Abstract

Introduction

What is a glacier?

A glacier is a mass of ice derived from accumulation of snow that moves under its own power due to gravity. In order for a glacier to form, snowfall in the winter (accumulation) must exceed melting in the summer (ablation). The ice that makes up the glacier forms when a body of snow becomes compacted under its own weight. The layers of snow are compressed and become dense layers of glacial ice. The advance or retreat of a glacier is dependent on its mass balance and whether or not the glacier is in equilibrium with the local climate. If snow accumulation is great than ablation, this give the glacier a positive mass balance and it will advance.If snow accumulation is less than ablation then this gives the glacier negative mass balanced and it will retreat. Where glaciers exist is influenced by the altitude and latitude of an area, the amount of precipitation that area receives, and the relief of the area. As a result, glaciers will only exist under specific conditions.

How do glaciers move?

There are three main ways in which glaciers can move. Internal deformation (creep) occurs when the weight of ice and gravity causes the ice crystals to deform. In this case, gravity is a sheer stress and the deformation of ice crystals is strain. Creep depends on the thickness of the ice as well as the slope of the glacier's bed. When ice moves by this mechanism it can undergo extensional flow (advancing) or compressional flow (retreating). Creep is also temperature-dependent, and warmer ice moves faster than colder ice. When the stresses on the ice are greater than the rate at which the ice moves it can cause fractures and faults on the ice surface. A glacier can also move by basal sliding and there are two forms of this: enhanced basal creep and regelation slip. For both of these it is key for there to be reduced friction. Water at the bed is essential for a gacier to slide, and there only needs to be a few millimeters of water present and basal sliding will occur. Another factor that effects basal sliding is how much debris is present and how large the debris is. Enhanced basal creep is best for causing ice to deform around large debris deposits, such as boulders. Regelation slip is best for deforming around small debris deposits. Regelation slip is the process of melting and refreezing of ice. If a glacier's bed is made of soft sediment then the ice will deform the bed and glacial flow will accelerate. This type of movement is called subglacial bed deformation. (Bennett & Glasser, 2009)

Figure 1: Diagram of glacial movement. Deformation is smallest at the bed
(Wiley, 1999 http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/Ryan_Cross/Ryancross.dir/Glacial%20flow.JPG)

Figure 2: Diagram of types of glaciers and their position in relation to pressure-melting point of ice
(http://www.homepage.montana.edu)

Types of glaciers

Glaciers can vary not only in size and location, but also in internal temperature which causes them to behave quite differently from each other. A Wet-based (temperate) glacier is one that is at pressure-melting point throughout. Wet-based glaciers often move by basal sliding and flow faster than cold-based glaciers. The bed of the glacier is made of soft sediments and is saturated with meltwater due to friction and geothermal heating. A cold-based (polar) glacier is one that is below pressure-melting point throughout. Cold-based glaciers are frozen to their beds and move slowly by internal deformation. An intermediate (subpolar) glacier is one which is at pressure-melting point at the bed, but not through the entire glacier. This experiment focuses on wet-based and cold-based glaciers.

What is an ice stream?

An ice stream is a type of glacier that moves much faster than the ice around it. In Antarctica ice streams only account for ~10% of ice volume, but these are not small features. They can be up to 50 km wide and 2000 m thick (British Antarctic Survey, 2007). A large amount of ice leaves the ice sheet through ice streams, in some areas over 1000 m of ice per year (British Antarctic Survey, 2007). The velocity of an ice stream is controlled by two major components: presence of water and bed type. The amount of water that is lubricating the base of the ice will effect the velocity at which the ice stream is moving. If the bed is made of soft, water-saturated deformable sediment this will also allow the ice stream to flow faster than if the glacier is sitting on hard bedrock (British Antarctic Survey, 2007). Ice streams may also have crevasses at their margins.

Figure 3: A Diagram showing different mechanisms that will increase flow velocity. An ice sheet consists of the ice sheet interior, an outlet glacier or ice stream, the grounding line and sometimes an ice shelf or ice tongue (a). If the ice shelf or ice tongue calves off then this creates a change in the downstream forces (b). The ice stream or outlet glacier will speed up and the elevation and mass of the ice sheet interior will decrease. If the bed of the ice sheet is lubricated, this will also cause the ice stream or outlet glacier to speed up (c). This also results in a decrease in elevation and mass.
(Bell, 2008 http://www.nature.com/ngeo/journal/v1/n5/box/ngeo186_BX1.html)

The West Antarctic Ice Sheet & East Antarctic Ice Sheet.

Antarctica is made of two ice sheets, the East Antarctic Ice Sheet (EAIS) and the West Antarctic Ice Sheet (WAIS), separated by the Transantarctic Mountains. The EAIS is considered stable and sits on a land mass. The WAIS is much smaller than the EAIS, but it does contain around six meters of sea-level rise in mass. On the other hand, the WAIS is considered to be very unstable. It is marine-based, sitting on the sea floor. The WAIS contains ice streams which are made of soft-sediment beds and meltwater. The ice streams drain the ice sheet, and cause the ice to flow faster in certain areas. The WAIS also contains cold-based ridges which causes the ice in that area to move slowly by internal deformation.

Figure 4: Map of Antarctica depicting EAIS, WAIS, and Transantarctic Mountains
(http://www.geos.ed.ac.uk/homes/des/antarctica_intro.html)

Figure 5: Model of West Antarctic Ice Sheet including ice streams
(http://www.wunderground.com/blog/RickyRood/)

Lambert Glacier

The Lambert Glacier is located in northern East Antarctica and is the largest glacier the world. It is responsible for draining up to 900,000 km² of ice each year (NASA, 2008). It contains multiple ice streams and flows into the Amery ice shelf. The geography of the area has only become known in the last few years after multiple research groups worked there in an attempt to figure out its thickness (NASA 2008).

Figure 6: Map of the Lambert Glacier area.
(Modified from Radarsat http://nsidc.org/data/radarsat/images/gallery/lambert_map.jpg)

Figure 7: Map of the velocity of ice flow in the Lambert Glacier area
(NASA, 2008 http://earthobservatory.nasa.gov/IOTD/view.php?id=1199)