Geology Reference
In-Depth Information
appropriate to mention here that the spacing between the
dendrites (or platelets), also known as brine layer spac-
ing, depends on the growth rate of the ice as presented in
detail in section 4.5.2. For the growth rate range between
of 6 and 18 mm/day, the spacing in the bulk of natural sea
ice ranges between 0.5 and 1.0 mm [
Nakawo and Sinha,
1984;
Sinha and Zhan,
1996]. However, the average spac-
ing between the dendrites in or just above the skeletal layer
is measurably less than that in the bulk of the ice. This will
be clarified and explained later in this subsection.
Figure 2.25 shows schematic diagrams of the cross‐sec-
tional views of the bulk ice, well above the transitional
area of the skeletal region and in the plane parallel to the
ice surface. It features details in a cross section of a colum-
nar grain at a right angle to the length of the column.
This diagram is applicable to columnar grains regardless
of whether the grains are randomly oriented or geometri-
cally aligned in the horizontal plane. The commonly seen
rows of aligned brine and air inclusions inside a grain,
parallel to the basal plane, are illustrated in Figure 25a.
Basal plane (or 0001) in hexagonal crystals like ice is the
plane in which the material can solidify more rapidly than
in other planes in the absence of any growth constraint.
Figure 25b shows the locations of the inclusions inside
a so‐called grain of sea ice. They are shown to be mostly
(but not all) along the subgrain (or subcrystal) bounda-
ries inside a grain. The subgrain boundaries correspond
to surfaces of small‐angle boundaries representing slight
mismatches in the
c
‐axis and/or
a
‐axis orientations of the
subgrain as a whole. Individual subgrains consist of pure
(a)
(b)
0.1mm
Figure 2.23
Photomicrographs of (a) irregular and (b) cylindri-
cal brine pockets at −30 °C exhibiting precipitated salt crystals
and air bubbles, [
Sinha
, 1977a].
Brine layer
spacing
(a)
(b)
Subgrain
boundar
y
G
rain
b
oundary
Pure ice
dendrites
Figure 2.25
Cross‐sectional views at right angle to the length
of a columnar grain in directionally solidified (DS) sea ice;
inclusions only are shown in (a) and presence or absence of
subgrain boundaries are shown in (b).
Figure 2.24
Sketch showing pertinent features of the skeletal
layer, about 10 mm in thickness, at the bottom of steadily grow-
ing sea ice grain and the zone above this layer.
