A pressure ridge, when consisting of ice, is a linear pile-up of

sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's o ...

fragments formed in pack ice by accumulation in the convergence between floes. Such a

pressure ridge A pressure ridge is a topographic ridge produced by compression. Depending on the affected material, "pressure ridge" may refer to: * Pressure ridge (ice), between ice floes * Pressure ridge (lava), in a lava flow * Pressure ridge (seismic), in ...

develops in an ice cover as a result of a stress regime established within the plane of the ice. Within sea ice expanses, pressure ridges originate from the interaction between floes,A ''floe'' is any individual piece of sea ice larger than . as they collide with each other.http://nsidc.org/cryosphere/seaice/index.html .Weeks, W. F. (2010) ''On sea ice''. University of Alaska Press, Fairbanks, 664 p. Currents and winds are the main driving forces, but the latter are particularly effective when they have a predominant direction. Pressure ridges are made up of angular ice blocks of various sizes that pile up on the floes. The part of the ridge that is above the water surface is known as the ''sail''; that below it as the ''keel''.These terms also apply for any floating ice feature, such as

iceberg An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...

s. Pressure ridges are the thickest sea ice features and account for about one-half of the total sea ice volume. Stamukhi are pressure ridges that are grounded and that result from the interaction between

fast ice Fast ice (also called ''land-fast ice'', ''landfast ice'', and ''shore-fast ice'') is sea ice that is "fastened" to the coastline, to the sea floor along shoals or to grounded icebergs.Leppäranta, M. 2011. The Drift of Sea Ice. Berlin: Springer- ...

and the drifting pack ice.Barnes, P.W., D., McDowell & Reimnitz, E. (1978). Ice gouging characteristics: Their changing patterns from 1975-1977, Beaufort Sea, Alaska. United States Department of the Interior, Geological Survey Open File Report 78-730, Menlo Park, U.S.A., 42 p.Ogorodov, S.A. & Arkhipov, V.V. (2010) Caspian Sea bottom scouring by hummocky ice floes. Doklady Earth Sciences, 432, 1, pp. 703-707.

Internal structure

The blocks making up pressure ridges are mostly from the thinner

ice floe An ice floe () is a large pack of floating ice often defined as a flat piece at least 20 m across at its widest point, and up to more than 10 km across. Drift ice is a floating field of sea ice composed of several ice floes. They may cau ...

involved in the interaction, but it can also include pieces from the other floe if it is not too thick. In the summer, the ridge can undergo a significant amount of weathering, which turns it into a smooth hill. During this process, the ice loses its salinity (as a result of brine drainage). This is known as an ''aged ridge''. A ''consolidated ridge'' is one whose base has undergone complete freezing. The term ''consolidated layer'' is used to designate freezing up of the rubble just below the water line. The existence of a ''consolidated layer'' depends on air temperature — in this layer, the water between individual blocks is frozen, with a resulting reduction in porosity and an increase in mechanical strength. A keel's depth of an ice ridge is much higher than its sail's height - typically about four times. The keel is also 2-3 times wider than the sail.

Thickness

One of the largest pressure ridges on record had a sail extending above the water surface, and a keel depth of . The total thickness for a multiyear ridge was reported to be .Johnston, M., Masterson, D. & Wright, B. (2009). Multi-year ice thickness: knowns and unknowns. Proceedings of the 20th International Conference on Port and Ocean Engineering under Arctic Conditions (POAC), Luleå, Sweden. On average, total thickness ranges between and , with a mean sail height that remains below .Strub-Klein, L. & Sudom, D. (2012). A comprehensive analysis of the morphology of first-year sea ice ridges. Cold Regions Science and Technology, 82, pp. 94-109.

Characterization methods

The physical characterization of pressure ridges can be done using the following methods: * Mechanical drilling of the ice, whereby augers designed for ice are driven through the ridge, and the core retrieved for analysis. * Surveying, whereby a level,

theodolite A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and ...

or a

differential GPS Differential Global Positioning Systems (DGPSs) supplement and enhance the positional data available from global navigation satellite systems (GNSSs). A DGPS for GPS can increase accuracy by about a thousandfold, from approximately to . DGPSs ...

system is used to determine sail geometry. * Thermal drilling – drilling involving melting of the ice. * Observation of the ice canopy by

scuba divers This is a list of underwater divers whose exploits have made them notable. Underwater divers are people who take part in underwater diving activities – Underwater diving is practiced as part of an occupation, or for recreation, where t ...

. * Upward looking sonars. * A series of

thermistor A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''. Thermistors are divided based on their conduction ...

s, to monitor temperature changes. *

Electromagnetic induction Electromagnetic or magnetic induction is the production of an electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1831, and James Cle ...

, from the ice surface or from an aircraft.

Interest for pressure ridges

From an offshore engineering and naval perspective, there are three reasons why pressure ridges are a subject of investigation. Firstly, because the highest loads applied on offshore structures operating in cold oceans by drift ice are associated with these features. Secondly, when pressure ridges drift into shallower areas, their keel may come into contact with the seabed, thereby representing a risk for subsea pipelines (see Seabed gouging by ice) and other seabed installations. Thirdly, they have a significant impact on navigation. In the Arctic, ridged ice makes up about 40% of the overall mass of sea ice.Wadhams, P. (2000). Ice in the Ocean. Gordon and Breach Science Publ., London, 351 p.

Notes

References