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Secondary calcite crystallization and oxidation processes of granite near the summit of Mt. McKinley, Alaska

Processus secondaires de cristallisation et d’oxydation du granite près du sommet du Mont McKinley, Alaska
Kenji Yoshikawa, Yositomi Okura, Vincent Autier et Satoshi Ishimaru

Résumés

Sur la montagne du Denali, Alaska (également connue sous le nom de Mont McKinley), les processus de désagrégation sont dominés par la cristallisation secondaire (calcite) et l’oxydation (vernis), avec une action limitée du gel. Pendant les étés de 2002 et 2003, deux sites d’étude (versants nord et sud d’une face rocheuse) situés à 5 710 m d’altitude sur le Denali ont montré un nombre limité de cycles gel-dégel actuels (30 et 63 cycles, respectivement) ; ces cycles peu fréquents sont semblables à ceux mesurés à l’intérieur des terres de l’Antarctique (46 cycles gel-dégel en 1994). Les conditions d’humidité de la roche ont été influencées par le rayonnement solaire et par la neige soufflée. La neige soufflée et sa fonte ont fourni une humidité abondante à la surface de la roche. Dans la tranche d’altitude de 3 000 à 3 500 m, la neige soufflée est la plus active sur les faces sud et sud-est où la température de la roche est la plus élevée. Le taux de saturation des fissures ou de surfaces poreuses est considéré comme minimal. La cristallisation secondaire et l’oxydation sont régies par l’évaporation de surface, l’apport d’humidité et les propriétés chimiques de la roche. Le paramètre le plus important dans le secteur d’étude est l’apport d’humidité, pour une même roche et un niveau de rayonnement solaire constant. Les particules de neige sont déplacées par les vents, apportant l’humidité à la surface de la roche où la neige s’entasse, principalement aux pieds des falaises et aux cols. La cristallisation secondaire et l’oxydation se sont ainsi plus développées aux pieds des falaises et au niveau des cols exposés au sud et au sud-est où la température est la plus élevée.

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Notes de la rédaction

Acknowledgments

The authors would like to thank the International Arctic Research Center (IARC) for financial support, the Japan Alpine Club and Hakusan Corporation for logistic support, and K. Irving and T. Saitoh for logistic support in the field.  The authors gratefully acknowledge the thorough reviews of Drs. A. Hequette, L. Hinzman, D. Sellier, S. Boatwright, and an anonymous reviewer for having improved this manuscript. The Thermal IR camera was made available by the Institute of Northern Engineering, University of Alaska Fairbanks.

Texte intégral

Introduction

1Denali (also known as Mount McKinley), part of the Alaska Range, is located at Lat. 63°04’15”N, about 330 km south of the Arctic Circle. The mountain is well-known as the highest in North America (6194 m ; 20,320 ft). Bedrock on Denali mostly consists of pink granite. Black shale covers the granite at the north of Denali Pass. The intrusive masses of the Alaska Range are part of a massive oval-shaped granitic batholith including most of Denali’s predominantly quartz monzonite. The age of the granite is about 57.3 Ma (Connor and O’Haire, 1988). The study area is affected by more severe climatic conditions and lower solar radiation compared with higher (>7000 m) mountains on Earth, such as those in the Himalayas and the Andes. The climatic conditions of this mountain, however, are not well known. The Japan Alpine Club (JAC), working with the Denali National Park Service, established a meteorological station just above Denali Pass (5710 m) in 1990 (e.g. Okura, 1994). The areas of higher altitude consist of arid and cold environments due to lower atmospheric pressure and strong winds. Despite the scarcity of liquid water, however, almost all exposed rocks are varnished and cracked. Frost action is the most frequently reported weathering process in cold regions for the maritime (Hall, 1987) and continental regions (Matsuoka et al., 1990 ; Yoshikawa et al. 2000). In addition, salt weathering and chemical weathering have been reported in polar regions (Johnston, 1972 ; Ishimaru and Yoshikawa, 2000). J. Hirschwald (1912) and W.W. Thomas (1938) suggest that the actual moisture value (Saturation Coefficient or S-value) is a major factor governing frost weathering. C. Tourenq (1970), and C. MacInnes and J.J. Beaudoin (1968) have tried to better understand the relationship between moisture saturation and weathering intensity. Other research concluded that weathering was influenced by the moisture distribution within the rock as well as the moisture saturation (Hall, 1986).

2A data logger-controlled weather station near Denali Pass (Okura, 1994) shows a mean air temperature of about -30 °C since 1990. Winds are generally west to north with a maximum wind speed exceeding 60 m/s (Okura, 1994). Relative humidity ranges from 60-80% in summer and around 55% in winter. Although air temperatures did not rise above freezing (> 0° C) during the study period (1990-2003), melting snow particles were observed on the meteorological station, as well as on most of the south-facing outcrops. The purpose of this study is to clarify the importance of non-frost action on rock weathering in a sub-polar high mountain environment. A study site was selected on Mt. Denali, in the Alaska Range. This paper describes the thermal and chemical properties using X-ray diffraction for energy dispersion spectrometry (EDS) of the rock surface, freeze-thaw cycles, and the varnish intensity in different geographical locations (north-, south-facing walls and elevation) using GIS.

Methods

3The air and the rock surface temperatures were measured on north- and south- facing rock walls (>80°) near the weather station situated at 5710 m (fig. 1). Bedrock surface temperatures were monitored with thermistor probes connected to a four-channel micro-data-logger (Onset computers H8 series) with external extra batteries. Two logger systems were installed to record rock temperatures. Each system was placed on a representative granite outcrop on north- and south- facing granite rock. A thermistor probe 6 mm in diameter was installed on the south- and north-facing walls and cemented with a mixture of rock dust and silicone rubber. Rock surface temperatures were measured from June 2002 to June 2003.

Fig. 1 – Map of study area indicating measuring points.
Fig. 1 – Carte des sites d'étude indiquant les emplacements de mesure.

Fig. 1 – Map of study area indicating measuring points. Fig. 1 – Carte des sites d'étude indiquant les emplacements de mesure.

Rock temperature has been measured on the north- and south-facing walls. Weather station : measured air temperature, ground temperature, and wind speed (map based on Swiss Foundation for Alpine Research and University of Alaska Press, 1990).
La température de la roche mesurée sur les versants nord et sud. Station météorologique : température de l'air,  température du sol et vitesse de vent (carte fondée sur la Fondation suisse pour les recherches alpines et Presse de l'université d'Alaska, 1990).

4Rock sample examination focused on chemical properties of the surface crystallization materials and varnish. Micro-scale weathering processes were subsequently analyzed at the geological survey of Hokkaido, Japan. The weathered crystallization and surface varnish materials were identified using X-ray diffraction analysis. Salts and clay minerals on the rock surface were identified using X-ray diffraction (XRD : Rigaku RAD-C) and energy dispersion spectrometry (EDS : JOEL JED-2110) to determine the influence of salt weathering or hydrolysis on surface minerals. The varnish film was viewed by optical microscopy, in transmitted light microscopy (SEM : JOEL JSM-5310LV) and analyzed using EDS to determine its distribution and elementary composition. SEM observation and XRD analysis were also used to identify potential water saturation and examine frost action at the pore space of the rock. Remote sensing analysis using Airborne Visible InfraRed Imaging Spectrometer (AVIRIS) was carried out in a granite area for determining the spatial distribution of calcite and varnish. Rock surface temperature variations were also determined using thermal infrared camera (ThermaCAM P40, FLIR) systems. The dataset for this study was collected on 13 June 1995. The AVIRIS instrument contains 224 different detectors, each with a wavelength sensitivity range of approximately 10 nanometers (nm), allowing it to cover the entire range between 380 nm and 2500 nm.

5Calcite (CaCO3) deposition and iron (Fe) oxidation are an ideal case for remote sensing because they are so ubiquitous on the exposed surface of Denali. Further, due to the high levels of the carbonate absorptions in the mid-infrared wave range, at least one wavelength — if not all — are saturated in reflectance. The carbonate absorption at 2350 nm is reduced in depth, the 1610 nm band is absent, and the UV absorption is weak. There are whole suites of iron oxides, iron hydroxides, iron sulfates and so on that are mostly unrecognizable, and many amorphous phases, all with similar electronic absorption bands in the visible and near-infrared. Iron oxides, hydroxides, and sulfates are also detected by spectroscopy at very low levels because of the strong absorption bands in the infrared. In nature, there appear to be many amorphous varnish films with similar levels of absorptions. Despite extensive snow cover, the AVIRIS imagery proved to be very useful for mapping weathering intensity in the Denali region. Another advantage of the study area is the uniformity and likely homogeneity of rock properties over the region. The low-altitude AVIRIS imagery with 1.8 m ground resolution was far superior to satellite imagery for discerning weathered surfaces. Due to the much lower flight altitude (~12,000 feet), atmospheric artifacts were largely absent from the low-altitude AVIRIS imagery. In general the low-altitude AVIRIS imagery yielded a good correlation between mapping results and elevation/slope aspect. The maps created using the three different bands (channels) should constitute useful tools to guide further geochemical and geological research.

6The thermal infrared camera, mounted on a Cessna 185 aircraft, was used for understanding the spatial distribution of the rock surface temperature. Thermal images (7.5-13 mm) were taken on 10 May 2005 in late afternoon, 18 :30-20 :00 Alaska Daylight Saving Time (+8h GMT), after the area had received more than 12 hours of solar radiation. Atmospheric transmission correction was automatically adjusted for the distance, atmospheric temperature and relative humidity.

7Snow pit and oxygen and hydrogen stable isotope snow sampling was carried out along the West Buttress standard mountaineering route (Kahiltna Glacier, Denali Pass to South peak) during June 2004 (Kanamori et al., 2004). Frequent Jet Stream circulation over the northern hemisphere brought strong winds, causing high sublimation conditions near the Mt. Denali summit. The degree of the sublimation and humidity was evaluated through calculation of a deuterium excess d (d-excess) parameter :

8d-excess = δ2H -8δ18O

9The d-excess is strongly affected as humidity takes place. When humidity is low, the vapor is strongly depleted and the deuterium excess d reaches or exceeds 10.

Results

Chemical properties

10Six samples (30- 1000 cm3) were collected from the rock wall. The wall shows reddish brown parts indicative of weathering, and pink fresh parts. The surficial reddish coloring ranged from 0.3-1.0 mm in thickness. The absence of clay minerals and salts suggested that hydrolysis and salt weathering are insignificant. On Mt. Denali, oxidation (varnish) is an important weathering process. The ferrous iron in minerals oxidizes to ferric iron. In contrast to the Fe+2 ion, which forms a structural relationship with other ions in the mineral, ferric iron concentrates in cracks in and between mineral grains. This ferric iron is generally termed “limonite”(Carroll, 1970). On the other hand, desert varnish in some studies is due to hematite (Fe2O3) coating (for example Engel and Sharp, 1958). In any case, both materials are produced by oxidation.

Fig. 2a – The surface of the varnish rock consists of Ca, Al, and Fe from X-ray diffraction for energy dispersion spectrometry (EDS) and SEM image.
Fig. 2a – La surface d’oxydation de la roche est constituée de Ca, Al, et de Fe, mesurés aux rayons X, par spectrométrie de dispersion d'énergie (EDS) et à partir d’images au MEB.

Fig. 2a – The surface of the varnish rock consists of Ca, Al, and Fe from X-ray diffraction for energy dispersion spectrometry (EDS) and SEM image.Fig. 2a – La surface d’oxydation de la roche est constituée de Ca, Al, et de Fe, mesurés aux rayons X, par spectrométrie de dispersion d'énergie (EDS) et à partir d’images au MEB.

11EDS analyses showed that the rock surface consists of primarily Si, Al, K, Fe, and S (see fig. 2a). Si, Al, K are contained in silicate minerals. Ferrous iron is contained in hypersthene, biotite, ilmenite, and magnetite. Our analyses also show that the margins of these crystals are altered. Therefore, the ferric iron in limonite and hematite seems to be generated from these crystals.

Oxygen and hydrogen isotope analyses

Table 1 – Stable isotope results and ‘d-excess’ from surface snow of climbing route.
Tableau 1 – Résultats d’analyse des isotopes stables et de l’excès de deutérium (d-excess) à la surface de la neige le long de l'itinéraire de l’ascension.

Table 1 – Stable isotope results and ‘d-excess’ from surface snow of climbing route.Tableau 1 – Résultats d’analyse des isotopes stables et de l’excès de deutérium (d-excess) à la surface de la neige le long de l'itinéraire de l’ascension.

12Isotope results show that the evaporation potential (sublimation) increases with altitude (table 1). The value of the d-excess at the summit of Mt. Denali was almost the same as that found in the middle of the Arabia Peninsula (Rozanski et al., 1993). The d-excess is strongly affected as sublimation takes place. The isotope results support SEM and EDS results as the relatively fresh conditions in the cracks show Ca and Fe concentrations, but the more weathered rock surfaces do not (fig. 2b). Snowmelt water would not remain longer on the surface of rocks under dry conditions, and the amount of snowmelt present at the rock surface is not sufficient to fill in cracks.

Table 1 – Stable isotope results and ‘d-excess’ from surface snow of climbing route.
Tableau 1 – Résultats d’analyse des isotopes stables et de l’excès de deutérium (d-excess) à la surface de la neige le long de l'itinéraire de l’ascension.

Table 1 – Stable isotope results and ‘d-excess’ from surface snow of climbing route.Tableau 1 – Résultats d’analyse des isotopes stables et de l’excès de deutérium (d-excess) à la surface de la neige le long de l'itinéraire de l’ascension.

Remote sensing analysis

Fig. 3 – A false color image of Mt. Denali Mountain, Alaska
Fig. 3 – Image en fausse couleur du Mont Denali, Alaska.

Fig. 3 – A false color image of Mt. Denali Mountain, Alaska Fig. 3 – Image en fausse couleur du Mont Denali, Alaska.

Red : band 140, Green : band 223, Blue : band 2. The dataset was flown on 13 June 1995.
Rouge : bande 140, vert : bande 223, bleu : bande 2. Les données sont dérivées d’un survol effectué le 13 juin 1995.

13Figure 3 is a false color image of Mt. Denali, Alaska. The bands used to create this image are Red Plane= band 140 (1690.38 nm), Green Plane = band 223 (2489.11 nm), and Blue Plane = band 2 (400.02 nm). This figure shows the spectra characteristics of both freshly exposed and severely weathered rock surfaces obtained from the AVIRIS data. Weathered rock indicated calcite and varnish (oxidation) according to absorbed wave windows, 2350 nm for calcite and 1650 nm for varnish (oxidation) respectively. Thermal infrared images indicated warm (> 0 °C) rock surface temperatures (7 °C) at below 4500 m a.s.l. on south- to southwest-facing slopes (fig. 4). These positive temperature areas and high Ca or Fe coating areas showed overlap over the entire mountain body. Slope aspect is one of the major parameters controlling weathering process and water supply. The geomorphological relationship between weathering severity, slope, and altitude are revealed using GIS (Arc/Info) techniques. Severely weathered (showing varnish and calcite) rock surfaces are observed at lower elevations (3000-3500 m a.s.l.) and on south- to southeast-facing rock walls. No correlations were observed between weathering severity and slope angles.

Fig. 4 – Thermtal IR image of south-facing wall of Denali, taken on 10 May 2005.
Fig. 4 – Image IR thermique de la face sud du Mont Denali, prise le 10 mai 2005.

Fig. 4 – Thermtal IR image of south-facing wall of Denali, taken on 10 May 2005.Fig. 4 – Image IR thermique de la face sud du Mont Denali, prise le 10 mai 2005.

Discussion

14Solar radiation heated rock surfaces during summer days, often raising surface temperatures on both the north- and the south-facing walls above 0 °C (fig. 5). At night, however, wall temperature dropped down to almost -20°C. The daily temperature fluctuations were wider (>35 °C at north-facing, and >50 °C at south-facing) than those found in inland Antarctica (around 20 °C) (Yoshikawa et al. 2000). No daily cycles occurred during winter periods at either north- or south-facing sites. An effective freeze-thaw cycle was defined as a fall below -2 °C on the rock surface followed by a rise above +2 °C (Matsuoka, 1990) (referred to as  “effective freeze-thaw cycle” in this text). Moisture was defined as a balance of water supply and evaporation from the rock surface. Water saturation of the rock has been shown to be an important factor (McGreevy, 1981). During the study period, 63 effective freeze-thaw cycles occurred on the south-facing wall and 30 cycles occurred on the north-facing wall. The number of effective freeze-thaw cycles at Ellsworth Mountain (80°20.77’S, 81°35.4’W), 1250-1400 m, in Antarctica was 46 during a year-long study in 1992-1993 (Yoshikawa et al. 2000). Such inactivity of frost weathering can  explain the limited debris deposits at the higher altitudes. Due to extremely cold temperatures (never above 0 °C) at the rock surface during winter, freeze-thaw cycles are only possible during summer months both in Antarctica and on Mt. Denali.

Fig. 5 – Rock temperature, north- and south-facing walls.
Fig. 5 – Température de la roche sur versants nord et sud.

Fig. 5 – Rock temperature, north- and south-facing walls.Fig. 5 – Température de la roche sur versants nord et sud.

15South-facing rock surface temperatures were comparable to those of the north-facing rock wall, mostly rising above 0 °C by midday in June 2003. Relatively high temperatures occurred as the vertical solar insulation heated rock surfaces during the day, even though air temperature was below 0 °C during the entire observation period. The daily air temperature usually varied between -25 °C and -50 °C on winter days, but were around -20 °C on summer days. Temperature variations in winter occurred due to breaking of polar air depressions. However, snowfall at this time of year did not supply moisture to the rock surface. The rock surface moisture was supplied in summer sunny days by snow blown by winds rather than snowfall. Wind and solar radiations therefore appear as important factors for generating weathering processes on Denali.

Conclusion

16Varnish coating and calcite deposits at the surface were regarded as relatively inactive at higher elevations on Denali, but still more effective than frost shattering. The number of effective freeze-thaw cycles that occurred on each instrumented (north and south) rock wall (63 and 30 times ,respectively) is similar to previously documented freeze-thaw cycles in inland Antarctica (46 times). Moisture conditions depended on solar radiation and snow blowing.  Blown snow and subsequent melting provided limited moisture to the rock wall. The most active snow blowing occurred on the south- to southeast-facing rock walls between 3000 and 3500m.  For snowmelt to occur, the snow particle temperature must be close to 0 °C, as thawing involves extra latent heat to become liquid. For melting snow particles, the air temperature is also an important consideration to take into account, as well as the rock surface temperature. Once snow melts, most of the liquid water cannot remain at the surface, and is not available for filling in cracks or pore spaces in the rocks due to the extreme evaporative and limited snow sources.

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Bibliographie

Carroll D. (1970) – Clay minerals : a guide to their X-ray identification. Geological Society of America Special Paper 126, 80 p.

Connor C., O’Haire D. (1988)Roadside geology of Alaska. Mountain Press Publishing Company, Missoula, USA, 251 p.

Engel C. G., Sharp R. S. (1958) – Chemical data on desert varnish. Geological Society of America Bulletin, 69, 487-518.

Hall K. (1986) – Rock moisture content in the field and the laboratory and its relationship to mechanical weathering studies. Earth Surface Processes and Landforms, 11, 131-142.

Hall K. (1987) – The physical properties of quartz-micaschist and their application to freeze-thaw weathering studies in the maritime Antarctic. Earth Surface Processes and Landforms, 12, 137-149.

Hirschwald J. (1912) Handbuch der Bautechnischen Gesteinsprufung. Borntaeger, Berlin, 923 p.

Ishimaru S., Yoshikawa K. (2000) – The weathering of granodiorite porphyry in the Thiel Mountains, inland Antarctica. Geografiska Annaler, 82A(1), 45-57.

Johnston J. H. (1972) – Salt weathering processes in the McMurdo dry valley regions of south Victoria land, Antarctica. New Zealand Journal of Geology and Geophysics, 16, 2, 221-224.

Kanamori S., Okura Y., Shiraiwa T., Yoshikawa K. (2004) – Snow pit studies and radio-echo soundings on Mt. McKinley. Bulletin of Glaciological Research, 22, 89-97.

MacInnes C., Beaudoin J. J. (1968) – Effect of the degree of saturation on the frost resistance of mortar mixes. Journal American Concrete Institute, 65, 203-207.

McGreevy J. P. (1981) – Some perspectives of frost shattering. Progress in Physical Geography, 5, 56-75.

Matsuoka N. (1990) – The rate of bedrock weathering by frost action : field measurements and a predictive model. Earth Surface Processes and Landforms, 15, 73-90.

Matsuoka N., Moriwaki K., Iwata S. Hirakawa K. (1990) – Ground temperature regimes and their relation to periglacial processes in the Sør rondane mountains, east Antarctica. Proceedings of the NIPR symposium on Antarctic Geosciences, 4, 55-66.

Okura Y. (1994)Mt. McKinley wind measurement expedition (in Japanese). Sangaku dai-nen Japan Alpine Club, Tokyo, 31 p.

Rozanski K., Araguas-Araguas L., Gonfiantini R. (1993) – Isotopic patterns in modern global precipitation. In Swart P.K., Lohmann K.C., McKenzie J., Savin S. (Eds.), Climate Change in Continental Isotopic Records. American Geophysical Union, Geophysical Monograph No. 78, Washington, D.C., 1-36.

Swiss Foundation for Alpine Research and University of Alaska Press (1990) Mount McKinley map, Scale 1 :50,000, Boston’s Museum of Science, the Swiss Foundation for Alpine Research and the Swiss Federal Institute of Topography 6th edition (surveyed by B. Washburn).

Thomas W.N. (1938) – Experiments on the freezing of certain building materials. Deparment of Scientific and Industrial Research, Building Research Station. Technical Paper 17, 146 p.

Tourenq C. (1970) – La gélivité des roches. Application aux granulats. Laboratoire des Ponts et Chaussées, Rapport de Recherche 6, 60 p.

Yoshikawa K., Ishimaru S., Harada K. (2000) – Weathering Processes for Paleozoic Marbles in the Independence Hills and Patriot Hills, Ellsworth Mountains, Antarctica. Physical Geography 21, 6, 568-576.

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Annexe

Version française abrégée

Sur le Mont Denali ou Mont McKinley, les processus de météorisation dominants sont la cristallisation secondaire (calcite) et l’oxydation (vernis), l’action du gel étant réduite. Les conditions climatiques régnant sur ce sommet sont cependant peu connues. Le Club Alpin Japonais, en collaborant avec les autorités du Parc National du Denali, a installé en 1990 une station météorologique juste au-dessus du col de Denali à 5 710 m (cf. Ogura, 1994 et fig. 1).

Les zones plus élevées sont sèches et froides en raison des basses pressions atmosphériques et des vents violents. En dépit de la rareté de l’eau à l’état liquide, presque toutes les surfaces rocheuses exposées sont recouvertes d’un vernis et fissurées. On sait que la gélifraction est le processus de météorisation le plus couramment évoqué dans les régions froides, qu’elles soient maritimes (Hall, 1987) ou continentales (Matsuoka et al., 1990 ; Yoshikawa et al., 2000). Pourtant, au cours des étés 2002-2003, deux études stationnelles effectuées sur une paroi rocheuse du Mont Denali exposée au nord et une autre exposée au sud ont révélé un nombre limité de cycles gel-dégel (respectivement, 30 et 63 : fig. 5). Ce nombre de cycles relativement faible est comparable à celui livré par des relevés réalisés en Antarctique continentale (46 cycles de gel-dégel effectif en 1994). L’humidité des roches semble influencée par la radiation solaire et la présence de congères. La fonte des congères fournit une grande quantité d’eau aux parois rocheuses. Lorsque la neige fond, la majeure partie de l’eau liquide ne peut se maintenir en surface et n’est plus disponible pour remplir les fissures ou les pores des roches en raison d’une évaporation poussée et d’un faible nombre de sources d’origine nivale.

Des mesures isotopiques montrent que le potentiel d’évaporation (sublimation) augmente avec l’altitude (tab. 1). La valeur de l’excès de deutérium (d-excess) au sommet du Denali est quasiment la même que celle enregistrée au milieu de la Péninsule arabique (Rozanski et al., 1993). Le d-excess est fortement affecté par la sublimation. Les résultats isotopiques s’accordent avec les résultats des observations au MEB et EDS pour faire des fissures des milieux relativement frais où se concentrent le fer et le calcium, au contraire des surfaces rocheuses davantage météorisées (fig. 2b). Les analyses d’éléments majeurs par microsonde électronique montrent que la surface rocheuse est constituée de Si, Al, K, Fe et S (fig. 2a). Si, Al, K proviennent des minéraux silicatés. Le fer ferreux est contenu dans l’hypersthène, la biotite, l’ilménite et la magnétite.

Les eaux de fusion nivale ne peuvent rester longtemps à la surface des affleurements rocheux dans ces conditions sèches et le volume d’eau de fonte présent à la surface ne permet pas de remplir les fissures. La figure 3 montre les caractéristiques spectrales des surfaces fraîchement exposées et de celles fortement météorisées, obtenues grâce à l’imagerie utilisant un spectromètre dans l’infra-rouge visible. Les roches météorisées sont caractérisées par la présence de calcite (fenêtre d’absorption de 2 350 nm) et d’un enduit (oxydation, fenêtre d’absorption de 1 650 nm). Des images infrarouges thermiques montrent des températures de surfaces relativement « chaudes » (7 °C) au-dessous de 4 500 m d’altitude sur les versants exposés au sud et sud-ouest (fig. 4). Ces zones de températures positives et celles des enduits Ca ou Fe se superposent sur l’ensemble du massif montagneux. La pente est l’un des paramètres majeurs contrôlant les processus de météorisation et la fourniture en eau. Les relations géomorphologiques entre le degré de météorisation, la pente et l’altitude sont révélées grâce à des techniques SIG. Les surfaces rocheuses sévèrement météorisées (montrant des enduits ferreux et de calcite) sont observées dans les étages inférieurs (3 000-3 500 m d’altitude) et sur les parois exposées au sud ou sud-est. Aucune corrélation entre la sévérité de la météorisation et la pente n’est apparue.

La cristallisation secondaire (calcite) et l’enduit sont contrôlés par l’évaporation de surface, les conditions d’humidité et les propriétés chimiques des roches. Le paramètre le plus important de cette étude est l’humidité, à conditions lithologiques et radiatives égales. Les particules neigeuses sont mobilisées par les courants-jets d’altitude, apportant de l’humidité à la surface de la roche, qui se concentre principalement sur la base des escarpements et les cols. Ainsi, la cristallisation secondaire de calcite et l’enduit d’oxydation se développent fortement dans ces zones, notamment là où les températures sont également moins froides (versants exposés au sud et sud-ouest).

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Table des illustrations

Titre Fig. 1 – Map of study area indicating measuring points. Fig. 1 – Carte des sites d'étude indiquant les emplacements de mesure.
Légende Rock temperature has been measured on the north- and south-facing walls. Weather station : measured air temperature, ground temperature, and wind speed (map based on Swiss Foundation for Alpine Research and University of Alaska Press, 1990).La température de la roche mesurée sur les versants nord et sud. Station météorologique : température de l'air,  température du sol et vitesse de vent (carte fondée sur la Fondation suisse pour les recherches alpines et Presse de l'université d'Alaska, 1990).
URL http://geomorphologie.revues.org/docannexe/image/147/img-1.png
Fichier image/png, 90k
Titre Fig. 2a – The surface of the varnish rock consists of Ca, Al, and Fe from X-ray diffraction for energy dispersion spectrometry (EDS) and SEM image.Fig. 2a – La surface d’oxydation de la roche est constituée de Ca, Al, et de Fe, mesurés aux rayons X, par spectrométrie de dispersion d'énergie (EDS) et à partir d’images au MEB.
URL http://geomorphologie.revues.org/docannexe/image/147/img-2.png
Fichier image/png, 734k
Titre Table 1 – Stable isotope results and ‘d-excess’ from surface snow of climbing route.Tableau 1 – Résultats d’analyse des isotopes stables et de l’excès de deutérium (d-excess) à la surface de la neige le long de l'itinéraire de l’ascension.
URL http://geomorphologie.revues.org/docannexe/image/147/img-3.png
Fichier image/png, 1,9k
Titre Table 1 – Stable isotope results and ‘d-excess’ from surface snow of climbing route.Tableau 1 – Résultats d’analyse des isotopes stables et de l’excès de deutérium (d-excess) à la surface de la neige le long de l'itinéraire de l’ascension.
URL http://geomorphologie.revues.org/docannexe/image/147/img-4.png
Fichier image/png, 822k
Titre Fig. 3 – A false color image of Mt. Denali Mountain, Alaska Fig. 3 – Image en fausse couleur du Mont Denali, Alaska.
Légende Red : band 140, Green : band 223, Blue : band 2. The dataset was flown on 13 June 1995.Rouge : bande 140, vert : bande 223, bleu : bande 2. Les données sont dérivées d’un survol effectué le 13 juin 1995.
URL http://geomorphologie.revues.org/docannexe/image/147/img-5.png
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Titre Fig. 4 – Thermtal IR image of south-facing wall of Denali, taken on 10 May 2005.Fig. 4 – Image IR thermique de la face sud du Mont Denali, prise le 10 mai 2005.
URL http://geomorphologie.revues.org/docannexe/image/147/img-6.png
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Titre Fig. 5 – Rock temperature, north- and south-facing walls.Fig. 5 – Température de la roche sur versants nord et sud.
URL http://geomorphologie.revues.org/docannexe/image/147/img-7.png
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Kenji Yoshikawa, Yositomi Okura, Vincent Autier et Satoshi Ishimaru, « Secondary calcite crystallization and oxidation processes of granite near the summit of Mt. McKinley, Alaska », Géomorphologie : relief, processus, environnement [En ligne], vol. 12 - n° 3 | 2006, mis en ligne le 01 octobre 2008, consulté le 30 avril 2017. URL : http://geomorphologie.revues.org/147 ; DOI : 10.4000/geomorphologie.147

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