Mer De Glace drains the north side of Mont Blanc. This is the largest glacier in this section of the Alps, it is 12 km long. The “sea of ice” terms not only refers to the size of the glacier, but also to the ogives, curved color bands formed at the base of the icefall. Today this sea of ice is slowing down as well as thinning and retreating. More people have been underneath this glacier than any other, thanks to the tunnel system that is drilled under the glacier and accessed from Montenvers Station. The glacier begins at 4200 meters and ending today at 1500 meters. During the Little Ice Age the glacier advanced to the floor of the Valle de Chamonix at 1000 meters. The 2300 meter retreat since that time has been recorded in paintings and photographs, note Zumbuhl and others (2008) University of Bern, at this tourists mecca. At the famous Montenvers Station the glacier has thinned 150 meters since 1820. The retreat has not been continuous as during the 1970s and ’80s, the glacier advanced 150 meters. However, from 1994-2008 it has lost more than 500 meters. This is more than the retreat of adjacent Glacier d’Argentiere and Taconnaz Glacier, but similar to that of Glacier Blanc on Barre Des Ecrins and Glacier du Tour. As reported to the World Glacier Monitoring Service the retreat was 162 meters from 1996-2000 and 208 meters from 2001-2005. The terminus as shown in 2004 ends in a small pro-glacial lake. In the image below, A indicates the moraine emplaced by the advance ending in the 1980′s, B indicates the tunnel entrance and C indicates the former height of the ice level at the 1980′s advance maximum. The glacier has lost 70 meters in thickness at the Montenvers Station during the last 20 years as it has retreated. This has resulted in the ongoing adjustment of the stairs leading to the tunnel system that goes under the glacier. The tunnels used to be accessed at the top of the stairs seen in the first image below. The changes at the terminus between 2003 (red) and 2009 (yellow) are evident in the images below, the lower section of the glacier below the ice tunnel is stagnant and simply melting away. The lake that had been at the terminus in 2003 is no longer in contact with the glacier in 2009.Below 2100 meters in the relatively low slope tongue of the glacier in the ablation zone repeat mapping indicate that the thinning rate increased dramatically from 1 ± 0.4 m.a−1 (years 1979–1994) to 4.1 ± 1.7 m.a−1 (2000–2003), according to Berthier and others (2004). This is the section in the midst of the ogives, which form due to annual changes in flow through the icefall section. Also notice that the largest tributary to the Mer de Glacier has little contribution today (first image below). There are 48 identifiable ogives in the 2009 Google Earth Imagery. The distance from the first to the last is 4.6 km, indicating an annual flow of just under 100 meters per year, second image below. A new paper by Berthier and Vincent (2012) identifies that 1/3 of the thinning in the lower section of the glacier is due to reduced flow from above and 1/3 to increased ablation of the glacier tongue. The velocity they observe in the ogive reach confirms the figures cited above determined just from ogives. In Figure 6 they note a 30-40 % decline in glacier velocity from 1985-2007
Mer de Glace, Glacier Retreat-A Receding Sea
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