Geology Reference
In-Depth Information
pieces (a few meters to tens of meters wide) due to wind
effect and/or oceanic conditions. While floating and mov-
ing, the fractured pieces may slide over each other to
form what is known as surface rafting (Figure 2.9). This
phenomenon is discussed in more details in section 2.4.,
but it suffices here to mention that rafting is a character-
istic of nilas. In fact, rafting is a useful surface feature to
identify nilas in remotely sensed images.
The second scenario of lateral ice growth is observed
under wind and turbulent ocean surface conditions. This
is commonly seen in open seas where the water surface
is usually more turbulent than lakes. When the ocean
surface is rough at the time of initial ice formation, tur-
bulence will not allow consolidation of the herded frazil
crystals into nilas. Instead, it causes frazil to undergo cyclic
compression following the wave action. If compressed
enough, herded crystals may bond with each other due to
the freezing of water between them. Eventually they may
form discs that may start as slush before the solidification
when water between crystals continues to freeze. It is, there-
fore, common to see the formation of small and thin pan-
cakes during the very beginning of freezing as shown in
Figure 2.10. Actually, these small features can also become
an integral part of the nilas leading to different shades
of grayness, as can be noticed in Figure 2.8.
The disc‐shaped features are called pancakes simply
because they are round or oval in shape. The diameter of
these discs range from a few fractions of a meter to sev-
eral meters. Often their edges are raised as results of rota-
tion and collision against each other by the ocean‐induced
motion. Figures 2.10, 2.11, and 2.12 illustrate different
stages of the growth of pancake ice. It can be seen that
the larger pancakes are formed by the consolidation of
smaller ones. Large pancakes may also break due to
wave actions. An example of cracked and separated frag-
ments of a large pancake is marked as (a) in Figures 2.12.
Pancakes
Dark nilas
Light nilas
Grease ice
Figure 2.10
Grease ice and small pancakes with diameters in
the range of 100-200 mm in Davis Strait in early November
1982; note the differential damping of the ocean waves in the
two ice regimes (photo by N. K. Sinha, unpublished).
Figure 2.8
Dark and light nilas in Hudson Strait, Canada, during
mid‐November, photograph taken from altitude of about 700 m
(Courtesy: Canadian Ice Service).
Figure 2.11
Freely floating and partially consolidated pancakes,
with diameters of 0.5-1.0 m, damping the ocean waves in Davis
Strait (photo by N. K. Sinha, unpublished).
Figure 2.9
Nilas with rafting and cracking in Nares Strait, north
of Baffin Bay, in April 1994 (photo by N. K. Sinha, unpublished).
