Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 288

1 km

Fig. 6.75 These beaded seif dunes run parallel to the mean south-
west wind blow obliquely to the longitudinal crests, nourishing first
one side of the crest and then the other. They also migrate over
immobile surfaces, in this case bedrock.

Fig. 6.76 Star dunes form when multidirectional winds cause mean
rotary sand transport over immobile desert surfaces.

80 m. There are signs from global satellite surveys that the
world's ice volume is indeed contracting. Of the ice-lands,
the Antarctic ice cap has about 86 percent by area,
Greenland has about 11 percent and the many valley and
piedmont glaciers make up the remainder. A further 20
percent of land area is affected by permafrost. In the Ice
Ages of Quaternary times a staggering 30 percent of the
Earth's surface was ice-covered, with vast areas of North
America and Europe subjected to glacial erosion and dep-
osition and even larger areas occupying the permafrost
zone. The major environments of glacier ice are:
Ice-sheets and their associated fast-moving outlet ice streams
and coastal ice shelves (Fig. 6.77)
Valley glaciers and their marine outlets called tidewater
glaciers (Fig. 6.78)
Piedmont glaciers - divergent, fan-like ice masses formed
after a valley glacier becomes unconfined.
Moving ice is an eroding and transporting system.
Motion is caused by deformation of the crystalline solid
phase due to its own body force under the influence of
gravity. The direction of movement is controlled by
regional pressure gradients caused by the 3D distribution
of ice mass and/or bedrock slope. Thus the radial flow of
a mound-like ice sheet will pay little attention to local or
even regional bedrock relief; ice sometimes moves uphill
relative to the bedrock surface. The slow flow of glacier ice
is usually measured in meters per year, with values between
10 and 200 m yr 1 for valley glaciers and 200-1400 m yr 1
for ice streams. Corresponding strain rates are also small.
Although usually slow and steady, spectacular glacier
surges occur periodically when ice velocity increases by an
order of magnitude and more.

linear cloud formations (cloud “streets”), whose persistence
and wavelength resemble linear dunes. Another explana-
tion for longitudinal dunes is that they arise when trans-
verse dunes are subjected to winds from two directions at
acute angles to each other. One dune becomes elongated,
later to become the nucleus of a new dune as the wind
reestablishes itself in its former mode. The resultant dune
has its long axis orientated parallel to the resultant of the
two wind azimuths. This theory is broadly supported by
flow visualization studies on longitudinal dunes of the
Sinai desert where the oblique incidence of seasonal winds
to crestlines causes leeside helical flow spirals to be set up.
Complex flow aeolian dunes include the spectacular star-
shaped dunes (Fig. 6.76) known as rhourds , which com-
monly range from 500 to 1000 m wavelength and from 50
to 150 m height. The forms have central peaks about
which curved crests radiate like vortex lines. They may be
spaced randomly, separated by immobile rock or gravel
substrates, or in rows, and seem to arise from the interac-
tion of multidirectional regional winds with, less certainly,
local winds due to convected air masses. The flow over
these forms is, not surprisingly, particularly complicated.

6.7.5 Glacial ice and the cryospheric
boundary layer

The frigid cryosphere makes up 30 percent of the Earth's
land surface. Ten percent of this is ice cover, representing
about 80 percent of surface fresh water: should all this ice
and snow melt then global sea level would rise by some