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دوشنبه ششم شهریور 1391 :: نویسنده : پانته آ

What are geohazards?

Hazard is a natural or man-made event or condition that has the potential to cause harm or loss to humans and/or damage to the natural and built environment. There are two definitions of geohazards:

1. A geohazard is a hazard of geological, hydrological nature which poses a threat to Man and his activities.

2. A geohazard is one that involves the interaction of man and any natural process on the planet.

Types of natural geohazards

Natural geologic processes are in constant operation on the planet. These processes are considered hazardous when they go to extremes and interfere with the normal activities of society. For instance, the surface of the Earth is constantly moving through plate tectonics, yet we do not notice this process until sections of the surface move suddenly and cause an earthquake. The geological hazards take many shapes and forms, from

violent earthquakes to the slow downhill creep of material on a hillside and the expansion of clay minerals in wet seasons. The principal natural geohazards are:

1. earthquakes (rapid hazards)

2. volcanic eruptions (rapid hazards)

3. landslides (rapid hazards)

4. tsunamis (rapid hazards)

5. subsidence (slow hazards)

6. coastal erosion (slow hazards)

7. coastal progradation (slow hazards)

8. soil erosion (slow hazards)

9. expansive soils (slow hazards)

The Earth is a naturally dynamic and hazardous world, with volcanic eruptions ejecting lava and ash, earthquakes pushing up mountains and shaking Earths surface, and tsunami that sweep across ocean basins at hundred of miles per hour, rising in huge waves on distant shores. Mountains may suddenly collapse, burying entire villages, and slopes are gradually creeping downhill moving everything built on them. Storms sweep coastlines and remove millions of tons of sand from one place and deposit it in another in single days. The slow but steady movement of tectonic plates on the surface of the Earth is the cause of many geologic hazards, either directly or indirectly.

Plate tectonics controls the distribution of earthquakes and the location of volcanoes and causes mountains to be uplifted. Other hazards are related to Earths surface processes, including floods of rivers, coastal erosion, and changing climate zones. Many of Earths surface processes are parts of natural cycles on the Earth, but they are considered hazardous to humans because we have not adequately understood the cycles before building on exposed coastlines and in areas prone to shifting climate zones.

A third group of geologic hazards is related to materials, such as clay minerals that dramatically expand when wetted, and sinkholes that develop in limestones. Still other hazards are extraterrestrial in origin, such as the occasional impact of meteorites and asteroids with Earth. The exponentially growing human population on Earth worsens the effect of most of these hazards. Most of the earthquakes on the planet are directly associated with plate boundaries. Single earthquakes have killed tens and even hundreds of thousands of people, such as the 1976 Tangshan earthquake in China that killed a quarter million people.

Most of the worlds volcanoes are also associated with plate boundaries.

Those volcanoes which situated above subduction zones at convergent boundaries are capable of producing tremendous explosive eruptions. Volcanic eruptions and associated phenomena have killed tens of thousands of people in this century, including the massive mudslides at Nevada del Ruiz in Colombia that killed 23,000 in 1985. Plate tectonics is also responsible for uplifting the worlds mountain belts, which are associated with their own sets of hazards, particularly landslides and other mass wasting phenomena.

Some geologic hazards are associated with steep slopes, and the effects of gravity moving material down these steep slopes to places where people live. Landslides and the slow downhill movement of earth material occasionally kill thousands of people in large disasters, such as when parts of a mountain collapsed in 1970 in the Peruvian Andes and buried a village several tens of miles away, killing 60,000 people. Coastal subsidence coupled with gradual sea-level rise is rapidly becoming one of the major global hazards that the human race is going to have to deal with in the next century, since most of the worlds population lives near the coast in the reach of the rising waters. Cities may become submerged and farmlands covered by shallow salty seas. An enormous amount of planning is needed, as soon as possible, to begin to deal with this growing threat.

Geologic hazards can be extremely costly, in terms of price and human casualties. With growing population and wealth, the cost of natural disasters has grown as well. The costs of natural geologic hazards are now similar to the costs of warfare demonstrates the importance of their causes and potential effects.





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