William W. Locke

Application of Auger Electron Spectroscopy (AES) to naturally weathered hornblende

Abstract The surface chemistry of naturally weathered hornblende has been analyzed using Auger Electron Spectroscopy. The high spatial resolution and depth profiling capabilities of this technique allow changes in the relative concentrations of cations to be determined over micron-scale areas and at depths resolvable on the sub-micron scale. These data indicate that during chemical weathering: 1) there is a systematic change in surface chemistry through a thickness up to 1200 angstroms, 2) complete cation depletion at the surface layer does not occur, 3) different components are leached to different depths to varying degrees, and 4) no new phase such as clay or smectite necessarily forms. A nonsteady state diffusion model is most consistent with these data.

Modelling of icecap glaciation of the northern Rocky Mountains of Montana

Abstract The extent of the lastglacial icecap over the northern Rocky Mountains of Montana has been inferred only from cursory correlation of its marginal deposits, which leaves several significant unresolved controversies. Those controversies all involve the maximum altitude and shape of that interconnected ice mass, and include (1) the source of the Two Medicine piedmont lobe (local or Cordilleran ice), (2) correlation of terminal deposits of the Flathead lobe (late lastglacial, early lastglacial, or prelastglacial), and (3) regional paleoclimate at last glacial maximum (dominant westerlies or local easterlies). Theoretical reconstructions of the glacial surface along major flowlines, constrained by ice-marginal features, nunataks and breached divides throughout the region, tentatively resolve those issues. The Two Medicine lobe was dominantly composed of northeast-flowing local ice, not ice of northwestern (Canadian cordillera) origin. Flathead lobe deposits include lastglacial deposits both supported by and without a component from the Swan Valley. There is evidence of only a regional westerly flow of moisture across the region. In addition, a proglacial lake 60 km long occupied the Swan Valley during the early stages of deglaciation. Although subject to revision from fieldwork, glacial process modelling remains one of the few geological arenas in which rigorous process modelling can be used to predict the evolution of form.

A 12,000‐year record of vertical deformation across the Yellowstone caldera margin: The shorelines of Yellowstone Lake

The 600 ka Yellowstone caldera exhibits several signs of unrest, the most evident of which is historic ground deformation including both uplift and subsidence. We document deformation in the area of the southeastern caldera across ∼12,000 years using the postglacial shoreline terraces of Yellowstone Lake. Raised shoreline elevations were interpreted from 230 leveling profiles surveyed across flights of terraces, with an accuracy of +/−0.5 m. Of about 11 recognizable terraces, the five most continuous raised shorelines were correlated around the lake basin to reveal deformation patterns. Shoreline ages are based on minimum- and maximum-limiting radiocarbon and obsidian-hydration dates. Each terrace is interpreted as representing an episode of uplift (∼1 kyr−1) of the caldera interior and subsequent subsidence, with little net volume change. This cyclic behavior may result from magma emplacement and subsequent withdrawal or cooling and crystallization, and/or episodic trapping and release of magmatic fluids (evolved during cooling and crystallization) in a self-sealing reservoir, as hypothesized by Dzurisin et al. (1990). Early postglacial shoreline deformation reveals substantial intracaldera subsidence, possibly reflecting cooling and loss of trapped hydrothermal fluids. Extension along north-south trending normal faults and related structures is also apparent in early to middle postglacial deformation both inside and outside of the caldera margin, including downwarping and local faulting of hot, weak intracaldera crust. Net deformation over the past ∼3 kyr has been dominantly up within the caldera interior and slightly down along the caldera rim, relative to the extracaldera region. This uplift is roughly similar to the historic pattern and may largely represent the effects of the most recent inflation episode. Subtraction of the total estimated magnitude of inflation in this episode suggests that the overall trend of postglacial deformation has been subsidence. The cause of this trend is undetermined but is most likely related to the effects of regional extension and long-term cooling within the Yellowstone caldera.

Origin and deformation of Holocene shoreline terraces, Yellowstone Lake, Wyoming

Geodetic surveys within the Yellowstone caldera have documented active uplift that is most likely caused by magmatic processes in the upper crust. Along the northeast shore of Yellowstone Lake, maximum relative uplift rates are 10 mm/yr for the period 1923-1975. However, information on deformation prior to historic instrumental records has been lacking. In this study, closely spaced data on elevations of postglacial shoreline terraces around the north end of Yellowstone Lake reveal complex tilting. Though most Holocene deformation is probably magma related, the pattern of shoreline tilting deviates significantly from the historic pattern of roughly symmetric inflation of the caldera. Along the northeast shore, where tilt directions of historic and shoreline deformation are similar, differential uplift of a > 2500-yr-old terrace is roughly 10 m; this gives a maximum uplift rate of 4 mm/yr. These unique Holocene terraces may exist due to episodic deformation because vertical movements affecting the lake outlet directly control lake level.

Pleistocene mountain glaciation in Montana, USA

Publisher Summary Montana is unique in hosting multiple Pleistocene examples of all glacier types including continental ice sheets, large mountain ice caps, small mountain ice caps and transection glaciers. The chapter reviews that uncounted valley, cirque, and niche glaciers also existed in more than 60 distinct mountain ranges. The distribution of these glaciers, with equilibrium-line altitudes rising from north-west to south-east, is consistent with control by moist air masses entering the north-western comer of the state. Prevailing winter westerly winds immediately south of the Cordilleran Ice Sheet at the glacial maximum are indicated. Mapped extents of Pleistocene glaciers in the mountains of western Montana were achieved from topographic map and aerial photographic interpretation, with limited field verification. Nowhere in Montana are the ages of glacial episodes well-constrained. Most Last-glacial moraines are correlated with the Pinedale glaciation elsewhere in the Rocky Mountains, rather than dated directly. Radiocarbon dates in the Yellowstone region and south of Glacier National Park and occurrences of Glacier Peak volcanic ash in the Flathead region show massive retreat or disintegration of the major ice caps prior to the Younger Dryas Chronozone. It is assumed that similar ages apply to the many mountain glaciers as well.

Obsidian-hydration dating of fluvially reworked sediments in the West Yellowstone region, Montana

Abstract This study evaluates obsidian-hydration dating in postglacial fluvial terraces cut into an outwash plain near West Yellowstone, Montana. Fluvial transport fractures obsidian grains. However, some old hydration rinds may be preserved, thus, a grain may record several fracturing events. The most recent fracturing event at West Yellowstone is recorded in surface sediments from all of the terraces, which were cut in a shorter period of time than the technique can discern. They formed about 19,000 ± 1000 yr ago, using published hydration-rate estimates and a mean rind thickness of 6.34 ± 0.14 μm (1 SE). Alternatively, the application of published hydration-rate constants for the Obsidian Cliff flow with an estimated effective hydration temperature of 1.4°C yield an age of 24,400 ± 1100 yr (1 SE). Thicker rinds record fracturing during Bull Lake glaciation and cooling cracks from the emplacement of several source flows. Much of the observed spread in rind thicknesses (6.34 ± 1.69 μm: 1 SD) is probably the result of chemically induced variations in hydration rate. Terrace ages based on a single rind would range from 13,000 to 39,000 yr (±1 SD). Therefore, it is inappropriate to (1) use a set of hydration-rate constants determined from a single sample to calculate ages for multiple artifacts or geological samples, (2) date an archaeological or geological event on the basis of a single artifact, or (3) generate a chronostratigraphy on the basis of individual dates as a function of depth. Multiple evaluations of source chemistry and hydration rates and multiple rind measurements are required to date fracturing events.

Teaching geomorphology through spreadsheet modelling

Geomorphology, like most scientific disciplines, has evolved from a descriptive stage through quantification to modelling of processes and outcomes. Indeed, high-resolution modelling has been termed the third branch of science for its ability to generate high-quality hypotheses, testable in turn through field mapping and laboratory experimentation. I teach the principles and basic applications of modelling using microcomputer spreadsheets. These serve as transparent boxes in which graphical output is generated in response to numerical input, and more importantly, in which the modeler (student) can see the equations and order of steps (unlike traditional black box models). The technology is widely available and familiarity with it can be assumed, required, or taught at the undergraduate level. Modelling can be taught through a series of spreadsheet exercises in which students, while learning topical material in a traditional sequence, progress from a simple, cookbook exercise through more realistic, process-based exercises to the level of addressing a previously unstudied research question. At each step the model can be used to generate testable hypotheses. I have worked with scarp evolution, stream longitudinal profiles, and glacier reconstructions, and plan to include eolian, soil development, mass wasting, and coastal processes in additional exercises. The first year of use of this technique in a junior-level Geomorphology course clarified the strengths and weaknesses of the modelling approach. The strengths involve students learning the capabilities and flaws of modelling, a stronger understanding of causeeffect relationships, and the effects of time on landforms. The weaknesses involve time commitment, supervision in model implementation, and limitations to student ability to independently pursue openended research.