Ice or frost wedging is a mechanical or physical weathering process. It happens when water repeatedly seeps into rock cracks and pores and expands upon freezing. The expansion creates internal pressure that widens cracks or pores, ultimately causing rocks to break or disintegrate.
Ice wedging happens because of the unique water property of expanding by about 9% when it freezes. This means frozen water or ice takes up more space than molecules in liquid water.
A simple experiment of freezing water in a glass bottle can prove it. The bottle will fracture or break when the water turns into ice since it will expand.
This weathering process is known as gelifraction, congelifraction, or freeze-thaw. A freeze-thaw cycle occurs when the temperature drops until the water freezes, then goes high until it melts.
Lastly, it can occur with other physical weathering processes like salt wedging, wetting and drying, thermal expansion, exfoliation/sheeting, or biological or chemical ones.
How does frost wedging weathering work?
Frost wedging occurs in places with adequate moisture and experience freeze-thaw cycles.
An example of frost wedging is when rain, dew, or fog water enters rock microcracks, cracks, pores, crevices, or layers. This water will become ice or crystallize when temperatures go below freezing.
Since ice takes up more space, it will apply pressure or stress to the walls of pores, cracks, crevices, or layers. This will widen them or even cause disintegration.
When temperatures rise, water will melt, i.e., thawing occurs. More water will enter the cracks or pores, which are now wider. Again, upon freezing, it will expand, enlarging them further.
Repeated freezing and thawing will progressively widen cracks or pores. Over time, it will make the rock break off, forming small angular clasts and large boulders. The large boulders will gradually break into smaller pieces, too.
Crystalline rocks like diorite, gabbro, or granite can undergo granular disintegration during ice wedging, forming individual mineral crystals.
Lastly, cracks, microfractures, and microstructure of a given rock influence the shape and size of shattered rocks.
Frost heave or ice lens formation
You are familiar with frost heaving. It displaces soil upward during winter, which is noticeable during spring thawing. This bulging damages roads and concrete slabs. Also, it will misshapen and make lawns spongy.
This same process can cause rock weathering. Some authors believe it is what drives frost wedging the most. Weathering from expansion due to freezing is only secondary.
It happens when ice lenses grow in pores or cracks by attracting films of unfrozen water or moisture from the surroundings. This growth of ice lenses will exert a lot of stress that will enlarge fractures or cause rocks to shatter.
Hydrofracturing
Sometimes, hydrofracturing can cause rocks to crack and shatter. It is like a confined or closed system where volume increases from freezing.
This volume increase can theoretically exert 200-250 MPa pressure. Such a huge can break any rock, with a tensile strength below 25 MPa.
How does it happen? Hydrofracturing happens when water-filled fissures or pores quickly freeze on the surface before the inner parts. This seals the surface of the pore or crack.
Expanding ice will then cause cryostatic or hydrostatic pressure. This pressure is transmitted equally through the pores or cracks with unfrozen water, which is more like a hydraulic jack. It is large enough and will cause rocks to shatter.
Lastly, hydrofracturing causes the shattering or cracking of rocks in unfrozen depths and from outward to inward.
Where is frost wedging most likely to occur
Frost wedging is more likely to occur in areas 1) near peaks in high mountains in the tropics and 2) middle and high latitudes, i.e., temperate and colder. These places often have moisture and freeze-thaw cycles. Also, they are near glaciers (periglacial).
Examples of composite volcanoes with high frost weathering in the tropics include Mt Kenya, Rwenzori, and Kilimanjaro in East Africa. Others are in Peru, Chile, Colombia, and Ecuador in the Andes.
Nights are cold in these mountains, and days warm. This causes freeze-thaw cycles. Also, they have moisture.
Examples in temperate or colder areas include Nevada in the Betic Cordillera of Spain, Spitzbergen, and the Swiss Alps.
Lastly, it can happen in high plateaus and low-latitude deserts with dew or where winter rains. Such will have enough moisture for freeze-thaw cycles.
Factors affecting ice wedging
Some of the factors that will affect the rate and extent of frost wedging include:
Temperatures
The temperature must meet the low minimum when water turns into ice, often at 0°C or 32°F, and rises until the ice melts. Otherwise, it will not happen.
1. Freezing speed
Rapid freezing speeds of at least 0.1°C per minute will favor effective cracking and shattering of rocks than slower ones.
2. Moisture content
Moisture content will influence frost weathering. For instance, partly empty cracks and crevices allow ice to expand without causing fracturing. This explains why cold, dry deserts in polar don’t experience much ice wedging or weathering.
Furthermore, shattering will occur in polar and alpine regions when at least 90 of the pores or cracks available have water.
3. Freeze-thaw cycle frequency
The higher the freeze-thaw cycle, the faster the frost wedging. Fragmentation occurs more due to the fatigue effect.
4. Rock properties
Rock type, texture, structure, strength, porosity, and water absorption capacity influence ice wedging.
For instance, metamorphic rocks and crystalline igneous resist frost wedging the best, while porous chalk, shales, and sandstones are the least resistant. Reasons include cementation and mechanical strength.
Also, it will be faster in rocks with a high density of microfractures or fractures like columnar joints from lava cooling or joints from bedding and extension of bedrock or along bedding planes.
Lastly, rocks with very low porosity hardly undergo gelifraction. For instance, it may not happen in sedimentary rocks with less than 6% porosity.
Frost wedging material
Frost wedging material will mostly remain in situ. However, erosional agents can transport and deposit the fragments as sediments.
However, in alpine or mountainous environments, debris on scree or talus slopes demonstrates frost weathering in the field.
In places where frost shattering or weathering is quick, it will form a blanket of angular rock fragments known as felsenmeer. Felsenmeer is a German name for a rock sea.
These fragments are common on slopes and summits of alpine environments that experience freezing and thawing.
For instance, these rubbles of angular fragments occur on Mount Washington in New Hampshire in the US. Also, they are at the summit of Mount Darling in Antarctica.
Ice wedging effects
Weathering from freeze-thaw cycles is why poorly insulated pipes will fracture during the freezing season. During these times, water freezes inside the pipes and expands. The resulting pressure fractures the pipes.
Also, it causes the porthole menace seen on highways and roads, especially in the northern parts of the USA. These places experience freezing and thawing.
Similarly, concrete will crack in colder areas that experience a freeze-thaw cycle. It happens when it seeps into crevices, pores, or cracks and expands when it freezes, widening cracks.
References
- Elorza, M. G. (2013). Geomorphology. Taylor & Francis.
- Huggett, R. J. (2011). Fundamentals of geomorphology (3rd ed.). Routledge.
- Dixon, J. C. (2004). Weathering. In Goudie, A(ed.) Encyclopedia of geomorphology (vol. 1, pp 1108-1109). Routledge
- Bierman, P. R., & Montgomery, D. R. (2014). Key concepts in geomorphology. W.H. Freeman and Company Publishers
- Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
- Plummer, C. C., Carlson, D. H., & Hammersley, L. (2016). Physical Geology (15th ed.). McGraw-Hill/Education, Inc