Geology Reference
In-Depth Information
W
s,
sail width
P
s,
sail porosity
A
s,
sail area
Sail
H
s
,sail height
α
s
Sea level
Consolidated
layer
α
k
Keel
H
k
,
keel depth
P
k,
keel porosity
A
k
,
keel area
Figure 2.47
Pressure ridges (indicated by arrows) shown in a
ridge zone in this image from the Beaufort Sea near Alaska on
April 1, 2007. Details of the ridge are shown in the insert (pho-
tographed by Dr. Pablo Clemente‐Colon of the National Ice
Center, Washington).
W
k
,
keel width
Figure 2.49
Geometrical idealization of a first‐year sea ice
ridge with definitions of characteristic parameters [
Timco and
Burden
, 1997].
relationships between the ridge parameters based on
analysis of 112 ridges in different sites in the Arctic are
presented in Figure 2.50.
Since both rafting and ridging are caused by collision of
ice floes/sheets, then one of the questions that are often
asked is: under what conditions does rafting or ridging
occur? This question has been addressed in a number of
studies including
Weeks and Kovacs
[1970],
Parmerter
[1975],
Hopkins et al
. [1999], and
Tuhkuri and Lensu
[2002].
The factors that determine the mode of deformation
include ice thickness, modulus of elasticity, geometry of
the leading edge of the sheet, and the degree of uniformity
of thickness distribution within each ice floe/sheet.
Generally speaking, conditions may be favorable for raft-
ing when the two ice pieces are thin enough and have more
or less the same thickness. In some cases, the two ice sheets
can continue pushing against each other, breaking and
abrading, until conditions become suitable for rafting.
Parmerter
[1975] developed an analytical model of rafting
based on the geometry of the two colliding sheets. Results
emphasized two findings: (1) two sheets can raft if their
thickness is not significantly different and (2) the probabil-
ity of rafting decreases with increased ice thickness and its
modulus of elasticity. On the other hand, ridging is more
likely to occur when the ice is relatively thick as mentioned
earlier. Upon abrasion and crushing of the two sheets,
their edges keep changing until conditions become right
for ridging. This implies a transitional mode between raft-
ing and ridging where broken ice pieces that accumulate at
the surface of one sheet can lift the other sheet above it.
Using a model that simulates motion of two‐dimen-
sional ice blocks,
Hopkins et al
. [1999] simulated rafting
and ridging between two identical sheets. They confirmed
the established conclusion that rafting was the likely
Figure 2.48
Upturned ice blocks that form a small pressure
ridge in the Fram Strait off the east coast of Greenland. The
ridge sail height is about 2 m and keel depth 6.5 m (photo taken
by D. Sudom of NRC of Canada).
[2007] found that ridging dominated the deformation pro-
cess of ice when its thickness is above 0.4 m. The Sea of
Okhotsk, however, is significantly warmer than the Arctic.
Extensive field surveys and statistical modeling on
ridging of first‐year (FY) and multiyear (MY) ice were
conducted by the National Research Council (NRC) of
Canada in the 1990s.
Timco and Burden
[1997] presented
a suite of geometrical parameters that characterize ridges
(Figure 2.49). These include sail height
H
s
, keel depth
H
k
,
sail width
W
s
, keel width
W
k
, sail area
A
s
, keel area
A
k
,
sail porosity
P
s
, keel porosity
P
k
, sail slope
α
s
, and keel
slope
α
k
. They compared those parameters from profiles
of 184 ridges reported in 22 different studies and devel-
oped exponential best fitting of data points. Table 2.3
summarizes the best‐fit power relationships along. The
number of data points (
n
) used in the regression and the
correlation coefficient (
r
2
) between the two parameters
in each equation are also given. The first four equations
apply to FY ice ridges. The last two equations (with the
letter m in the subscript) apply to MY ice ridges. Average
values of ridge angles as well as simple empirical linear
