What is a glacier?
Figure 1. Serrano glacier (Photo: Thomas Shaw).
Glaciers are rivers of ice which form from the persistence of fallen snow over a long period of time that eventually compresses into thickened ice masses. Official classifications may vary, but a glacier is usually considered as an ice mass with an area ≥100 m2 and a depth of ice approximately ≥50 m 1. Some glaciers are as small as football fields, while just parts of those in Antarctica can be as large entire cities. In fact, you could walk the length of Echaurren Norte, one of the glaciers with longest record in Chile 2 and cover a distance similar to one loop around Costanera Center. Conversely, O’Higgins glacier in Patagonia is roughly 45 km long 3, equivalent to the horizontal distance between Costanera Center and cerro El Plomo.
Figure 2. Left: O’Higgins glacier (Photo: Roberto Araya). Right: Echaurren Norte glacier (Photo: DGA).
The survival of glaciers is dependent on the balance of how much the ice is melting or ‘ablating’ and how much new snow falls and stays around long enough to densify and form into glacier ice. The balance of these two processes is called the glacier mass balance and is determined principally by the temperature of the air and the amount of precipitation (in the form of rain, snow or sleet). Temperatures are colder at higher altitude on the glacier because of the lower air pressure, and so less ice is melting. At the same time, precipitation will more likely fall as snow and stay longer because of the lower temperatures. We refer to this higher region as the accumulation zone.
Figure 3. A schematic for glacier mass balance. Source: Daniel Colonia.
However, the lower section the glacier (the part we normally see more of) is called the ablation zone, which refers to ablation or loss of mass from the glacier. Here, temperatures will be (relatively) higher so less precipitation will fall as snow, and snow that does fall will melt and disappear much faster, allowing more of the glacier ice underneath to melt as well! The altitude on the glacier surface where we start to accumulate more than we melt is referred to as the equilibrium line altitude. If you visit a glacier at the end of the summer (approximately March/April here in Chile), look for this line where the snow starts. This line indicates the ‘health’ of the glacier and the higher it is, the worse the glacier is doing, because we are melting much more than we are replacing with snow (which can eventually become more ice!).
Figure 4. Simple schematic of the equilibrium line and mass balance of glaciers in connection with climatic conditions. Source: Thomas Shaw.
Cited information:
1 Is there a size criterion for a glacier? USGS.gov: https://www.usgs.gov/faqs/there-a-size-criterion-a-glacier?qt-news_science_products=0#qt-news_science_products.
2 Masiokas, M. H., Christie, D. A., Le Quesne, C., Pitte, P., Ruiz, L., Villalba, R., … Barcaza, G. (2016). Reconstructing the annual mass balance of the Echaurren Norte glacier (Central Andes, 33.5° S) using local and regional hydroclimatic data. Cryosphere, 10(2), 927–940. https://doi.org/10.5194/tc-10-927-2016.
3 MOTOKI, A, ORIHASHI, Y, NARANJO, J et al. (2006). «Geologic reconnaissance of Lautaro Volcano, Chilean Patagonia». Revista geológica de Chile (en inglés) (Rev. geol. Chile. [online]) 33 (1): pp. 177-187. ISSN 0716-0208. doi:10.4067/S0716-02082006000100008.
Written by Thomas Shaw.
Edited by Equipo Glaciar.