Geology Reference
In-Depth Information
2
Ice Physics and Physical Processes
Development of a sea ice cover at any previously ice‐
free location encompasses a number of phases: (1) nucle-
ation of ice crystals and initial formation, including
consolidation that depends on the prevailing weather and
atmospheric conditions, (2) vertical growth or “ice conge-
lation,” (3) motion and deformation depending on sea
and wind states, (4) melt and decay, and (5) aging. Wind,
current, and wave‐induced motion and associated strain
(hence stress) stimulate deformation in different forms.
Cracking, rafting, raised edges, and piling are common
forms in thin floating ice with thickness less than 0.3 m,
known as young ice. Formation of ridges and rubble
fields are common in thicker ice. Grounded and/or land‐
fast ice (i.e., fastened to the shore) are obviously less
affected by these elements. Shore fast ice is, however, sub-
jected to the effects of tides, forming cracks, hinges, and
rubbles near the shorelines.
This chapter focuses on the first two phases with its
major processes of initial ice formation, lateral and verti-
cal growth, entrapment of inclusions, and the continuous
desalination processes. It also includes a brief presenta-
tion on the synopsis of ice motion, deformation, and
decay. Aging of ice is addressed in Chapter 5.
Formation and growth of ice entails three major stages.
The first is the initial formation when minute ice crystals
are nucleated in the seawater and grow sufficiently to rec-
ognizable sizes and shapes in the form of needles or tiny
discs, called frazil ice. The frazil crystals are usually sus-
pended in water and are often herded by wind action. At
this stage of ice formation, the water has a slushy consist-
ency. The ice particles are loose and isolated and have not
yet frozen to the point of consolidation. The second stage
is marked by lateral growth of the spicular crystals to
form small rounded discs or flat patches, and the crystals
have coagulated to form a soupy layer on the surface,
known as grease ice. These two stages are well known to
the Inuit as
qinu
(in Inuktitut) as described in Chapter 1. In
the third stage, ice congelation starts when it grows vertically
(i.e., thickens). These three stages are addressed in section 2.1.
on initial ice formation and section 2.2. on ice growth.
One of the key processes in sea ice formation and
growth is the rejection of salts originally dissolved in the
seawater. This takes place mainly at the ice‐water inter-
face as water solidifies, adding to the volume of pure ice
at the interface and growth of thickness of the ice sheet.
During the initial stages of solidification, the rejection of
salts is associated with the nucleation of crystals of pure
water molecules. The crystal lattice has no room to
accommodate any salt molecules. As these crystals grow
in size and numbers, the entire surface is covered with
solids with arms or dendrites at the ice‐water interface
(section 2.3.1.). This convoluted geometry, in turn, causes
some of the rejected salts to be entrapped into the gaps
between the ice discs or plates and eventually become
enclosed within the volume between the plates as brine.
This is what has become known as “brine pockets.” Brine
pockets could be interconnected or merge to form brine chan-
nels during the early growth period. At this stage, the
brine continues to drain rapidly to the underlying seawa-
ter (with also some expulsion to the surface depending on
the ambient conditions). The desalination processes con-
tinue throughout the lifetime of a sea ice cover. These
processes are addressed in details in section 2.3.3.
Since ice is one of a very few crystalline substances
for which the solid phase is less dense than its melt, it
always floats on the water surface. Unless this floating ice
becomes grounded to the shore land in the form of land‐
fast or simply fast ice, it may undergo a complex motion
at different scales, leading to several forms of deforma-
tion of the ice cover. Mobility and deformation of sea
ice are presented in section 2.4., while ice decay is
addressed briefly in section 2.5. Ice classes and regimes are
