A027

NON-UNIFORM DISTRIBUTION OF VOLCANIC HAZARDS, MOUNT RAINIER. D.R. Zimbelman, G.O.Logic, hazard@gorge.net, R.J. Watters, Univ. Nevada, J.K. Crowley, USGS, R.O.Rye, USGS. [GEO]

All of the geologic features necessary for large edifice collapse events are in place at Mount Rainier volcano. However, results from geologic mapping, remote sensing, geotechnical, and geochemical studies show that geologic features controlling the most potentially destructive volcanic hazard types occur non-uniformly. In a general sense, the east and west sectors of the volcano are more deeply dissected by glaciers, have hosted numerous historic debris avalanches and flows, and contain active fumaroles, large areas of hydrothermally altered rock, near-vertical fracture systems, and radial dikes. In contrast, the north and south sectors generally lack these features and have hosted fewer collapse events. Rainier’s young summit cone contains two overlapping, east-west-aligned craters and active fumaroles with magmatic components, suggesting that the geologic controls on the active hydrothermal system generally mimic controls on past events. Risk mitigation efforts will be most effective when they reflect the non-uniform distribution of the geologic controls on volcanic hazards, a consideration that increases in importance closer to the volcano.

A029

SIGNIFICANCE OF ROCK STRENGTH RESULTS AND SLOPE STABILITY CALCULATIONS TO EDIFICE AND FLANK INSTABILITY AT MOUNT RAINIER. R. J. Watters and S. D. Bowman, University of Nevada, Reno; D. R. Zimbelman, G. O. Logic, Washington; J. K. Crowley, USGS, Reston, Virginia. [GEO]

Rock strength results from field and laboratory studies permit using the strength of surface rocks to determine the confined rock mass strength for potential failure surfaces and the corresponding slope factor of safety (FOS). The strength results indicate that non-uniform rock mass strengths, consisting of relatively stronger and weaker rock masses, are normal on the volcano. Slope stability calculations from two areas where historical instability has occurred and one from one relatively stable area provide examples: 1) The Sunset Amphitheater area, where instability is common within altered and faulted rock; 2) Little Tahoma Peak, where major discontinuities and altered rock exist and was the site of a large rock avalanche in 1963; and 3) The Willis Wall, a ca. 1000m. headwall where slope angles exceed 50 degrees. Preliminary stability calculations indicate that the lowest FOS values, between 1.0 - 1.3, occurs at Little Tahoma Peak and Sunset Amphitheater, and the highest FOS values, 1.2 - 1.6, occur in the Willis Wall region. These results indicate that Sunset Amphitheater and Little Tahoma Peak are marginally stable and at high risk for future instability.

A040

SEISMICITY STUDIES AT MOUNT RAINIER. Seth Moran, USGS-Alaska Volcano Observatory, and Steve Malone, Geophysics Program, Univ. of Washington, Seattle. [GEO]

The Pacific Northwest Seismograph Network installed two seismometers on the flanks of Mount Rainier in 1989, and another at Camp Muir in 1993. Data recorded by these and other nearby seismometers over the last decade were used to investigate the nature of seismicity occurring directly beneath Mount Rainier, as well as the nature of the "plumbing system" beneath the volcano. An average of 1-2 high-frequency "volcano-tectonic" (VT) earth- quakes occur directly beneath the summit in a given month. VT earthquakes occur in several clusters located from 2 km above to 1 km below sea level, well below the inferred base of the volcanic edifice. Many of the computed focal mechanisms are normal, with most stress axes deviating significantly from the regional stress field. These characteristics are most consistent with earthquake occurrence in association with hydrothermal circulation within and below the base of the edifice, which serves to weaken rock and/or reduce effective stress to the point that gravity-induced slip can occur. These and other events were also used to invert for P-wave velocity structure beneath Mount Rainier. The resulting images show a broad region of slightly reduced velocities at depths of 4-14 km, possibly indicating the presence of a volume of hot rock with small amounts of magma and/or magmatic fluids. This volume could be the source for heat and magmatic fluids that feed existing surface fumarole fields.

A045

3-D EDIFICE STABILITY OF MOUNT RAINIER, WA. Mark E. Reid, Dianne L. Brien, and Thomas W. Sisson, all at USGS, Menlo Park, CA. [GEO]

Massive slope failures have affected the edifice and surrounding valleys of Mount Rainier, and the consequences of future failures may be severe. We use a 3-D slope stability model to examine edifice instability resulting from the interplay between topography, potential failure surfaces, and the 3-D distribution of rock strength. We evaluate the potential for large (> 0.1 km3) gravitationally-induced landslides using three scenarios. Our initial scenario, using homogeneous rock properties, examines instability induced by topography alone. In this case, the least stable portion of the edifice is predicted in the steep, north-facing Willis Wall, an area where few large landslides have originated. Our second scenario incorporates variations in strength caused by the hydrothermal alteration of volcanic rocks. Using geologic mapping, we divide the edifice into four units: strong basement rocks; relatively strong, fresh volcanic flows and breccias; slightly weaker, lightly altered rocks; and weak, highly altered rocks. In this scenario, the west-facing Sunset Amphitheater is predicted as the least stable; many past failures have originated from this area. In our third scenario we build on the second scenario by adding the 3-D geologic complexity of the large subsurface Osceola failure scar, where newer volcanic rocks overlie older altered rocks. This model predicts some reduced stability of the eastern flank, although the overall least stable region is still the Sunset Amphitheater area.

A047

THE BURROUGHS MOUNTAIN LAVA FLOW, MOUNT RAINIER, WA. KR Stockstill, TA Vogel, LC Patino, Michigan State Univ., E. Lansing, TW Sisson, USGS, Menlo Park. [GEO]

The andesite of Burroughs Mountain is representative of large volume andesite-dacite lavas erupted from flank vents on Mount Rainier. The 3.4 km3 lava conformably overlies andesitic block and ash-flow tuffs. The aims of our study are to understand the processes that create voluminous andesitic magmas and that lead to their explosive or effusive eruption. Samples of the lava and juvenile block and ash-flow clasts are chemically similar, despite differing eruptive styles, and range from 56.4 to 64.3 wt.% SiO2 and 2.4 to 4.9 wt.% MgO. Absence of geographic compositional zoning in the lava and similarity with the pyroclastic deposits suggests that ascent and degassing processes, rather than a zoned magma reservoir, led to the differing eruptive styles. Our working model is that the pyroclastic flows erupted from a summit vent, and the lava then erupted from a radial dike, having vented its gas complement through the previously-opened summit conduit system. The major element compositions of lava and tuff samples vary systematically, consistent with fractional crystallization, but incompatible trace element contents vary widely at constant apparent degrees of differentiation. This scattered variation of incompatible trace elements indicates that the magma reservoir was assembled from multiple parent magmas, perhaps from differing sources or involving differing degrees of assimilation, and that there was insufficient time for the magma to become homogenized or develop systematic compositional zoning before it erupted.

A054

BURIED FORESTS OF MOUNT RAINIER VOLCANO- EVIDENCE FOR EXTENSIVE HOLO-CENE INUNDATION BY LAHARS IN THE WHITE, PUYALLUP, NISQUALLY, AND COWLITZ RIVER VALLEYS. Patrick T. Pringle, WA Dept. Of Natural Resources, Div. of Geology and Earth Resources, Olympia, WA. [GEO]

Buried trees exist in every major river valley that drains Mount Rainier. Sediments from lahars and related flooding have buried forests as far downstream as Puget Sound and Toledo. The subfossil wood can be dated using 14C and/or dendrochronology, and can provide valuable paleoclimate data. 14C ages of recently exhumed forests in the Duwamish River valley, for example, indicate that lahars inundated significant areas of that valley about 1600? and 1100 years ago and suggest that eruptions of Mount Rainier have been more frequent and resultant flooding and rates and amounts of aggradation more extensive than previously thought. The presence of earlywood cells under bark of buried trees at Orting shows that the Electron Mudflow occurred in the spring about 600-500 years ago.

A056

MOUNT RAINIER SUMMIT CAVES VOLCANIC ACTIVITY. F. Le Guern, E Ponzevera, LSCE-CNRS, Yvette, France, W. Lokey Assistant Chief-Special Operations, Governor’s office of Emergency Services, Sacramento California, R.D. Schroedel Pierce County Dept. of Emergency Management, Tacoma, Washington. [GEO]

In 1997-1998 we explored the caves and took samples of the volcanic gases on the summit of Mount Rainier. In the eastern crater we mapped 700 meters of caves. The main fumaroles, in this crater, are located at the eastern entrance (airplane cave). Air circulated downwards in the eastern branch and upwards in the northern and southern branches. Very few fumaroles were observed deep within the cave and the air circulation kept the atmosphere safe to breathe. The CO2 content measured in the fumaroles was around 1% and the CO2 concentration in the cave atmosphere was close to 300 ppm. No sulfur was detected in the gases. In the western crater155 meters were mapped. Fumaroles with sulfur crystal formation at a temperature of 86 degree were located and sampled in the cave lake. The atmosphere in this cave contains 0.3% of CO2 and 2 to 5 ppm H2S giving a rotten eggs odour to the atmosphere. These concentrations are below the toxic admitted concentrations. Samples of soil minerals resulting from rock alteration by the volcanic gases were taken in both caves. Thanks to the cooperation of the National Park Service staff, the Mountain Climbing Rangers, the Tacoma Mountain Rescue Unit and Pierce County Emergency Management we have been able to work a week on the summit.

A061

HYDROTHERMAL INDICATORS IN STREAMS AND SPRINGS AT MOUNT RAINIER. David Frank, U.S. Environmental Protection Agency, Seattle, Washington. [GEO]

A surface water survey during 1993-98 sought evidence of the effects of hydrothermal processes and materials. The choice of indicator parameters was guided by a conceptual model of transport of hydrothermal fluids from the upper part of the cone outward toward areas of leakage on the lower flanks. This model considered hot acid sulfate-chloride water to be neutralized by reaction with andesite and cooled by dilution with cold ground water prior to discharge into surface waters. An alternative model considered dissolution of hydrothermally mineralized rock by cold water. Easily measured indicators that accommodate both models include temperature, pH, electrical conductivity, sulfate, and chloride. The geochemical characteristics of four sites stand out from among over 50 that were sampled within or proximal to the outcrop area of Mount Rainier andesite. Two sets of thermal springs with elevated sulfate and chloride occur near Paradise and Winthrop Glaciers. Cold neutral water with elevated sulfate and chloride discharges from Winthrop Glacier. Cold acid sulfate water drains from hydrothermally altered debris covering the snout of Tahoma Glacier.

A062

PROBABILITIES OF MOUNT RAINIER TEPHRA ERUPTIONS. Manuel Nathenson U.S. Geological Survey, Menlo Park, California. [GEO]

Most analyses of eruption probabilities assume that the probability of a volcanic eruption may be treated as a Poisson process. Time histories for some volcanoes match this assumption well. In some cases, however, the time history contains disparate time intervals between eruptions, with some being short and others being much longer. Mullineaux’s (1974) data for ages of tephra layers at Mount Rainier have three intervals >2000 years punctuated by seven that are <600 years. The exponential distribution for a Poisson process for the probability that an eruption will occur within some time period does not represent clumped data well. An alternate distribution is the double exponential. The basic notion is that there are two states, one involving short intervals and a second involving long intervals. The probability of an eruption occurring in each of these states is governed by an exponential distribution. This double-exponential distribution matches the available data well and resolves a conceptual problem for volcanoes with clumped eruption time intervals. Probability for a 30-year time period calculated using the double exponential of tephra eruptions for Mount Rainier is twice that for the single exponential.

Mullineaux, D. R., 1974, Pumice and other pyroclastic deposits in Mount Rainier National Park, Washington, U.S. Geological Survey Bulletin 1326, 83 p.

A063

SULFATE ANOMALIES AND LOADS OF SELECTED STREAMS DRAINING MOUNT RAINIER. Robert H. Mariner, U.S. Geological Survey, Menlo Park, CA. [GEO]

One way to determine which parts of Mount Rainier have been weakened by volcanic fluid/rock interaction is to look at the alteration products being transported by streams draining the mountain. Oxidation of fumarolic gases produce acid-sulfate waters which alter and bleach rock. Highest SO4 concentrations occur in Tahoma Creek, South Puyallup River, South Mowich River, and the West Fork of the White River. With some time lag, sulfate concentration varies inversely with discharge rate in Tahoma, South Mowich, and the West Fork of the White, but is almost constant and independent of discharge in the South Puyallup. Quantifying the sulfate load in these predominately glacial streams is difficult because of the large diurnal change in discharge and the generally poor gaging conditions. Measurements of sulfate loads at low-flow (Fall) conditions in 1997 and 1998 are consistent for Tahoma Creek (~5,000 kg/day), South Puyallup River (~900 kg/day), and West Fork of the White River (~3,000 kg/day). Anomalous sulfate concentrations and high sulfate loads of the aforementioned streams show that the west and north sides of Mount Rainier have been weakened by hydrothermal alteration.

A075

THE GEOLOGIC HISTORY OF MOUNT RAINIER VOLCANO, WASHINGTON. TW Sisson and MA Lanphere, USGS, Menlo Park, CA. [GEO]

Mount Rainier (MR) volcano is composed of andesite and dacite lava flows with subordinate pyroclastic flow deposits. Unlike nearby Mount St. Helens, lava domes and pumice falls are of neglible volume. Average MR lava and pumice are 61.8 wt.% SiO2 andesite and 64.2 wt.% SiO2 dacite, respectively. Pyroclastic flows are not distinct in composition from lavas. Modern MR began to form close to 500 ka atop the extensively eroded remains of an ancestral volcano that shed the lahars and pyroclastic flows of the voluminous Lily Creek formation, dated at 1.2 and 1.3 Ma. Eruptive activity moderated after ~1 Ma, but did not cease entirely, and then increased substantially subsequent to 500 ka, leaving a voluminous and nearly continuous volcanic record through the Holocene. Two episodes of heightened eruptive activity, 500-420 ka and 280-190 ka, marked the emplacement of an ~E-W radial dike system, the eruption from flank vents of the most voluminous individual lavas, and (probably) the formation of most flank hydrothermal alteration. Lavas erupted between 40 and 6 ka are confined to the upper S flank of MR and may fill a S-facing collapse crater. Volume estimates indicte an integrated eruptive rate of 0.4 km3/ka (±0.1) for the S-flank lavas. The altered and faulted ENE flank of MR collapsed at 5.6 ka, forming the Osceola mudflow. Subsequent eruptions nearly filled that crater and volume estimates for it indicate a similar integrated eruption rate of 0.35 km3/ka (±0.1).

A076

AN AUTOMATED LAHAR-DETECTION SYSTEM FOR THE CARBON AND PUYALLUP RIVERS, WA. Thomas L. Murray, Andrew B. Lockhart, Edward W. Wolfe, all U.S. Geological Survey, Vancouver, Tom R. Sharp, Pierce County Department of Emergency Management, Tacoma, Scott E. Fielding, City of Orting. [GEO]

To reduce the risk from lahars in the Carbon and Puyallup River valleys from a catastrophic flank collapse at Mount Rainier, the U.S. Geological Survey and the Pierce County Department of Emergency Management have begun a 2-year pilot project to install and operate an automated lahar detection system. With the cooperation of the City of Orting, Plum Creek Logging, and Champion Pacific, a set of 5 lahar sensors was placed in each drainage in 1998. As a lahar front passes the sensors, the ground vibration from the flow will trigger each sensor sequentially. Additionally, two of the sensors in each set act as "dead-men". They will be destroyed by any sizeable lahar and the system will note the cessation of their transmissions. Data from the sensors are telemetered to computers at the Washington Department of Emergency Management at Camp Murray and the Tacoma - Pierce County Law Enforcement Support Agency in Tacoma. The computers monitor the data and will sound an alarm when the proper sequence of events indicates that a lahar is en route. It is estimated that a lahar could reach the present confluence of the Puyallup and Carbon River valleys in as little as 30 minutes after detection.

A084

ANDESITIC SILL COMPLEX IN THE MOWICH LAKE AREA, NORTHWESTERN MOUNT RAINIER NATIONAL PARK, WASHINGTON. Paul E. Hammond, Department of Geology, Portland State University, Portland, OR 97207, Keith A. Brunstad, Department of Geology, Portland State University, Portland, OR 97207, Peter R. Hooper, Department of Geology, Washington State University, Pullman, WA 99164, Robert A. Duncan, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, and Bruce K. Nelson, Department of Geological Sciences, University of Washington, Seattle, WA 98195. [GEO]

To compare various lava-flow sequences previously designated as Fifes Peaks Formation in the central Cascade Range, we examined strata so named in Mount Rainier National Park. Mapping in the Mowich Lake area reveals not lava flows but a prominent andesitic sill complex about 60 km² in area with an exposed thickness of about 1 km. Sills are 0.5 to 150 m thick and traceable distances to 2 km. Dikes, 5 to 50 m thick, striking E-W to generally NW-SE, and plugs up to 100 m in diameter cut the sills, have similar composition, and were feeders to the complex. These feeders, and an underlying dioritic pluton at least 2 km² in area, are concentrated in a 4 km² area east of Mowich Lake, forming the probable intrusive center of the complex.

Four samples of sills produced ⁴⁰Ar-³⁹Ar isochron ages of 21.2 to 22.4 Ma; the mean age is 21.6±0.2 Ma. Three samples contain excess argon, based on non-atmosphere ⁴⁰Ar/³⁶Ar intercepts. The underlying diorite has an age of 16.5±0.1 Ma. The age of the sills clearly distinguishes the complex from the 26.4 Ma and compositionally distinct lava flows of the type Fifes Peaks Formation.

All textural gradations occur among the rock types, from aphanitic to phaneritic andesite to microdiorite. Phenocrysts compose 30 to 45% of the rocks, and are dominantly plagioclase, followed in order by orthopyroxene, clinopyroxene, and ilmenite-magnetite.

Pertinent compositions are 56-67% SiO₂, 0.7-1.4% TiO₂, 5.1-8.3% FeO*, 1.6-5.2% MgO, 0.3-3.3% K₂O, 0.13-0.29% P₂O₅, 320-970 ppm Ba, 5-87 ppm Rb, 260-710 Sr, 135-270 ppm Zr, 8-15 ppm Nb, and 17-32 ppm Y. The rocks are classified as medium-K, calc-alkaline in composition. They are well evolved magmatically; Mg# range from 20 to 35.

Isotopic analyses of three samples are the same, within error, and average ¹⁴³Nd/¹⁴⁴Nd = 0.51287 and ⁸⁷Sr/⁸⁶Sr = 0.70371. This is consistent with a common parental magma of depleted-mantle origin and little contamination by older crust.

Many sills and the underlying dioritic pluton have features indicative of multiple magmatic injections in their emplacement. Sills show crude columnar jointing from top to bottom margins, sharp but irregular intrusive contacts, locally abundant inclusions, and fluidal layering. The layering is defined by concentrations of minerals or grain sizes; some sills show disrupted and folded layering. Many sills show internal zones or layers of platy jointing. Some sills show differences in composition across their outcrop width. A few sills in the area of the intrusive center are "bulbous" in shape; that is, their thickness is 0.2 to 0.5 their lateral extent. In the basin east of Knapsack Pass a 100-m cap of breccia and underlying fluidally layered and disrupted aphanitic and phaneritic andesite grades downward into porphyritic microdiorite. A NW-striking dike of porphyritic quartz diorite, intruding the sills in the basin, is the youngest intrusion.

Host rocks are mainly lithic tuff-breccia, followed by rhyolitic tuff, volcanic sedimentary rock, and one 90-m sequence of scoriaceous basaltic lava flows. These rocks are about 24 Ma. The tuff-breccia, in particular, appears to have been water bearing and only partially consolidated during emplacement of the sills. Most rock is hydrothermally altered and bleached. Thermally metamorphosed rock is present only along contacts of the larger intrusions. The complex was probably emplaced at shallow depth, possibly less than 2 km, in the upper part of the Stevens Ridge Formation.

Lava flows along the White River valley north of the park are compositionally similar to the sill complex and may have been erupted in the same magmatic process that emplaced the complex. Because it is neither compositionally correlative nor age-equivalent to the type Fifes Peaks Formation, the sill complex should be named separately.

A086

HOLOCENE ERUPTIVE HISTORY OF MOUNT RAINIER. James W. Vallance, McGill University, and Susan Donoghue, University of Hong Kong. [GEO]

In the Holocene time, Mount Rainier erupted 11 vesicular tephras and 20 to 25 nonvescicular tephras. The vesicular tephras contain pumice, scoria, or both and range in volume from 1 to 100 million cubic meters. The sand- and silt-sized tephras comprise a mixture of juvenile black vitric pyroclasts, dark gray lithic (juvenile and accessory) fragments, and subordinate amounts of pumice. Analysis of glass in vitric pyroclasts shows an unusually wide range of pyroclast compositions within eruptives and between eruptives. These fine-grained tephras are widespread and represent a significant part of the Holocene eruptive history of Mount Rainier. The silt-sized tephras are glass rich and up to four appear to be associated with pyroclastic flows. Many of the sand-sized tephras correspond with periods of effusive cone building. Some of the nonvescicular tephras are part of eruption sequences that include vesicular tephras, and other represent discrete eruption. Mount Rainier has erupted ca. 20 times beginning ca. 9700 B.P. Multiple eruptions occurred from 6800 to 5000 B.P. About 5000 B.P. phreatomagmatic eruptions culminated in the formation of the Osceola Mudflow. Cone building ensured between 5000 and 4500 B.P. More recent notable eruptions include 4 to 5 between 2600 and 2200 B.P., two ca. 100 B.P., one ca. 500 B.P. and one ca. AD 1850.

A097

LAVA AND ICE INTERACTION AT MOUNT RAINIER, WASHINGTON. David T. Lescinsky, Arizona State University, Tempe, Tom W. Sisson, U.S. Geological Survey, Menlo Park, CA, Jonathan H. Fink, Arizona State University, Tempe, AZ. [GEO]

Numerous lava flows on Mount Rainier show evidence supporting interaction with glacial ice. The most notable of these are the thick ridge-forming flows that extend radially onto the lower flanks of the volcano. Great thicknesses (up to 400 m) and steep margins of these flows are the result of lava flowing into ice canyons within glaciers and ice sheets. Water and steam associated with melting of the ice rapidly quenched the sides of these flows producing glass and narrow (5-10 cm diameters) columnar fractures. Some lavas have columnar fractures on their upper surfaces indicating that they travelled some distance subglacially. Such indicators of lava-ice interaction can be used to determine the presence and approximate thicknesses of glaciers during past eruptions. Future lava flows at Mount Rainier will likely interact with ice and snow. Eruption observations at glaciated volcanoes suggest that lava flows are less hazardous than explosive eruptions. Lava melts ice causing increased runoff, but since melting is relatively slow few floods occur. However, when lava flow fronts collapse they produce hot avalanches and pyroclastic flows that can rapidly scour and melt snow and ice generating dangerous debris flows.

A102

GLACIER ADVANCES NEAR MOUNT RAINIER AT THE LAST GLACIAL/INTERGLACIAL TRANSITION. Jan T. Heine, University of Washgington, Seattle. [GEO]

The most recent example of global and rapid climatic change occurred at the transition from the last glaciation to the current interglaciation (ca. 15,400 to 11,600 cal yr BP). One key element to our understanding of rapid climate change is the geographic distribution of short-term climatic oscillations. A comparison of climate and glacier behavior in the Pacific Northwest with the relatively well-established sequences from northwestern Europe allows to test whether climatic events occurred throughout the northern hemisphere or not. Glaciers in the vicinity of Mount Rainier advanced twice during this time. Radiocarbon dates obtained from lake sediments adjacent to the corresponding moraines show that the first advance occurred before 13,200 cal yr BP. During the North Atlantic Younger Dryas event, between 12,900 and 11,600 cal yr BP, glaciers retreated near Mount Rainier, probably due to a lack of available moisture, but conditions may have remained cold. The onset of warmer conditions occurred at about 11,600 cal yr BP. Organic sedimentation lasted for at least 700 years before glaciers readvanced between 10,900 and 9950 cal yr BP. Glaciers in the vicinity of Mount Rainier seem to have advanced in response to regional or local shifts in late-glacial climate. The evidence does not support the view that glacier behavior in the Pacific Northwest paralleled that in the regions surrounding the North Atlantic.

A118

SURFACE ELEVATION MEASUREMENTS ON NISQUALLY GLACIER, MOUNT RAINIER, WA., 1931-1998. Driedger, C.L., U.S. Geological Survey, and Samora, B.A., Mount Rainier National Park. [GEO]

Between the mid-1800’s and the 1920’s, Nisqually Glacier receded about one kilometer. This retreat fueled concern among water managers that future glacier runoff would be insufficient to fill the reservoir at the newly completed hydroelectric facility at LaGrande. In 1931 Tacoma City Light began a series of transverse surface elevation measurements in an effort to measure the extent of thinning. These annual-to-semi-annual measurements have been continued by the U.S. Geological Survey and by private contract for the National Park Service, and are the longest continuous series of glacier measurements in North America. Measurements indicate that increased snowpack produces zones of locally thickened ice near the head of the glacier that propagate down valley as kinematic waves over a period of years. The greatest thickening during the period of measurement occurred between 1931 and 1945 when the glacier thickened by about 50% near 2,800 meters of altitude. This and subsequent thickenings during the mid-1970’s to mid-1980’s produced waves that advanced its terminus. Glacier thinning occurs during intervening periods. Between 1994 and 1997 the glacier thickened by 17 meters at 2,800 meters altitude, indicating probable glacier advance during the first decade of the 21st century.

Last Updated:Thursday, 06-May-2004 13:22:14 Eastern Daylight Time
http://www.nps.gov/archive/mora/ncrd/symposium/geo.htm
Author: Natural & Cultural Resourses Division,
Mount Rainier National Park