Sea Ice Types ICE TYPE SALINITY DENSITY POROSITY (at -5 deg C) UNIAXIAL COMPRESSIVE STRENGTH (as deduced from porosity) FRAZIL Not applicable Not applicable Unconsolidated ice Strength negligible GREASE ICE Not applicable Not applicable Unconsolidated ice Strength negligible SLUSH Not applicable Not applicable Unconsolidated ice Strength negligible NILAS 15 ppt 0.92 Mg m -3 0.15 8.8 - 10.8 MPa PANCAKE 11 ppt 0.92 Mg m -3 0.11 9.2 - 11.0 MPa YOUNG ICE Not available Not available Not available Not available 1st YEAR ICE 9 ppt 0.92 Mg m -3 0.09 9.6 - 11.2 MPa 2nd YEAR ICE Not available Not available Not available Not available MULTI-YEAR ICE Not available Not available 0.03 8.2 - 9.7 MPa (from http://www.gi.alaska.edu/~eicken/he_teach/GEOS615icenom/struc/strength.htm) Porosity or relative brine volume Vb / V can be calculated from the density r in Mg m -3 and the salinity of ice Si in ppt: Vb / V = r Si / F1 (T) where F1 (T) = 91.3 Mg m -3 at -5ºC (temperature dependent coefficient)   Frazil Ice: Frazil is the first stage of sea ice. The upper layers of the ocean water starts to freeze into fine needles or plates that are suspended in the water and float. This type of ice is only formed in turbulent, supercooled (= below its equilibrium freezing point) water, like the ocean. The air temperature has to be significantly less than -1.8ºC (the approx. freezing point of sea water) for this to occur. Grease Ice: This is the next stage of the ice where single frazil crystals coagulate and sinter into an oily looking surface layer. This layer is very thin but nonetheless can smooth out small surface waves. A picture of this type of ice can be found here. Slush: is snow mixed with water in the water column while slush ice is snow on top of the ice that either melts or get flooded ("overflow") and refreezes into a solid layer. This material is structually weaker then water ice. Nilas: First dark nilas developes. It is a thickened layer of grease ice and still transparent. It turns into light nilas as it thickens further because it is able to reflect light. This layer is less than 10cm thick and bends with larger surface waves. picture Pancake Ice: This ice is formed from frazil or grease ice in rough water where the ice surface layer starts to break up and the separated pieces bump into each other and form what looks like pancakes. The plastic deformation on the outside edges leads to raised rims around the perimeter and further freezing thickens and stabilized the plates. They usually don't get bigger than 1 to 2 meters. picture Young Ice: This clasification is used for ice build under clam conditions and precedes the nilas stage. Thickness of young ice is 10 - 30 cm. picture First Year Ice: This term is used for consolidated pancake ice (rough conditions) and young ice thicker than 30cm (calm conditions). Once a continuous ice cover has formed, congelation or columnar ice starts to grow at the bottom of this sheet. While pancaking is mostly a dynamic ice growth process, congelation refers to the thermal growth that takes place under quiescent conditions underneath the already excisting ice cover. Any roughnesses on the underside of the ice that developed during the dynamic growth get smoothed out in the process. First year sea ice can reach thicknesses between 1 and 2.5 meters. picture Second Year Ice: This ice has survived one summer - meaning one melting season. The melting snow layer sent a freshwater flux through the brine channel network and replaces the highly concentrated brine during late spring/early summer. Properties of this type of ice differ from first year ice in that the brine drainage has progressed and salinities in the top layer of the ice are close to zero. The density in second and multi-year ice is lower and as a consequence the ice has a higher freeboard (level of the snow surface above the water surface). Since the relatively fresh water in the brine channels of multi-year ice has a higher freezing point, the porosity of this ice is higher and hence it is stronger. This is one of the reasons why ice breakers avoid this type religiously if possible. picture Multi-Year Ice: If the ice survives more than 2 melting seasons it is called multi-year ice. This ice eventually reaches an equilibrium thickness at which the growing rate in the winter equals the melting rate in the summer and therefore no net gain or loss is happening. Multi year ice is mainly found in the Arctic and has a maximum life expectancy of about 7 years before it gets flushed out into Fram Strait. For a general depiction of all ice types see the "Fly over the ice edge" movie.