Page 540 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

504 part III The earth–atmosphere interface
Animation
Beach Drift, Coastal Erosion
Surf zone
Sand movement
(a) Longshore currents are produced as waves approach the surf zone and shallower water. Longshore drift and beach drift result as substantial volues of material are moved along the shore.
▲Figure 16.11 longshore current and beach drift. [(b) Bobbé Christopherson.]
(b) Processes at work on the shore
of Ile du Havre-aux-Maisons, Magdalen Islands, Québec.
This process, called beach drift, moves particles along a beach with the longshore current by shifting them back and forth between water and land with each swash and backwash of surf. Littoral drift is the term for the com- bined actions of the longshore current and beach drift.
You have perhaps stood on a beach and heard the sound of littoral drift as the myriad sand grains and sea- water mix in the backwash of surf. These dislodged ma- terials are available for transport and eventual deposition in coves and inlets and can represent a significant vol- ume of sediment.
Tsunami A series of waves generated by a large under- sea disturbance is known as a tsunami, Japanese for “harbour wave” (named for the large size and devastat- ing effects of the waves when their energy is focused in harbours). Often, tsunami are reported incorrectly as “tidal waves,” but they have no relation to the tides. Sudden, sharp motions in the seafloor, caused by earth- quakes, submarine landslides, eruptions of undersea volcanoes, or meteorite impacts in the ocean, produce tsunami. They are also known as seismic sea waves since about 80% of tsunami occur in the tectonically active region associated with the Pacific Ring of Fire. However, tsunami can also be caused by nonseismic events. Often, the first wave of a tsunami is the largest, fostering the misconception that a tsunami is a single wave. However, successive waves may be larger than the first wave, and tsunami danger may last for hours after the first wave’s arrival.
Tsunami generally exceed 100 km in wavelength (crest to crest) but are only a metre or so in height. They travel at great speeds in deep-ocean water—velocities of
600–800 km · h−1 are not uncommon—but often pass un- noticed on the open sea because their long wavelength makes the rise and fall of water hard to observe.
As a tsunami approaches a coast, the increas- ingly shallow water forces the wavelength to shorten. As a result, the wave height may increase up to 15 m or more, potentially devastating a coastal area far be- yond the tidal zone, and taking many human lives. In 1992, a 12-m tsunami wave killed 270 people in Casares, Nicaragua. A 1998 Papua New Guinea tsunami, launched by a massive undersea landslide of some 4 km3, killed 2000. During the 20th century, records show 141 damaging tsunami and perhaps 900 smaller ones, with a total death toll of about 70000. No warning system was in place when these tsunami occurred.
On December 26, 2004, the M 9.3 Sumatra–Andaman earthquake struck off the west coast of northern Suma- tra along the subduction zone formed where the Indo- Australian plate moves beneath the Burma plate along the Sunda Trench. (Please review the Chapter 13 opening map to find this trench along the coast of Indonesia in the eastern Indian Ocean Basin.)
The earthquake caused the island of Sumatra to spring up about 13.7 m from its original elevation, trig- gering a massive tsunami that travelled across the Indian Ocean (Figure 16.12). Energy from the tsunami waves trav- elled around the world several times through the global ocean basins before dissipating. With Earth’s mid-ocean mountain chain acting as a guide, related large waves ar- rived at the shores of Nova Scotia, Antarctica, and Peru.
The total human loss from the Indonesian quake and tsunami may never be known but exceeded 150000 people. In response to this tsunami, the Indian Ocean
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