Among the variety of forms, reliefs, landscapes with which we are gratified our beautiful planet, the most beautiful and impressive, no doubt, count the glacial landscapes.
Today, only 2% of surface water is in the form of ice, but this small percentage is enough for 10% of that surface is covered by ice. This percentage largely corresponds to the great inland ice, also called inslandsis , Greenland and Antarctica.
Outside these areas, can be found linked to high altitude, high mountain, associated with the most incredible demonstration of Earth's internal forces, such as the great Cordilleras. Andes, Alps, the Himalayas.
However, Quaternary Geology tells us that thousands of years ago glaciers were related to lower latitudes and altitudes. Glaciers are and have been major shapers and builders of relief. They were and are powerful architects. Think of the majestic valleys U, moraines and other landforms that we see in a trip to the mountain. And this ability is linked with an edifying quality of glaciers, ice, maybe we go unnoticed in a first observation: the ability of ice to flow. And although the ice is a solid, is able to "flow" just like a liquid. And this quality can be explained from a rheological point of view, the science of materials is the study of relations between the stresses and strains.
First, we will deal with how to form a glacier.
NOURISHES WHAT IS A GLACIER?
a first approximation, a glacier can be defined as large accumulations of ice. We all know from personal experience how little it takes for the ice to melt at room temperature.
therefore, that the ice will keep for enormous periods of time, we need appropriate environmental conditions (low temperatures). Such conditions are achieved over the so-called "level of perpetual snow" or snow line is defined as the altitude from which a portion of the snow in winter, "survives" the summer and is able to withstand up the following winter. The end result is that from this level, the accumulation of snow prevail on their merger, and the net balance is that the content of snow will increase over time (technically, in reality, this takes place over the so-called equilibrium line, which is not conceptually the same as snow line, but from a practical point of view can be considered synonymous).
therefore above the limit of perpetual snow, strong water inflows are greater than outflows, and this snow will experience processes that turn into ice, ice that can persist for thousands of years.
The height at which places the snow line will vary with the latitude where we are, to the extent that varies with latitude insolation and annual maximum temperatures. Thus, while in the Kilimanjaro stands at around 5500 meters, in our latitudes is approximately at 3000 meters and 400 meters in Iceland, whereas if we move further north (or south, depending on the hemisphere) above 70 º, this altitude is roughly the sea level.
Another prerequisite for the development glaciers is solid precipitation and snow. Except some exceptional regions, this is not a problem on our planet, because the snow is a more or less daily.
However, what makes the snow on the glacier ice? Let's start.
freshly fallen snow is formed in a 5% snowflakes (with their irregular shapes, corners, edges ...) and 95% by air. Therefore has very low density (about 0.05 grams per cubic centimeter), and from a geological point of view, it can be considered as a sediment wind, not unlike, for example, by an accumulation of grains of sand on a beach (saving the difference in density). In fact, on the surface of freshly fallen snow, wind can also form sand waves, called ripples minidunas or that are used to seeing on the sand of the beach.
After a snowfall, if temperatures remain below freezing, the air is soaking the pores between the flakes causes the sublimation of the ends of the snowflakes, taking place this water condensation in the center of flakes of snow so that is losing its pointed shape gradually gaining a more granular. This causes the granular snow, avalanches so many problems gives skiers. Granular snow an increase of packing and density and some expulsion of intergranular air. As this process continues, the packaging is increasing, the expulsion of air becomes more clear and finally reached the state of firn or snow powder, snow grains are practically small areas of a few millimeters in diameter, porosity reduces to 50% and the density reaches 0.4. The snow is what we found at the end of winter, the snow that has persisted. The accumulation of more snow on the neve, for example the following year, resulting in increased pressure "cryostat" which is an increase of compaction and expulsion of air. The result is that the pores lose communication and small bubbles air remains isolated from each other. The density reaches 0.80. Thus, the firn is compressed, resulting in an aggregate of interlocking ice crystals, was born on the glacial ice, and process that has taken place is called diagenesis, which is nothing more than a process of recrystallization. All this processing can take from a few years, to 100, depending on the circumstances in which we find ourselves. Thus, from a "sludge" (snow) has caused a rock solid: ice.
MECHANICAL BEHAVIOR OF ICE
Like any material, the ice when subjected to efforts to respond. The main effort is to act on the glacial ice will be its own weight, the force of gravity.
In the 50's, Glenn through experimental studies showed that the rheological model which best describes the behavior of ice is that of a pseudoplastic material. Expressed in a simplified way the behavior of ice by the equation e = K x T-exp (n) where e is the deformation of ice, T and K acting effort n different constants for each glacier.
At first, when the ice is thin (less than 50 meters), will behave as a conventional solid, with an effort will undergo elastic deformation until it exceeds its elastic limit and break. This is what happens in the first 50 meters of the glacier, where we can see bergschrund morphologies such as (deep cracks) or seracs (large ice blocks bounded by fracture surfaces, for which have suffered a relative motion) raising much respect among the mountaineers.
However, once the mass of ice acts an effort equivalent to a column of ice over 50 meters tall, ice behavior conformed to that of a plastic substance: as we increase the effort the material will experience less resistance to deformation, and small increments of effort, strain rates increase sharply. We see therefore that in this situation the glacier flows and moves.
So far we have seen the mechanics of ice. Now is the time to consider the mechanisms involved in this mechanism, ie, as the deformation develops. MECHANISMS
DESPLZAMIENTO
We have seen that a material, at first sight completely solid, such as ice, in front of large efforts and long-term is able to "flow" of travel.
The ways in which a glacier moves can be classified into 2 groups of mechanisms.
- The so-called mechanisms creep or flow, low flow caused by internal deformation of the ice mass.
- sliding mechanisms, namely the displacement of large masses of ice on a surface, without such masses experiencing internal deformation.
In practice, it is very difficult to say which of these 2 mechanisms caused the movement of a particular glacier, as in nature (always so complex), glaciers suffered both mechanisms. Within
flow mechanisms are: the pressure distribution (or diffusion phase change), which is that in every bean ice produces a diffusion of water molecules from the area in which it operates the most comprehensive effort to areas where it operates the maximum extensional stress, leading to a common orientation in all the crystals and a net flow in the direction of efforts to understand (this process is reflected in the equation of Glenn with constant values \u200b\u200bof K very high), another mechanism of intergranular flow are adjustments, rotation and displacement of the grain boundaries of crystals in relation to its adjacent crystals .. This is reflected by the constant n values \u200b\u200bclose to 1. Besides these two, there are other more complex mechanisms, such as recrystallization (Growth and development in solid ice crystals, being a process during which the mechanical energy stored in the ice mass during deformation, is consumed during the growth of new crystals, free of internal strain and the minimum free energy surface for the new conditions), or deformation of the crystal lattice of ice crystals. Within
glides, we have the basal slip (in case this was the mechanism that moves the glacier, the constant n from the equation of Glenn would much higher values \u200b\u200bthan 1), which consists of desplzamiento " to block "on the surface of the glacier the bedrock. This mechanism is more important in warmer climates, where the base of the glacier can be found near the melting point at that pressure, so that the existence of liquid water can "lubricate" and faclitar movement. Another mechanism is called slip slip by refreezing, which involves melting the base of the glacier ice up "of an obstacle (for increased pressure) and freezing" ice below. "
So far, all I could say about the movement of glaciers. Once in motion, the glaciers are able to remove large blocks of ice, and transported to areas far away, are able to dig incisions or grooves on the substrate along which originates and dug huge valleys, and lead to deposits of many types (sub-glacial, moraine fluvioglaciares ...). They are therefore very important highlight modeling agents, not only in areas where they currently remain, but also at low altitudes and areas where there is currently no glaciers, glacial morphologies can look amazing legacy: thus, for example, here in Spain , we can see all sorts of ways glaciers in areas such as Pyrenees, Picos de Europa, Gredos or Béticos, even when there are virtually no glaciers in these areas.
And all this dynamism is able to do a solid, seemingly static, as the ice when it starts moving under its own weight. Perhaps as I said Deborah (biblical figure) mountains (in this case, the glaciers) move in the eyes of God.
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