Glacial Landscapes of the Val D'hérens (Valais, Switzerland)
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20 march 2020 Glacial landscapes of the Val d’Hérens (Valais, Switzerland) Walter Wildi, Pauline Gurny-Masset, Mario Sartori Section des sciences de la Terre et de l’environnement Université de Genève Rue des Maraîchers 13, CH-1205 Genève © 2015 Document on Internet http://www.unige.ch/forel/fr/services/guide/valdherens/ 1 Glacial landscapes of the Val d’Hérens (Valais, Switzerland) Preface : the valley of the pyramids Walter Wildi, Pauline Gurny-Masset, Mario Sartori We cannot imagine the Alps without their snow-white mountain peaks in front of a blue sky. And the glaciers tongues creeping down into the valleys in a ceaseless flow, which remains invisible to the naked eye. These Content mountains attracted the first tourists and climbers in the 19th century. During the same period, glacier advance in the valleys and on the pastures demonstrated to researchers the need for a better understanding of the Preface : the valley of the pyramids 2 glacier world. 1. Introduc:on to glaciology 4 1.1 From snow flakes to ice 4 In the early 19th century J.P. Perraudin, hunter and explorer of mountain 1.2 Ice balance of a mountain glacier 5 crystals, observed the advance of the Giétroz Glacier, invading pastures and 1.3 The glacial system 6 building new moraine ridges (Fig. 2). Alerted by Perraudin, Ignaz Venetz, 1.4 Melt water 9 engineer of the Canton Valais, discovered the formation of a glacial lake that 1.5 Glaciers in movement 11 emptied in 1818 , flooding the upper Vallée de Bagne. 1.6 Glacial erosion 12 1.7 Sediments of the glacial landscape 13 2. Glacial history 2.1 Major climate changes at the origin of Alpine valleys and landscapes 14 2.2 Medieval Climate Op:mum and the LiPle Ice Age 16 3. The glaciers of the Val d’Hérens 18 4. From the last glacia:on :ll modern :mes and from Geneva to Ferpècle: the way back «home» of Alpine glaciers 20 5. From Ferpècle to the Mont Miné Glacier: a travel back to the LiPle Ice Age 28 6. Field trip: the Mont Miné Glacier from the LiPle Ice Age Figure 1:the pyramids of Euseigne are the key emblem of the Val d’Hérens :ll modern :mes 31 and its glacial history. These “demoiselles" (ladies) wear a hat corresponding to the residues of glacial ground moraine of the last ice age, at the meeting Bibliography and links 35 point of the Val d’ Hérence and Val d' Hérémence glaciers . 2 During the annual meeting of the Swiss Society of Natural Sciences (SHSN) in The guide finally proposes field visits on the theme of glacier return in the Lucerne, Ignaz Venetz emitted the revolutionary idea that glaciers may have Alps at the end of the last glaciation. Four sites along the road from Sion to had in the past a much larger extension than that observed by himself and Evolène illustrate this period. Finally, a field trip follows the way of the his contemporaries. Louis Agassiz took up the idea of an extension of retreating Mont Miné end Ferpècle glaciers from the Little Ice Age till now. glaciers outside the Alps. In his speech to the SHSN in 1837, published in 1840, he depicted the climate change responsible for the extension of an ice cap that would have covered most of the northern hemisphere. Between 1840 and 1845, the glacial theory was adopted worldwide. This is why and how was started the development of the “glacial theory” and the science of glaciers called “glaciology”. The latter is based, at least partly, on glacier observations in Valais. Since then, things have changed. Glaciers had advanced in the valleys during the 16th century, at the beginning of the period known as the "Little Ice Age”. After a number of fluctuations they reached their last maximum extension in 1850. Afterwards, glacier tongues retreated again. And even though they have not yet reached today the reduced position they occupied in Middle Ages times, one observes the trend of current climate change towards a return to a warmer climate period. And because any change is a challenge in people’s mind, there is a strong public interest for the glaciers and their transformation. In the Val d’Hérens there are neither the longest glacier tongues, nor the highest mountain peaks of the Valais. But this valley offers, through its landscapes, glaciers and rivers, and also its flora, extraordinary conditions for the observation of natural processes and landscape changes. The valley reveals to our eyes ideal didactic conditions to study the facts and processes of the recent glacial history. This guide presents a brief introduction to the main elements of glaciology, from ice formation mechanisms to erosion of the bedrock through ice and Figure 2: the Gietro Glacier between Mont Pleureur and Mont Mauvoisin, water. The following chapter focuses on the ancient glaciations and the next glacial lake (Val de Bagnes, VS), painting by H. C. Escher, le 23 July 1818. one on the imprints left by the warm Middle Ages period and the history of Drawing and watercolor, 26 x 26,5 cm , collection ETH Zurich (n° 223 = Inv. C the Little Ice Age. XII 13b). 3 1. Introduc,on to glaciology 1.1 From snow flakes to ice H Water chemistry and physics O • Chemical formula: H2O, dihydrogen monoxide, hydrogenol, hydrogen hydroxide, dihydrogen oxide. H • At sea level, water is found in 3 states according to temperature Figure 3: structure of water and ice. conditions: http://www.ressources.cfadf.com/pensee/chimie6.htm gas > 100°C < liquid > 0 °C solid vapour water ice Ice forms at a temperature below 0°C from water molecules and forms a • Density: varies according to temperature, pressure and state: crystalline lattice (Fig. 3): every oxygen atom (in red on the figure) is surrounded 3 99 °C: 958.4 kg/m (liquid, at 100 °C: evaporation) by four similar atoms, with the intercalation of a hydrogen atom. As shown in 3 4 °C: 999.973 kg/m (liquid, highest density) figure 3 (right) oxygen and hydrogen groups are arranged in layers. Within these 3 0 °C: 999.841 kg/m (liquid) layers, the distance between atoms is smaller than the distance between two 3 0 °C: 917 kg/m (solid: dilatation of ca. 10% when freezing) layers. Similar layered crystalline lattices are known from phylosilicates, such as -1 • Fusion (melting) energy of ice is 333 kJ·kg clay minerals and mica. Figure 4: main processes of ice formation in a mountain glacier: Burying and compaction of snow Fusion (melting) - recrystallization of snow (cold glaciers) (temperate glaciers) Snow flake Snow flake Partial fusion (melting) Burying under fresh snow and fractioning in small Recrystallization 10 cm particles of snow. into fine ice crystals. Figure 5: artificial ice tunnel at the Mer de Glace, Chamonix (France): laminated ice with white layers of air bubbles and rock Recrystallization debris. Transformation to a dense into larger ice 4 granulate crystals 1.2 Ice balance of a mountain glacier accumula$on'zone' abla$on'zone' snow'''' snowfield' glacier'tongue' The «accumulation zone» of a glacier is the area where the ice balance is positive during rimaye,'bergschrund' a hydrological year (October 1st till September 30th). This zone is limited with respect to young'firn' old'Firn' the «ablation zone» by the «equilibrium line». The ice balance corresponds to the clima$c'snow'line' bed'rock'''' difference between accumulation and ablation (ice melting) expressed in the equivalent water'saturated'area' Grundwasser' glacier'mill' ice' volume of water over a hydrological year. When the ice balance is negative, the glacier crevasse' surface'water'' (melt'water)' «retreats» (figs. 6 and 7), which means that its tongue is shortening. The mass balance has also to consider the glacier movement: glacial ice is moving through viscous flow and glacial'tunnel" fault' sliding from the snowfield to the valley, i.e., from the accumulation to the ablation zone. ground'water' thermal'ground'water' ! Figure 6: section of an Alpine glacier: fresh snow accumulates on the glacier surface during the cold season (autumn – spring) and melts away at low altitudes during summer. In elevated positions snow transforms to firn and consolidates into «old firn» and ice through melting and freezing processes. Figure 8: glacial tunnel at the front of the Mont Miné Glacier, October 2012, on a cold day The water of a glacier originates from precipitations, and eventually from avalanches. The loss of water may be due to sublimation (direct evaporation from solid snow), or to ice melting and water runoff on the glacier, within the glacier or at its base through Figure 7: main parameters of the ice balance and glacier the glacier stream. front posiAon: temperature, precipitaAon and geothermal heat flow 5 1.3. The glacial system Figure 9a-b: glacial system of the Zinal Rothorn (view from the Sorebois Alp– Zinal, Val d’Anniviers) a Zinal Rothorn Le Besso 2 2 8 See next page for explanation of numbers 1 to 10. glacial)cirque) 2) frost)wedging)morphologies) bergschrund) rimaye) 6 1) screes,) snowfield) nunatak,) 9 rockfalls) avalanches) horn)5))) snow)and)firn) 1) verrou) 4) lateral)moraine)snowfield) ice) clima<c)snowline) seracs) 8) ribs,) ice)avalanches) ogives) accumula<on)zone 7) 7) crevasses) 9) solifluxion) ) 9) glacial)polish) glacier)tongue) 3) ))6) roche)moutonneé) ))))))))))abla<on)zone) sandur,)flood)plain) frontal)moraine) 10) old)) 9) lateral)moraine) overdeepened) glacial)valley) lake)sediments) ! 6 Figure 10: the glacial system and its components glacial)cirque) 2) frost)wedging)morphologies) a) Snowfields and seracs of the bergschrund) c) Tongue of the Ferpècle Glacier rimaye) Ferpècle Glacier 1) screes,) snowfield) nunatak,) 1 rockfalls) avalanches) horn)5))) snow)and)firn) 1) verrou) 4) lateral)moraine)snowfield) 1 8 ice) clima<c)snowline) seracs) 8) 8 ribs,) ice)avalanches) ogives) 4 accumula<on)zone 7) 7) crevasses) 9) solifluxion) 3 ) 9) glacial)polish) glacier)tongue) 3) ))6) roche)moutonneé) ))))))))))abla<on)zone) sandur,)flood)plain) frontal)moraine) 10) old)) 9) 3 lateral)moraine) 6 overdeepened) glacial)valley) lake)sediments) ! d) Exit of the sub-glacial river at the front e) Crevasses and glacier mill on the f) Seracs and ice avalanches of the Mont of the Mont Miné Glacier, ground moraine tongue of the Ferpècle Glacier Mine Glacier 8 3 7 9 3 9 ! Snowfields 1 are located in the glacial cirques 2 covered with snow down to the climaJc snowline.