Page 535 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 16 Oceans, Coastal Systems, and Wind Processes 499
CriTiCAlthinking 16.1
Coastal Sensitivity to Sea-Level Rise
View the map “Sensitivity of the coasts of Canada to sea- level rise” at ftp2.cits.rncan.gc.ca/pub/geott/ess_pubs/210/ 210075/gscbul_505_e_1998_mn01.pdf. Sensitivity in this context is defined and explained briefly in the upper right corner of the map. Which regions have the highest sen- sitivity index values? if you visit or live near a coast, what is the value for that location? What characteristics of the coast in that area determine its sensitivity index value? •
Coastal System Actions
The coast is the scene of complex tidal fluctuations, winds, waves, ocean currents, and occasional storms. These forces shape landforms ranging from gentle beaches to steep cliffs, and at the same time sustain delicate ecosystems.
Tides
Tides are complex twice-daily oscillations in sea level, ranging worldwide from barely noticeable to a rise and fall of several metres. They are experienced to vary- ing degrees along every ocean shore around the world. Tidal action is a relentless and energetic agent of geo- morphic change. As tides flood (rise) and ebb (fall), the daily migration of the shoreline landward and sea- ward has significant effects on sediment erosion and transportation.
Tides are important in human activities, including navigation, fishing, and recreation. They are of special concern to ships because the entrance to many ports is limited by shallow water, and thus high tide is required for passage. Conversely, tall-masted ships may need a low tide to clear overhead bridges. Tides also occur in large lakes but are difficult to distinguish from changes caused by wind in those bodies of water because the tidal range is small. Lake Superior, for instance, has a tidal variation of only about 5 cm.
Causes of Tides Tides are produced by the gravita- tional pull of both the Sun and the Moon (Figure 16.6). Chapter 2 discusses Earth’s relation to the Sun and Moon and the reasons for the seasons. The Sun’s influ- ence is only about half that of the Moon’s because of the Sun’s greater distance from Earth, although it is a sig- nificant force. Figure 16.6 illustrates the relationship between the Moon, the Sun, and Earth and the generation of variable tidal bulges on opposite sides of the planet.
The gravitational pull of the Moon tugs on Earth’s at- mosphere, oceans, and lithosphere. The Sun also exerts a gravitational pull, to a lesser extent. Earth’s solid and fluid surfaces all experience some stretching as a result of these forces. The stretching raises large tidal bulges in the atmosphere (which we can’t see), smaller tidal bulges
▲Figure 16.6 The cause of tides. gravitational relations of Sun, Moon, and earth combine to produce spring tides (a, b) and neap tides (c, d). (Tides are greatly exaggerated for illustration.)
Animation
Monthly Tidal Cycles
Full
Earth
Solar tide New moon
Lunar tide
Solar tide
(a)
Lunar tide (b)
Lunar tide Earth
(c)
Lunar tide (d)
Earth moon
Earth
First-quarter moon Solar tide
Solar tide
Third-quarter moon
in the ocean, and very slight bulges in Earth’s rigid crust. Our concern here is the tidal bulges in the ocean.
Gravity and inertia are essential elements in under- standing tides. Gravity is the force of attraction between two bodies. Inertia is the tendency of objects to stay still if motionless or to keep moving in the same direction if in motion. The gravitational effect on the side of Earth facing the Moon or Sun is greater than that experienced
Neap tide Spring tide