transectional relationships in two instances suggest mantling of preexisting topography by younger, perhaps volcanic, material. Brightness of several extremely localized patches in large craters is attributed to enhanced backscatter owing to multiple reflections relative to surrounding plains and craters. These patches are generally "bluer" than typical mercurian plains, and some are surrounded by material which is "redder" than typical plains. Chemical alteration of crustal rocks, perhaps related to fumarolic activity along impact-induced fractures, is the preferred explanation for these uniquely mercurian features.</p

A thermodynamical model for rainfall-triggered volcanic dome collapse

Dome-forming volcanic eruptions typically involve the slow extrusion of viscous lava onto a steep-sided volcano punctuated by collapse and the generation of hazardous pyroclastic flows. We show an unequivocal link between the onset of intense rainfall and lava dome collapse on short time scales (within a few hours) and develop a simple thermodynamical model to explain this behavior. The model is forced with rainfall observations from the Soufriere Hills Volcano, Montserrat, and suggests that when the dome is in a critical state, a minimum rainfall rate of approximately 15 mm hr-1 for 2-3 hr could trigger a dome collapse

Some comparisons of impact craters on Mercury and the Moon

Although the general morphologies of fresh mercurian and lunar craters are remarkably similar, comparisons of ejecta deposits, interior structures, and changes in morphology with size reveal important differences between the two populations of craters. The differences are attributable to the different gravity fields in which the craters were formed and have significant implications for the interpretation of cratering processes and their effects on all planetary bodies