Geology Reference
In-Depth Information
(a)
(b)
Figure 5.39
(a) Airstrip indicated by two parallel lines in the left and PCSP campsite on the right top corner, and
(b) air photo of housing complex on Hobson's Choice ice island (photos by N. K. Sinha, unpublished).
sensitivity in strength is very similar to that observed in
equi‐axed as well as transversely isotropic freshwater lake
and river ice. In fact, very similar rate sensitivity has also
been observed for in situ borehole indentation (BHI)
strength of MY pack ice (of Figure 5.38) for the old rub-
ble field associated with the ice island [
Sinha
, 1991]. The
BHI strength increases from about 12 MPa at indentation
rate of 3 × 10
−3
/mm · s to rather high strength of about 40
MPa at the rate of 2 × 10
−1
/mm · s. Thus the ice islands can
indeed be extremely hazardous for ships navigating in the
northern waters. Detailed studies on the analysis of
medium‐scale indentation strength and failure processes
of ice island MY ice are given in
Frederking et al
. [1990a,
1990b] and
Jordaan et al.
[1992].
Snowfall / accumulation
5 km
Glacier
expansion
Calving
Ice shelf
Core
Sea
Land
Sea Floor
Figure 5.40
Schematic representation of an ice shelf; the scale
applies to the Ward Hunt Ice Shelf in the Canadian High Arctic
[
Barrette and Sinha
, 1994/96].
strain‐induced recrystallization changes the habit of the
crystals. Consequently, relatively small grains with diame-
ters of a few millimeters are commonly observed. This is
not the case for the bottom ice of ice shelves. The absence
of shear deformation near the bottom of ice shelves float-
ing on seawater prevents the ice from any strain‐induced
recrystallization. However, as the tongue of the glacier
spreads on top of the seawater, it starts accumulating snow
and hence snow-ice at the top surface year after years and
saline ice starts to develop at the water‐glacier ice interface.
The bottom of ice shelves consist of layers of sea ice. This
layer may also contain frazil ice crystals formed in the
water and floating up to the bottom of the ice sheet.
In April-May 1986, using an ice coring auger, two full‐
depth ice cores with a diameter of 76 mm were obtained
from the Hobson's Choice ice island. These were sampled
from an undisturbed area near the PCSP base shown in
Figure 5.38. One core (designated as 86‐1) was drilled to
a depth of 38.67 m below the crest of a hummock, and
the second core (designated as 86‐2) was drilled to a depth
of 33.97 m below the bottom of the adjacent depression.
The core sites were 100 m apart and the elevation differ-
ence was 2.11 m. All specimen depths refer to the datum
represented by the top of core 86‐1; thus, the bottom of
5.2.2. Shelf Ice and Arctic Ice Islands
An ice shelf generally forms where one or more land gla-
ciers spread out onto sea to produce a laterally widespread,
relatively flat floating body of ice [Thomas, 1979], as shown
schematically in Figure 5.40. There are many ice shelves in
the Antarctic and several of them are huge. A very brief
description of the Antarctic ice shelves will be given in sec-
tion 5.2.4 while presenting the Canadian Earth observa-
tion satellites Radarsat‐1, 2 and SAR images obtained
with the latter. In the Arctic, the Ward Hunt Ice Shelf is
probably the most known feature. It covers a triangular
area of about 440 km
2
and is located in the northern part
of Ellesmere Island at latitude 83.2° and between meridi-
ans of longitudes 72.0° and 78.5° (Figure 5.35).
In temperate glaciers the deposited snow undergoes
morphological changes, and the grains tend to grow fairly
rapidly because they are at the melting temperature. They
may start off as characteristically millimeter in size, but
they tend to grow at greater depths. Near the bottom of
glaciers, where the shear deformation is usually observed,
