Page 220 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

▲Figure 7.3 Sublimation of a mountain snowpack. Blowing snow enhances sublimation in the Canadian Rockies near lake louise, Banff National Park, Alberta. [Mick House/Alamy.]
(branching or treelike forms). Ice crystals demonstrate a unique interaction of chaos (all ice crystals are dif­ ferent) and the determinism of physical principles (all have a six­sided structure).
As temperatures descend further below freezing, ice continues to expand in volume and decrease in den­ sity to a temperature of –29°C—up to a 9% increase in volume is possible. Pure ice has 0.91 times the density of water, so it floats. Without this unusual pattern of density change, much of Earth’s freshwater would be bound in masses of ice on the ocean floor (the water would freeze, sink, and remain in place forever). At the same time, the expansion process just described is to blame for highway and pavement damage and burst water pipes, as well as the physical breakdown of rocks known as weathering (discussed in Chapter 13) and the freeze–thaw processes that affect soils in cold regions (discussed in Chapter 17). For more on ice crystals and snowflakes, see www.its.caltech.edu/~atomic/snowcrystals/ primer/primer.htm.
In nature, the density of ice varies slightly with age and the air contained within it. As a result, the amount of water that is displaced by a floating iceberg varies, with an average of about one­seventh (14%) of the mass exposed and about six­sevenths (86%) submerged beneath the ocean’s surface (Figure 7.4). With underwater portions melting faster than those above water, icebergs are inherently unstable and will overturn.
▲Figure 7.4 The buoyancy of ice. A bergy bit (small iceberg) off the coast of Antarctica illustrates the reduced density of ice compared to cold water. Do you see the underwater portion of the bergy bit? [Bobbé Christopherson.]
CriTiCaLthinking 7.1 Iceberg Analysis
examine the iceberg in Antarctic waters in Figure 7.4. in a clear glass almost filled with water, place an ice cube. Then approximate the amount of ice above the water’s surface compared to the amount below water. Pure ice is 0.91 the density of water; however, because ice usually contains air bubbles, most icebergs are about 0.86 the density of water. How do your measurements compare with this range of density? •
Water, the Liquid Phase Water, as a liquid, is a non­ compressible fluid that assumes the shape of its con­ tainer. For ice to change to water, heat energy must increase the motion of the water molecules enough to break some of the hydrogen bonds (Figure 7.2b). As dis­ cussed in Chapter 4, the heat energy of a phase change is latent heat and is hidden within the structure of wa­ ter’s physical state. In total, 80 calories* of heat energy must be absorbed for the phase change of 1 g of ice melt­ ing to 1 g of water—this latent heat transfer occurs de­ spite the fact that the sensible temperature remains the
*Remember, from Chapter 2, that a calorie (cal) is the amount of energy required to raise the temperature of 1 g of water (at 15°C) by 1 C° and is equal to 4.184 joules.
Georeport 7.1 Breaking Roads and Pipes
Road crews are busy in the summer repairing winter damage to streets and freeways in regions where winters are cold. Rainwater seeps into roadway cracks and then expands as it freezes, thus breaking up the pavement. Perhaps you have
noticed that bridges suffer the greatest damage; cold air can circulate beneath a bridge and produce more freeze–thaw cycles. The expansion of freezing water is powerful enough to crack plumbing or an automobile radiator or engine block. Wrapping water pipes with insulation to avoid damage is a common winter task in many places. Historically, this physical property of water was put to use in quarrying rock for building materials. Holes were drilled and filled with water before winter so that, when cold weather arrived, the water would freeze and expand, cracking the rock into manageable shapes.
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