Crystallization history of Obsidian Dome,Inyo Domes,California

Samples obtained by U.S. Department of Energy research drilling at the 600-year-old Obsidian Dome volcano provide the rare opportunity to examine the transition from volcanic (dome) to plutonic (intrusion) textures in a silicic magma system. Textures in the lavas from Obsidian Dome record multiple periods of crystallization initiated in response to changes in undercooling (T) related to variable degassing in the mag-ma. Phenocr)ysts formed first at low T. A drastic increase in T, related to loss of a vapor phase during initial stages of eruption, caused nucleation of microlites. All of the lavas thus contain phenocrysts and microlites. Extrusion and subsequent devitrification of the dry (0.1 wt% H₂O) magma crystallized spherulites and fine-grained rhyolite at high T. A granophyric texture, representing crystallization at a moderate T, formed in the intrusions beneath Obsidian Dome. Textures in the intrusion apparently represent crystallization of hydrous (1–2 wt% H₂O) rhyolitic magma at shallow depths.     

Shallow degassing events as a trigger for very-long-period seismicity at Kīlauea Volcano,Hawai‘i

The first eruptive activity at Kīlauea Volcano’s summit in 25 years began in March 2008 with the opening of a 35-m-wide vent in Halema‘uma‘u crater. The new activity has produced prominent very-long-period (VLP) signals corresponding with two new behaviors: episodic tremor bursts and small explosive events, both of which represent degassing events from the top of the lava column. Previous work has shown that VLP seismicity has long been present at Kīlauea’s summit, and is sourced approximately 1 km below Halema‘uma‘u. By integrating video observations, infrasound and seismic data, we show that the onset of the large VLP signals occurs within several seconds of the onset of the degassing events. This timing indicates that the VLP is caused by forces—sourced at or very near the lava free surface due to degassing—transmitted down the magma column and coupling to the surrounding rock at 1 km depth.     

Scheelite skarn granitoids: An evaluation of the roles of magmatic source and process

The compositions, mineralogies, textures, and isotopic characteristics of granitoids associated with scheelite skarns indicate these plutonic rocks cannot be uniquely described in terms of source materials, although most show distinguishing features of I-type granites^*. Scheelite skarn granitoids exhibit variable evidence for crustal contamination (primarily in their Sr isotopic ratios), but there is no correlation between degree of contamination (as measured by compositiona, mineralogical, and isotopic data) and size or abundance of associated scheelite skarns. Scheelite and Cu skarn-associated granitoids are generally similar, which implies similar sources for these granitic rocks. Textural and bulk compositional data, however, suggest that scheelite skarn granitoids are different from Cu skarn granitoids by virtue of greater degree of differentiation and by crystallization in a comparatively deep plutonic environment. Consideration of relevant phase equilibria indicates that magmatic water does not exsolve until very late in the crystallization of a scheelite skarn granitoid. Through this means, tungsten is concentrated in exsolved magmatic fluids by a combination of large degree of fractional cystallization and magmatic equilibration with a very Cl-rich exsolved aqueous phase. In consequence, the search for scheelite skarns should be based not on the search for plutons with appropriate chemical compositions (major, trace, or isotopic), but rather for plutons displaying mineralogical, textural, and general geologic features which point to crystallization in a highly fractionated, relatively deep environment. Large-scale tectonic features which might give rise to crystallization in this environment include crustal overthickening, which could be caused by collisional (accretionary) events.     

Oxyanion Concentrations in Eastern Sierra Nevada Rivers – 2. Arsenic and Phosphate

Water samples were collected from the Truckee River-Pyramid Lake system, the Walker River-Walker Lake system, and the Carson River, all located in eastern California and western Nevada, U.S.A., at three different times (i.e., summer 1991, spring 1992, and autumn 1992) over a two year period. The concentrations of As, Na, Cl, PO4, and pH were measured in these river samples and the associated terminal lakes. Arsenic values ranged from below 13 nmol/kg near Truckee, California to 160 nmol/kg at Nixon, Nevada in the Truckee River, from 40 nmol/kg in the headwaters of both West and East Walker Rivers to 270 nmol/kg below Weber Reservoir on the main branch of the Walker River, and from <27 nmol/kg to 234 nmol/kg for the lower Carson River system. Arsenic concentrations in Steamboat Creek (0.91 mol/kg–1.80 mol/kg) in the Truckee River catchment are above the U.S. EPA drinking water maximum contaminant level of 0.67 mol/kg, as are the As concentrations in both Pyramid Lake (1.33 mol/kg–1.57 mol/kg ) and Walker Lake (13.7 mol/kg–18.7 mol/kg). Sources of As for all three rivers include weathering of As-rich rocks and/or regolith and input of high-As geothermal spring waters, both processes primarily, although not exclusively, adding As to the headwater regions of these rivers. Steamboat Hot Springs (29 mol/kg As 54.5 mol/kg), for example, is identified as a source of As to the Truckee River via Steamboat Creek. The high As concentrations in Pyramid and Walker Lakes are likely due to (1) desorption of arsenate from aquatic particulate matter in these high pH waters (9.0 pH 9.5), (2) limited biologic uptake of arsenate, and (3) evaporative concentration of the lake waters. Evaluation of molar PO4}/As ratios of river waters and geothermal spring waters (e.g., Steamboat Hot Springs), indicates that phosphate is substantially enriched in Steamboat Creek as well as the mid to lower reaches of the Walker and Carson Rivers. These regions of each river are dominated by agricultural interests and, additionally, in the case of Steamboat Creek, residential areas and golf courses. Our data strongly imply that phosphate-rich agricultural return flow has likely added P to these streams and, consequently, increased their respective P:As ratios.     

Mining-induced seismicity in faulted geologic structures: An analysis of seismicity-induced slip potential

Relationships between the locations of mining-induced seismic events, local fault structure, and mine geometry were examined in a deep hard-rock mine in northern Idaho. Stopes experiencing rock bursts and other large seismic events were found to fall into two structural regimes: the Silver Vein, and the N48°W Trend, a steeply dipping plane of seismic activity that is subparallel to major local steeply dipping faults which bound blocky structures. The N48°W Trend also intersects a shaft that was seriously damaged when fault gouge was expelled into the opening during a 3-month period of high seismic energy release. Models of stress interaction are used to support the hypothesis that mining-induced deformation was mobilized along a 1.5 km length of the N48°W Trend. Specifically, numerical models are used to simulate rupture of seismic events and estimate induced changes in the quasi-static stress field. A Coulomb failure criterion is used with these results to estimate the spatial variation in potential for slip on planes parallel to local faulting. Increases in the potential for slip on fault planes subparallel to the N48°W Trend are consistent with activation of deformation along its 1.5 km length. For events with constant seismic moment, stress drop is shown to be far more important than source dimension in elevating slip potential along the observed plane of seismic activity     

An analytical solution for rapidly predicting post-fire peak streamflow for small watersheds in southern California

Following wildfires, the probability of flooding and debris flows increase, posing risks to human lives, downstream communities, infrastructure, and ecosystems. In southern California (USA), the Rowe, Countryman, and Storey (RCS) 1949 methodology is an empirical method that is used to rapidly estimate post-fire peak streamflow. We re-evaluated the accuracy of RCS for 33 watersheds under current conditions. Pre-fire peak streamflow prediction performance was low, where the average R² was 0.29 and average RMSE was 1.10 cms/km² for the 2- and 10-year recurrence interval events, respectively. Post-fire, RCS performance was also low, with an average R² of 0.26 and RMSE of 15.77 cms/km² for the 2- and 10-year events. We demonstrated that RCS overgeneralizes watershed processes and does not adequately represent the spatial and temporal variability in systems affected by wildfire and extreme weather events and often underpredicted peak streamflow without sediment bulking factors. A novel application of machine learning was used to identify critical watershed characteristics including local physiography, land cover, geology, slope, aspect, rainfall intensity, and soil burn severity, resulting in two random forest models with 45 and five parameters (RF-45 and RF-5, respectively) to predict post-fire peak streamflow. RF-45 and RF-5 performed better than the RCS method; however, they demonstrated the importance and reliance on data availability. The important parameters identified by the machine learning techniques were used to create a three-dimensional polynomial function to calculate post-fire peak streamflow in small catchments in southern California during the first year after fire (R² = 0.82; RMSE = 6.59 cms/km²) which can be used as an interim tool by post-fire risk assessment teams. We conclude that a significant increase in data collection of high temporal and spatial resolution rainfall intensity, streamflow, and sediment loading in channels will help to guide future model development to quantify post-fire flood risk.     

Forum The observational side of volcanology

American Scientist , I think. One panel shows an Einstein-like figure in an easy chair with a pencil and pad of paper; this panel is labeled Big Science. The other panel shows the headquarters of a high-tech company and is labeled Little Science. Think about it. Science builds on testable ideas, often qualitative in nature, that commonly arise from observations of natural phenomena. Technology confirms or denies those ideas and helps to quantify them. Both are important, and there is considerable feedback, but fundamentally the ideas drive the technology. Hence the cartoonist had it right, despite society’s common perception of what is big and what is little. Big bucks do not equal big science. Volcanology is a science, the study of volcanoes. Ideas are key to our understanding of how and why volcanoes erupt. Many of these ideas are formulated from direct observations of volcanoes and their products before, during, and after eruptions. Observational volcanology may seem old-fashioned today but remains one of the most stimulating endeavors I know. If not big science, at least it is moderate science. And rather simple, too. All you need are your eyes, ears, nose, and brain, together with suitable equipment for the situation (often only a hammer or spade). In many instances simple observations and related measurements provide fundamental information about how volcanoes work. I described three such instances in Chapter 21 of USGS Bulletin 1966 and elaborated there my feelings about the importance of field observations for monitoring volcanoes and the concept of keeping monitoring, i.e., repeated direct observation, as simple as practical. I am disheartened by the recent deaths of volcanologists in the field but encouraged by the general understanding that the volcanologic community has shown. No one wants the death rate to continue unchecked, but no one is seriously suggesting cutting back on field observations by volcanologists either. The best way to reduce fatalities is to understand the volcano better. The best way to understand the volcano better involves field observations as well as electronic sensors. Meanwhile, it is well to remember that volcanology is the study of volcanoes, and that purely scientific, curiosity-driven motives are as justified as those designed purely to mitigate risks, and I think more valuable in the end. Curiosity leads to understanding, and understanding is the paramount goal of the science as well as the soundest basis for reducing risk. Volcanologists who are curious will get themselves into trouble and sometimes die because of it. It is often stated that we must weigh the potential benefits and risks before doing something that may be perceived as risky. Of course we must, but it is mathematically impossible to solve one equation with two unknowns, and generally the potential benefits and risks are both unknowns. In the end it comes down to common sense, which varies among individuals and in any case is far from foolproof. Let is be no other way, and let us praise the curious as we mourn the dead.     

The complex filling of alae crater,Kilauea Volcano,Hawaii

Since February 1969 Alae Crater, a 165-m-deep pit crater on the east rift of Kilauea Volcano, has been completely filled with about 18 million m³ of lava. The filling was episodic and complex. It involved 13 major periods of addition of lava to the crater, including spectacular lava falls as high as 100 m, and three major periods of draining of lava from the crater. Alae was nearly filled by August 3, 1969, largely drained during a violent ground-cracking event on August 4, 1969, and then filled to the low point on its rim on October 10, 1969. From August 1970 to May 1971, the crater acted as a reservoir for lava that entered through subsurface tubes leading from the vent fissure 150 m away. Another tube system drained the crater and carried lava as far as the sea, 11 km to the south. Much of the lava entered Alae by invading the lava lake beneath its crust and buoying the crust upward. This process, together with the overall complexity of the filling, results in a highly complicated lava lake that would doubtless be misinterpreted if found in the fossil record.     

Analytical and numerical models to explain steady rates of spring flow

Flow from some springs in former glacial lakebeds of the Upper Midwest is extremely steady throughout the year and does not increase significantly after precipitation events or seasonal recharge. Analytical and simplified numerical models of spring systems were used to determine whether preferential ground water flow through high-permeability features in shallow sandstone aquifers could produce typical values of spring discharge and the unusually steady rates of spring flow. The analytical model is based on a one-dimensional solution for periodic ground water flow. Solutions to this model suggest that it is unlikely that a periodic forcing due to seasonal variations in areal recharge would propagate to springs in a setting where high-permeability features exist. The analytical model shows that the effective length of the aquifer, or the length of flowpaths to a spring, and the total transmissivity of the aquifer have the greatest potential to impact the nature of spring flow in this setting. The numerical models show that high-permeability features can influence the magnitude of spring flow and the results demonstrate that the lengths of ground water flowpaths increase when high-permeability features are explicitly modeled, thus decreasing the likelihood for temporal variations in spring flow.