Basic study page for Glaciers and Global warming's effect on glaciers A Glacier Story Over the past 60 to 100 years, glaciers worldwide have tended toward retreat. Alpine glaciers, which are typically smaller and less stable to begin with, seem particularly susceptible to glacial retreat. Whether this is due to a predictable climate trend or because of increased human impacts on global climate remains to be determined. The Franz Josef Glacier in New Zealand retreated rapidly over a 15 year period during the 1950s and 1960s. What makes a glacier? Where are glaciers found? How does the climate affect glaciers? If you want to know the answers to these questions and more, the Glacier Story will take you on a quick tour. In regions where average temperatures hover below zero degrees Celsius, glaciers grow with each snowstorm. Compressed by overlying snow, buried layers slowly grow together to form a thickened mass of ice. The pressure created from overlying snow squashes snow grains together. Individual grains eventually metamorphose, growing to the size of rock salt. If these enlarged crystals survive one melt season, they are considered firn. Looking down into a crevasse, is just one way of glimpsing the sheer immensity of a glacier -- most glaciers have accumulated and compressed so much snow that they are hundreds or even thousands of feet thick. Firn grains are generally four to 16 times the size of the original snow crystal and increase in size as the weight of the overlying snow increases. As the grains grow, they slowly snuff out pockets of existing air between the grains. Over time, individual firn grains are pressed together to form larger crystals, ultimately forming slabs of glacier ice. When the mass of compressed ice reaches a critical thickness of about 18 meters, it begins to deform and move. Its sheer girth, in combination with the forces of gravity, causes a glacier to slowly move, or flow. Glacier ice flows down mountain valleys, fans across plains, and spreads into the sea. As a glacier moves over the ground surface, friction causes the underside of the glacier to move more slowly while overlying glacier ice moves unimpeded. Most of the world's glaciers are found at the poles, but glaciers exist on all of the world's continents, even Africa. Glaciers require very specific geographical and climatic conditions. Most are found in regions of high snowfall in winter and cool temperatures in summer. These conditions assure that the snow accumulating in the winter remains throughout the summer. Such conditions typically prevail in the polar and high alpine regions. The amount of precipitation (whether in the form of snowfall, freezing rain, avalanches, or wind-drifted snow) is important to glacier survival. In areas such as Siberia and parts of Antarctica, where low temperatures meet glacier growth requirements, the lack of adequate precipitation prevents glacier development. In areas of glacier growth, upon reaching a critical mass, the slabs of ice begin to flow and dramatically impact the surrounding environment. The great weight and slow movement causes glaciers to reshape the underlying and surrounding landscape. Acting as an enormous push broom, the ice erodes the land surface, carrying broken rocks and soil debris far from their place of origin. Glaciers slowly push earth and rock forward as they advance and leave these same materials behind in the form of moraines and other glacial deposition features as they retreat. Glacier National Park, located in Montana, U.S.A., shows a variety of glaciated valleys. Part of this mountain has literally been cut away by a glacier that has long since retreated. In the northern half of North America, glacial remnants from the last ice age may be reincarnated as vegetated hillsides. Views from an airplane window over the midwestern states and provinces reveal lines of eskers and herds of drumlins dotting the landscape. Throughout advance and retreat, glacial debris (till) is jostled in all directions. Till is thrust forward with the glacier, brushed aside as the glacier pushes past less mobile objects, such as a mountainside, or drawn along on the glacier's journey. As large glaciers retreat, the underlying ground surface is typically scoured of most materials, leaving only scars on the underlying surface. Glacier retreat, melt, and ablation, result from increasing temperature, evaporation, and wind scouring. Ablation is a natural and seasonal part of glacier life. As long as snow accumulation equals or is greater than melt and ablation, glacier health is maintained. Over the past 60 to 100 years, glaciers worldwide have tended toward retreat. Alpine glaciers, which are typically smaller and less stable to begin with, seem particularly susceptible to glacial retreat. Whether this is due to a predictable climate trend or because of increased human impacts on global climate remains to be determined. Unattributed photographs at the World Data Center for Glaciology, Boulder http://nsidc.org/glaciers/story/page1.html Tierra Del Fuego Glaciers Tierra Del Fuego is a 150-mile long peninsula off the southern tip of South America, separated from the mainland by the Strait of Magellan. It is divided into Chilean and Argentinean halves, and its principle mountain range, a west-east range known as the Cordillera Darwin, stretches across both halves. The Cordillera is the continuation of the Patagonian Andes, and rises along the southern shore. The range holds beautiful ice-packed and misty mountains, with large glaciers pouring down into the sea. The range is dominated by two high peaks, both situated in the Chilean southwestern portion of the peninsula, within the boundaries of the Parque Nacional Alberto de Agostini. Impressive peaks, beautiful forests, lakes, glaciers, and abundant bird life are highlights of the park. The island is vulnerably situated at the point where two of the world's major oceans meet, and is consequently stormy for much of the year. The city of Ushuaia, Argentina is located on the southern shore, and is the southernmost city in the world. The two dominant peaks of Tierra Del Fuego are Monte Darwin (8,163 ft./2,488 m.) and Monte Sarmiento (7,546 ft./2,300 m.). For a list of the individual mountains in the Tierra Del Fuego, click below. http://www.peakware.com/encyclopedia/ranges/tierradelfuego.htm Glaciers: What Types of Glaciers are There? Ice Sheets Found only in Antarctica and Greenland, ice sheets are enormous masses of glacial ice and snow that cover over 50,000 square kilometers. The ice sheet on Antarctica is over 4200 meters thick in some areas, covering nearly all of the land features except the Transantarctic Mountains that poke up above the ice. Ice Shelves Ice shelves occur when ice sheets extend over the sea, floating on the water. In thickness they range from a few hundred meters to over 1000 meters. Ice shelves surround the entire continent of Antarctica. The largest shelf is the Ross Ice Shelf , covering over 500,000 square kilometers. Ice Caps Ice caps are miniature ice sheets, covering less than 50,000 square kilometers. They form primarily in polar and sub-polar regions, occupying high and relatively flat regions. Mountain Glaciers These glaciers develop in high mountainous regions, often flowing out of icefields that span several peaks or even a mountain range. The largest mountain glaciers are found in Arctic Canada, Alaska, the Andes in South America, the Himalayas in Asia, and on Antarctica. The Chickamin Glacier in British Columbia, Canada, is a typical mountain glacier. Several glaciers flow into it, and the landscape is nearly covered with ice and snow. Valley Glaciers Commonly originating from mountain glaciers or ice fields, these glaciers spill down valleys, looking much like giant tongues. Valley glaciers tend to be very long, often flowing down beyond the snow line, sometimes reaching sea level. Piedmont Glaciers Piedmont glaciers occur when steep valley glaciers spill into relatively flat plains, where they spread out into bulb-like lobes. The Malaspina Glacier in Alaska, covering over 5,000 square kilometers is one of the most famous examples of this type of glacier. Cirque Glaciers Cirque Glaciers are named for the bowl-like hollows they occupy, which are called cirques. Typically, they are found high on mountainsides and tend to be wide rather than long. Hanging Glaciers Also called ice aprons, these glaciers cling to steep mountainsides. Like cirque glaciers, they are wider than they are long. Hanging glaciers are common in the Alps, where they often cause avalanches due to the steep inclines they occupy. Tidewater Glaciers As the name implies, these are valley glaciers that flow far enough to reach out into the sea. Tidewater glaciers are responsible for calving numerous small icebergs, while not as imposing as Antarctic icebergs, can still pose problems for shipping lanes. Lamplugh Glacier, in Glacier Bay, Alaska, shows the snout of a typical tidewater glacier. Because the surface of the ice is heavily crevassed and jagged, small bits of ice have broken off, are seen floating in the water. http://www.digistar.mb.ca/minsci/geology/types.htm How Glaciers Form and Flow Glaciers are more-or-less permanent bodies of ice and compacted snow that have become deep enough and heavy enough to flow under their own weight. Today, glaciers are found in mountainous regions or in the very cold areas around the poles, and cover only about 10% of the Earth's surface. During past glacial periods this area increased considerably so, although active glaciation is very limited in Europe now, and non-existent in the UK, much of the landscape of northern Europe shows evidence of past glacial activity. How glaciers form and flow Glaciers develop where the temperatures are cold enough to allow snow to accumulate over a period of years. Favorable conditions are found around the poles and at high altitudes in lower latitudes, i.e. mountainous regions such Northern Scandanavia and the Alps. Enough snow must fall each winter to ensure that it doesn't all melt in the summer. This way, the amount of snow lying on the ground gets deeper each year as new snow is added to the remains of last years fall. On north facing slopes it may survive all year without melting, whereas all the snow may melt on south facing slopes ( in the northern hemisphere ). This is because north facing slopes get much less direct sunshine than south facing ones, and thus remain cooler. North facing corries (French Alps), showing small snow patches remaining in mid summer Fresh falling snow is made up of microscopic ice crystals and has a very delicate structure, nothing like solid ice found in glaciers, but after it lands it goes through a series of alterations to become hard ice. In fact, very cold, and thus dry, snow will hardly stick together at all under normal conditions and would never be any use as ice unless it was changed. As snow accumulates its structure changes. Newly fallen snow is very light and porous,with a density from 50 to 300 kg/m, but as the snow becomes buried by subsequent snowfalls the ice crystals that make up the snowflakes partially melt and sublimate, particularly at the delicate points of the flakes. The vapour then condenses, and over time the flakes change into granular ice crystals called firn, which has a density greater than 500 kg/m3. As more time passes the firn becomes buried even deeper under more fresh snow and firn, and the weight above it compresses it into solid ice with a density of approximately 900 kg/m3. This is the typical 'blue' ice of glaciers. The time needed to change snow in to glacial ice in this way depends on several factors such as how warm or cold it is and how much new snow is added each year. In fact, it can take anywhere from 5 years in an ideal site, to over 3000 years. The solid ice mass that is produced, unless it is on a very steep slope, wont start to move until its thickness approaches approximately 50 metres. This is because it is only when the ice is about 50m thick, that the pressure is enough to make the lowest, or basal, ice undergo plastic deformation, ie: it begins to flow like a soft or liquid plastic. Side view near the snout of an Alpine glacier, with a 30m wall of ice rising above the lateral moriane. The ice flows very slowly, moving away from the place where the pressure is greatest. This will be the place where the ice is thickest. The ice will even flow up hill if the pressure is great enough. In many regions the areas of accumulation of snow and ice are at high altitudes so the glaciers tend to flow down hill under the influence of gravity. The speed of the flow varies considerably, and in some cases, especially at high latitudes where it can be very cold, the glacier may be frozen to the bed rock so that it flows only by deformation under its own weight. It is as if the ice bends and slowly flows away like a block of toffee. Glaciers that are frozen to the underlying rock surface are called "cold glaciers". In a cold glacier the ice at the bottom of the glacier doesn't move at all, but the speed increases towards the surface of the glacier as the ice deforms. The fastest moving ice is that above about 50 metres from the surface of the glacier. The area of deformation in a glacier is also known as the "region of shear" because layers of ice are sliding over each other. In less cold regions the basal ice of the glacier may melt due to the pressure of the ice and firn above it. This allows the glacier to slide over the underlying land. These glaciers are not frozen to their bed and are called "warm glaciers". In warm glaciers the maximum velocity at the surface is greater than in cold glaciers because it is the sum of the velocity at which the glacier is sliding over the bed and the velocity due to internal deformation. The velocity of ice flow in all glaciers varies considerably, depending on factors like the thickness of the ice, the slope of the surface over which the glacier is advancing, and the amount of meltwater that is available to lubricate the base of the glacier. Some glaciers creep so slowly that their movement can only be seen over a period of years; others advance at several meters per day. When the ice becomes thick enough to flow, the glacier will begin to move away from its source. It will keep advancing as long as there is enough ice in the source area to keep supplying it. Whether it will advance along the valley, remain stationary or retreat again depends on the balance between the amount of new ice collected in the source area and the rate of melting (called ablation) in the rest of the glacier. The accumulation of ice is controlled by the annual snowfall and melting rates. The melting rate is mainly influenced by the temperatures and the amount of debris covering the ice surface. The ablation, or wastage, of a glacier takes place due to several processes with include sublimation, melting and, where the glacier ends in a water body, by calving of icebergs from the glacier snout. Melting is the most important process in almost all glaciers. The boundary between the zones of accumulation and ablation is roughly the same as the limit of year round snow. In other words, the point up the mountain where it becomes cold enough for snow not to melt during the summer. This is called the Snow Line. If the rate of snow accumulation in the source area is greater than the rate of wastage then the glacier will advance, but if the rate of accumulation is less than the rate of wastage the glacier will retreat. When accumulation and ablation are equal, they balance each other, and the snout of the glacier look as if it isn't moving. In fact, ice is always flowing towards the snout of the glacier whether it is advancing, retreating or seeming to remain stationary. Ice can deform under pressure, but most of the ice isn't all that good at it. Unless the conditions are just right, it's more likely to crack than deform. This is especially so where the ice encounters a sudden increase in gradient and is stretched faster than deformation can accommodate. This results in fractures developing in the ice, often narrowing at depth and widest at the surface because the ice is being stretched with a point of rotation at or near the glacier base. These cracks, which generally run at 90° to the glacier are called crevasses. Through crevasses meltwater and rock debris can be transported to the base of the glacier where they aid in the erosion of the bed rock. Crevasse with rock debris at its lip Crevasses are often the most obvious features on the ice surface, appearing either on their own or in groups. They present a danger to climbers and others who have to cross the ice since they are not only deep and smooth sided, they can be covered over by fresh snow that gives way under foot. http://www.zephryus.demon.co.uk/geography/resources/glaciers/origin.html GLACIER BAY PAGE http://sdcd.gsfc.nasa.gov/GLACIER.BAY/hall.science.txt.html
