Page 440 - Global Tectonics

422 CHAPTER 13

A similar deposit is bauxite, a residual deposit
enriched in aluminum hydroxide, which provides the 13.3 NATURAL
vast majority of the world’s aluminum. This forms by
the in situ weathering of aluminosilicate minerals on HAZARDS
stable peneplaned topography in a wet tropical climate
by the intense leaching of alkalis and silica. Bauxite only
forms within 30° of the equator and requires high rain- The most obvious natural hazards resulting from tec-
fall and high ambient temperatures. It is mined in tonic activity are earthquakes and volcanic eruptions.
Jamaica, northern Australia, and China. However both may cause tidal waves either directly, or
Evaporites form in an arid climate by the evapora- indirectly, by triggering major slides or slumps from
tion of seawater in semi-isolated basins which receive steep slopes at the continental shelf edge or on volcanic
periodic marine influxes. They cannot develop by the islands. The largest tidal waves, or tsunamis, are caused

evaporation of a single isolated body of water, as this by earthquakes on faults that displace the ocean fl oor,
could not explain the vast observed thicknesses of typically in the vicinity of ocean trenches and associated

evaporite deposits. The sequence of minerals precipi- with subduction of ocean floor (Fig. 9.5). A recent
tated is calcium carbonate and sulfate, sodium chloride, example was the South Asia tsunami arising from the

and finally magnesium or potassium minerals. Evapo- Indonesian earthquake of December 26, 2004. Volcanic
rites are important commercially in the chemical indus- eruptions on volcanic islands, particularly if they are
try, particularly for the potash salts. They are also explosive, as in the case of island arc volcanoes, can also
important in the generation of hydrocarbon traps produce very damaging tidal waves. The eruption of
because, being of low density, they mobilize after burial Santorini in the Aegean, in the 17th century BC, that
and rise through the sedimentary layers. Such haloki- severely damaged that part of the Minoan civilization
nesis provides fault traps along the sides of rising salt on Crete, and the eruption of Krakatoa in Indonesia in
masses and anticlinal traps in the layers above the 1883, are classic examples. Detailed mapping of the

masses which are folded during the ascent. This is an ocean floor has revealed major debris slides off conti-
important process in the North Sea and Gulf of México, nental slopes, such as the Storega slide off Norway, and
for example, which are underlain by salt deposits of around volcanic islands such as those of the Hawaiian
Permian age. chain. As most of these slides are pre-historic one
cannot be sure that they were all triggered by earth-
quakes or igneous activity but it seems very likely that
they were.
13.2.5 Geothermal power Several hundred thousand earthquakes are recorded
by the Global Seismograph Network every year. Most
Geothermal energy can be effectively utilized for power of them are so minor that they go undetected by people
generation when the vertical thermal gradient is several and less than 1%, approximately 1000, cause damage.
−1
times its mean value of about 25°C km , producing Earthquakes are most common on or in the vicinity
near-surface temperatures above 180°C. This condition of plate boundaries and in the other zones of deforma-
is achieved at constructive and destructive plate tion shown in Fig. 5.10. Fortunately damaging, large

margins, as exemplified by the geothermal power plants magnitude earthquakes are relatively rare, on average

in Iceland and the North Island of New Zealand, one or two a year, and they are confined to these
respectively. Anomalously high geothermal gradients major earthquake zones. In any one geographic area
are also present in intra-plate areas where they are therefore the interval between large, damaging earth-
frequently associated with granitic plutons. The normal quakes may be several hundred years. The frequency
geothermal gradient can be utilized for lower energy of occurrence is inversely proportional to the magni-
power generation, such as for space heating, wherever tude, thus small magnitude earthquakes are very
a thick pile of permeable sediments allows the circu- common particularly in the main earthquake zones.
lation of fluids to depths of several kilometers. An Earthquakes also occur in plate interiors, away from

example of this type is the Paris Basin, where space the main earthquake areas, as a result of the stresses
heating for over 20,000 dwellings is provided by deep and strains set up within plates by plate driving forces.
fl uid circulation. Such earthquakes often occur on pre-existing faults that