- A natural hazard is an event that causes loss of human
lives and properties. They are caused by extreme weather conditions and earth
movements.
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Natural
hazard
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Places
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High risk of drought
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Chad, Western USA, Australia, Ethiopia
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High risk of flood
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Northeastern China, Venice, Pakistan, The Netherlands, Thailand
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Frequent earthquakes
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Japan, Taiwan, Central China, Indonesia
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Active volcanoes
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Iceland, The Philippines, Indonesia, Hawaiian Islands, Peru, Chile,
Italy
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Structure
of the Earth
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Crust
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- Made up of rocks and soil which is not uniform in thickness, some
parts are thicker, forming the continents; others are thinner, forming the
ocean-floors.
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Mantle
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- Made up of very hot molten rocks called magma. - The upper layer is
upper mantle.
- The lower layer is lower mantle.
- Semi fluid
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Core
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- Made up of heavy metals under high temperature and high pressure.
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Unstable
zones on the earth
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- Circum-pacific belt
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- Mid-Atlantic belt
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- Mediterranean belt
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How does the crust move?
- The crust is not a single piece, it is broken into pieces
called plates. The narrow zone between two moving plates is called plate
boundary. The narrow zone between two moving plates is called plate boundary.
- Plates move slowly because of convection currents of magma
in the mantle. The plates sometimes move away, move towards and slide past each
other along the plate boundaries.
- Earthquakes, volcanic eruptions and fold mountains are
caused because of plate movements.
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Plate movements and plate boundaries
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Constructive plate boundary
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Destructive plate boundary
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Conservative plate boundary
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The plates move away from each other.
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The plates move towards each other.
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The plates move past each other.
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What are earthquakes? Why do earthquakes occur?
Earthquakes are the violent shaking of the earth’s crust.
Earthquakes occur where plates move towards (destructive plate boundary) or
move past (conservative plate boundary) each other. Such plate movement creates
pressure and causes the rocks to break. At the crack or the focus, shock waves
spread out. The earth’s surface above will shake and earthquake is resulted.
The magnitude of an earthquake is measured on the Richter
scale. The higher the scale, the greater the damage, the lower the frequency of
occurrence.
Why are there volcanic eruptions? How do volcanoes form?
When plates move, magma rises through cracks and stored in
the magma chamber of the earth’s crust. When there is too much pressure in the
magma chamber, the volcano erupt, lava, ash and gases come out. Magma rises and
flows out as lava layer by layer. Lava cools to form volcanoes.
What are the hazardous effects of earthquakes and volcanic eruptions?
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Earthquakes
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Volcanic eruptions
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1. People and livestock are killed and
injured.
2. Infrastructure (e.g. buildings,
bridges and roads) is damaged
3. Water, electricity and gas supplies
are cut
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4. Landslides may occur
5. broken gas pipes and electricity wired cause fires
6. Dams collapse and cause flooding
7. earthquake on ocean floors cause tsunami
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4. lava and mud flows bury fields and settlements
5. lava flows cause fire that burn forests and thus destroy the
ecosystem
6. dust and ash from volcanoes lower the air quality/visibility
7. plane engines are jammed by dust and ash, this causes planes to
crash
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Feasible measures to reduce risk of natural hazards.
1. Prediction
- Watch out for nature’s weird/abnormal behaviors
- Check the temperature and pressure of volcanoes
- Study the history of earthquakes/volcanic eruptions
2. Planning
- Set up evacuation route with adequate road signs
- Avoid dangerous land use in hazard prone areas
- Earthquake-proof buildings/infrastructure
- Set up monitoring and warning systems
- Prepare for relief work, shelters, and rescue team
- Earthquake drills
What are the factors affecting the degree of damage caused by
earthquakes?
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1. Nature of the hazard
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- Magnitude
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The higher the magnitude, the greater the damage.
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- Time of occurrence
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If earthquake occurs in midnight, higher death toll would be resulted.
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2. Background of the affected
area
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- Population density
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Higher
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Lower
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The higher the population density, the higher the casualty
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- Type of country
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Less developed country
e.g. Haiti, Iran
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More developed country
e.g. New Zealand, Japan
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- Level of economic
development
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Lower
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Higher
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- Level of technology
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Lower
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Higher
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Higher level of economic development implies the country has more
resources and more advanced technology to cope with earthquakes, hence less
loss would be resulted.
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- Level of
awareness/preparedness of people
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Lower
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Higher
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Higher level of awareness/preparedness refers people know how to
prepare for and respond to earthquakes.
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Why do people still live in hazard prone areas?
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Benefits
of a place for living
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Other
reasons
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1. Volcanoes are tourist attraction.
2. Gentle slopes at the bottom of volcanoes are suitable for farming
and building houses.
3. Volcanic ash make the soil fertile.
4. Useful minerals (e.g. diamonds, gold, iron) are found in volcanic
areas.
5. Steam and heat in the volcanic areas provide geothermal power.
6. Many big cities which are affected by earthquakes have a lot of
money and technology. People think that the benefits outweigh the
disadvantages.
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1. Optimistic views of people on hazards.
2. People believe that their governments have enough money and high
technology to cope with the hazards.
3. People prefer to stay because of family and social ties.
4. People have no choice because there are strict migration policies
in other countries.
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