1. Understanding crevasses: Introduction
A glacier is a mass of snow and ice which has been or is in motion
Snow falls and over time metamorphoses to ice and flows from higher to lower altitudes
The ice flows over the bedrock topography exerting different stresses on the ice which can result in crevasses

2. Glacier types Morphological classification Temperature types
Plateau glacier
Valley glacier
Cirque
Hanging glacier
Ice stream
Shelf ice
Temperature types
Warm glacier: constant around zero degrees throughout the ice mass (eg. Most European glaciers)
Cold glacier: temperature below zero (glaciers on Svalbard and Antarctic)
Some glacier massifs can consist of both types.

3. Glacier motion Can be compared with cold sirup Movement due to gravity
Internal movements
Gliding along the bottom
Both processes occur together
Deformation due to realignment and movement of individual crystals
Temperature dependent
Ice thickness dependent
Bedrock angle
Bedrock topography
Access to water
More in summer
Daily cycle

4. Crevasses
Crevasses form when the tension formed by the flow causes some ice to move faster than ice around it.
Variations in terrain
Marked roughness in the bedrock (not always obvious in Antarctic)
Widening or narrowing in the valley or ice stream
Ice stream flowing through slow moving ice
Hinge zone (tidal motion)
< 30m deep in European glaciers, 100m(?) in Antarctica
Types of crevasses
Marginal crevasses (at ice-rock boundary or fast ice – slow ice boundary)
Transverse crevasses (steeper slope / narrowing / hinge zone)
Longitudinal crevasses (widening / submerged ridge
Radial crevasses
Bergschrund

9. Snow metamorphosis
Snow is continuously subject to a process of change from new snow to glacier ice
Mechanical degradation
Wind
Weight of the snow pack
Time
Chemical degradation
Evaporation from crystal points
Freezing onto flat parts of the crystal
Melt water
Density
New snow = 0.1 – 0.3 g/cm
Firn = 0.55 g/cm
Glacier ice = 0.8 g/cm
In Europe the process takes 5-6 yrs
In Antarctica the process takes ca. 100 yrs
Chemical construction
Depth hoar builds up in snow bridges weakening them

10. Snow bridges
Crevasses are often covered with snow and impossible to spot from the surface
Support strength is dependent on:
Snow thickness
Snows density
Ice layers
Air temperature
Crevasse shape
Support strength
Strongest : Early in the season (Antarctic spring, November)
Weakest : End of season (February)
Daily cycle : stronger at night(?)
Weakening:
Snow evaporates /sublimes during the summer (warm days, solar radiation, wind)
Tiny amounts of melt water percolates down the side of crevasse walls destroying the bonds between the snow bridge and crevasse wall (hinge zone)
Uneven driving or sledges swinging up and down over hummocks
Strengthening:
Snow fall
Wind action compacting the snow
Freezing of melt water (?)
Mechanical intervention and physically reinforcing the crossing point
Repeated driving of heavy vehicles over the same tracks

14. Cross crevasses perpendicular
This crevasse was approached at ca. 60°

16. Detecting crevasses in snow covered terrain
Depressions in the snow surface
Low sun light throws shadows on the snow
Depressions can collect dust
Often easier to spot from a distance
Aerial reconnaissance provides a good picture of the surface particularly late in the season
GPR –ground penetrating radar

17. Summary Where are crevasses Snow bridges
Hinge zone : shelf ice-inland ice boundary (also edge of ice rises)
Ice streams : fast ice-slow ice boundary
Glacier ice-rock boundary
Terrain features : steeper, direction change, narrowing, widening, submerged ridge or peak, other irregularities, and nunataks
“Unexpectedly” in Antarctica (hidden terrain features!)
Easier to spot from distance / from air / GPR survey
Snow bridges
Antarctic snow bridges generally weak
Strongest in November, and weakest in February (at least in the hinge zone)
Probing surface will not necessarily identify weaknesses due to hard crust layers
Inspect crevasses to find narrowest crossing point
Always cross crevasses perpendicular