As glaciers flow, they crack and fracture in predictable ways. This brittle failure of the glacier ice yields insights into glacier strain, both in the present day and in the past. Because glaciers flow and ice is younger at the top of the glacier than at the bottom, these structures have been called the ‘writings in a glacier’s history book’, where the movement, strain and deformation is frozen in the ice1.
Crevasses are also important for moulin formation, calving icebergs at floating ice margins, and shearing at ice-stream margins. Melt-driven crevasse propagation is an important mechanism for delivering meltwater to the base of a glacier, which may result in seasonal accelerations as basal sliding increases2. The way in which ice fractures is also a control on how ice shelves break up and rapidly collapse.
The first key glacier structure results from the annual layering of the ice through cycles of melting and snowfall. These layers are known as Primary Stratification or Sedimentary Stratification. The layers start of parallel to the ice surface, and become deformed through ice flow. The dark and light layers comprise coarse-clear ice (summer ice – pore spaces infilled with refrozen meltwater) and white bubble-rich ice (winter snow).
Primary stratification may be visible in the field or on high-resolution remotely sensed images in the ablation parts of a glacier. On the map of Lachman Glacier below, stratification has been deformed by ice flow.
Longitudinal surface structures
Longitudinal surface structures (may be called longitudinal foliation or flow stripes) are common on glaciers and have a planar or layered structure that develops in ice during ice flow. The layers are characterised by variations in the ice crystals (e.g., coarse-clear and white bubble-rich). Longitudinal surface structures are prominent features, visible both in the field and on satellite images, and they have a variety of wavelengths and sizes, perhaps reflecting different mechanisms of formation. There are two predominant ways in which they form: deformed primary stratification, and during compressional, accelerating flow.
1. Deformed primary stratification
The first theory states that compression in the upper part of the glacier deforms the primary stratification3. Compression and shearing of these sedimentary layers forms elongated structures on the ice surface. These features tend to be more subtle and smaller-scale.
This foliation occurs the primary stratification is compressed in the upper part of the glacier3. Prolonged compression results in the transposition of the early layering, resulting in elongated longitudinal foliation. Primary stratification, debris layers, and crevasse traces are all elongated and incorporated in this process4. Compression and shearing of these primary layers is critical to the formation of longitudinal foliation.
2. Compressional, accelerating flow
The second theory states that longitudinal surface structures form in two main areas5:
- In zones of compressional, accelerating ice flow in the upper parts of glaciers;
- In zones of confluence between ice flow units.
In the compressional, accelerating ice flow, such as the area of confluence between two adjoining ice flow units, longitudinal extension occurs in the horizontal plane. This creates a hollow or depression at the ice surface, which is extended during ice flow. The downstream extent of this larger ‘flow stripe’ is determined by the transverse compressional forces. Flow stripes formed in this way are typically narrower, more persistent and are well defined on the ice surface5.
The figures below shows these larger-scale longitudinal surface structures forming on the surface of Antarctic Ice Streams (from ref. 5) (images (c) Neil Glasser).
Why do crevasses form?
Crevasses tell us the story of the glacier. Movement, strain and deformation are frozen in the ice. The analysis of crevasse patterns can tell us about the ice-dynamical processes experienced by the glacier. They can also be used during feature tracking to tell us about ice velocity.
Crevasses form when the elasticity threshold of ice is exceeded, and brittle failure occurs1. There are three kinds; tensile, opening stresses, fracturing and sliding, and tearing6. The tensile strength of the ice depends on the water content, temperature of the ice, the ice density and the ice structure.
Crevasses frequently occur at the lateral margins of a glacier, where lateral stresses against the valley walls resists flow, at steep sections, at the ice front, or at the head of the glacier.
Crevasses occur in response to changes in longitudinal stresses. Longitudinal extension in the upper part of the glacier, where ice is stretching, results in transverse crevasses. Here, net accumulation drives ice particles downwards, and ice is laterally compressed. As ice is incompressible, it must accelerate, opening up large crevasses arcing up-flow.
In the lower parts of the glacier, the ice is compressed in the longitudinal direction as ice velocities slow and ablation becomes the dominant process. The ice also splays out, resulting in across-flow extension and the opening up of splaying crevasses, orientated parallel to ice flow.
Controls on crevasse depth
Crevasse depth is controlled by the compressive stresses within the ice. With depth, ice overburden pressure increases, forcing the crack shut. This is illustrated in the figure below (after Benn and Evans, 2010). In temperate valley glaciers, crevasses are frequently no more than 20-30m deep7. Crevasses may be deeper in cold, stiff glaciers.
Water can create extra pressure, making crevasses deeper when they are water-filled (note that this is very important for ice-shelf collapse by hydrofracture in Antarctica!). In the lower figure below, the ice overburden pressure is opposed by water pressure, forcing the crevasse to open deeper.
Understanding crevasse patterns
Crevasse form at right angles to the principle stresses on a glacier. In the upper part of the glacier, with extending flow, the principle stress in the centre of the glacier is parallel to glacier flow. The crevasses are therefore parallel to glacier flow, forming transverse crevasses.
In the centre part of the glacier, where there is strong lateral drag against the valley walls, you may see chevron crevasses. They form through simple shear against the valley sides and take the form of linear fractures angled obliquely upvalley from the margins towards the centreline.
Splaying crevasses typically form in the lower parts of the glacier, in the zone of compression. The crevasses are approximately parallel to ice flow.
We can use the crevasses to make inferences about shear stresses and deviatoric stresses within a valley glacier.
Ogives form when ice passes through an ice fall. These arcuate structures are associated with valley glaciers, and form alternating bands of dark and light ice (summer and winter ice).
Further reading about ogives on this blog.
Rock fall onto glaciers from valley sides may result in significant debris cover. This is important as debris affects the rate of surface melt; a thin layer may accelerate melt by absorbing radiation and warming the ice. A thicker layer will insulate the ice, preventing ablation.
Medial moraines originate from point sources where rocks fall onto the ice surface. Typically medial moraines will delimit two flow unit boundaries, where two separate flow units of ice join together.
There are many excellent photographs of glacier structures on Glaciers Online. See also:
1 Herzfeld, U. C., Clarke, G. K. C., Mayer, H. & Greve, R. Derivation of deformation characteristics in fast-moving glaciers. Computers & Geosciences 30, 291-302, (2004).
2 Zwally, H. J. et al. Surface Melt-Induced Acceleration of Greenland Ice-Sheet Flow. Science 297, 218-222, (2002).
3 Hambrey, M. J. & Lawson, W. in Deformation of Glacial Materials Vol. Vol. 176 (eds Alex J. Maltman, B. Hubbard, & M. J. Hambrey) 59-83 (Geological Society of London, Special Publication, 2000).
4 Goodsell, B., Hambrey, M. J., Glasser, N. F., Nienow, P. & Mair, D. The structural glaciology of a temperate valley glacier: Haut Glacier d’Arolla, Valais, Switzerland. Arctic, Antarctic and Alpine Research 37, 218-232, (2005).
5 Glasser, N. F. & Gudmundsson, G. H. Longitudinal surface structures (flowstripes) on Antarctic glaciers. The Cryosphere 6, 383-391, (2012).
6 Benn, D. I., Warren, C. R. & Mottram, R. H. Calving processes and the dynamics of calving glaciers. Earth-Science Reviews 82, 143-179, (2007).
7 Benn, D. I. & Evans, D. J. A. Glaciers & Glaciation. 802 (Hodder Education, 2010).