Unit 6: Managing natural hazards
6.1 What is a natural hazard?
It’s a physical event that has the potential to cause loss of life or injuries and damage
property and infrastructure. They can be short or long term.
The most common classification is:
● Geological hazards: volcanic eruptions, earthquakes
● Climatic hazards: tropical cyclones, droughts, floodings
They can also be classified based on the magnitude/ intensity, speed, duration or frequency.
People and property need to be put in risk by the physical event for it to be called a natural
hazard
For a natural hazard to be considered a natural disaster, it has to meet a few conditions:
- a report of 10 or more people killed
- a report of 100 or more people affected
- a state of emergency declared by the government of the affected country
- a request by the relevant national government for international assistance
The impact of a natural disaster on a community will depend on:
- length of time people are exposed to the natural hazard
- vulnerability of the people affected
- people’s ability to cope with the effects
6.2 What causes earthquakes and volcanic eruptions?
To understand the movement of the Earth’s surface, it's important to understand the theory
of plate tectonics .
There are two types of crust:
Oceanic crust (sima) Continental crust (sial)
Made out of basalt Made out of granite
Thinner (6 km approximately) Thicker (35 km approximately but can be
over 100 km)
Denser (heavier) 3 g cm−3 Lighter, 2.6 g cm−3
Younger Older
It can sink and is continually being renewed It cannot sink an is neither destroyed or
and destroyed renewed
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, ● The plates are made up of crust and upper mantle (lithosphere). The surface of the
earth is divided into seven major and eight minor plates.
● The largest plates are the Antarctic, Eurasian and North American. Plates can be
continental, oceanic or a mixture or both. Heat from the core creates convection
currents in the magma of the mantle and these cause plates to move. Where the
convection currents rise to the surface the plates move away from each other. Where
convection currents sink, plates move towards each other. The place where two
plates meet the plate boundary that can move towards or sideways.
● Most earthquakes and volcanic eruptions occur along or close to plate boundaries,
some volcanoes are found above “hot spots” or earthquakes can occur wherever the
crust moves.
Plate boundaries
Constructive (divergent):
Two oceanic plates pulling away from each other, magma rises to the surface because of
convection currents. The magma solidifies when in touch with the cold ocean water, turns
into lava, and forms a new basaltic ocean crust (sea-floor spreading/ridge push). Small
earthquakes are triggered, the new ocean crust builds up to form mid-ocean ridges and can
also form submarine volcanoes (shield or basic volcanoes) usually have non-explosive
eruptions because there is little pressure build up.
If two continental plates move away from each other, a rift valley is formed as the central
block of land drops between the faults.
Deconstructive (converging):
Two tectonic plates moving towards each other.
Continental and oceanic plates: They move towards each other because of convection
currents. The denser, oceanic plate is forced down under the lighter, continental plate. This
is called subduction, and takes place in the subduction zone. An ocean trench is formed
where the oceanic plate is subducted.
As the oceanic plates move downwards, pressure and friction trigger severe earthquakes in
the benioff zone. The heat produced by friction turns the descending plate into magma and
then disintegrates. The magma then starts to rise and it can erupt through a weakness in the
crust as an explosive composite (strato) or acidic volcano. Fold mountains are formed on the
continental plate as the sediments get pushed upwards and folded.
Two oceanic plates: The older and denser plate is subducted, the magma that rises upwards
and erupts at the surface forms a chain of volcanic islands (island arc)
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6.1 What is a natural hazard?
It’s a physical event that has the potential to cause loss of life or injuries and damage
property and infrastructure. They can be short or long term.
The most common classification is:
● Geological hazards: volcanic eruptions, earthquakes
● Climatic hazards: tropical cyclones, droughts, floodings
They can also be classified based on the magnitude/ intensity, speed, duration or frequency.
People and property need to be put in risk by the physical event for it to be called a natural
hazard
For a natural hazard to be considered a natural disaster, it has to meet a few conditions:
- a report of 10 or more people killed
- a report of 100 or more people affected
- a state of emergency declared by the government of the affected country
- a request by the relevant national government for international assistance
The impact of a natural disaster on a community will depend on:
- length of time people are exposed to the natural hazard
- vulnerability of the people affected
- people’s ability to cope with the effects
6.2 What causes earthquakes and volcanic eruptions?
To understand the movement of the Earth’s surface, it's important to understand the theory
of plate tectonics .
There are two types of crust:
Oceanic crust (sima) Continental crust (sial)
Made out of basalt Made out of granite
Thinner (6 km approximately) Thicker (35 km approximately but can be
over 100 km)
Denser (heavier) 3 g cm−3 Lighter, 2.6 g cm−3
Younger Older
It can sink and is continually being renewed It cannot sink an is neither destroyed or
and destroyed renewed
Página 1 | 8
, ● The plates are made up of crust and upper mantle (lithosphere). The surface of the
earth is divided into seven major and eight minor plates.
● The largest plates are the Antarctic, Eurasian and North American. Plates can be
continental, oceanic or a mixture or both. Heat from the core creates convection
currents in the magma of the mantle and these cause plates to move. Where the
convection currents rise to the surface the plates move away from each other. Where
convection currents sink, plates move towards each other. The place where two
plates meet the plate boundary that can move towards or sideways.
● Most earthquakes and volcanic eruptions occur along or close to plate boundaries,
some volcanoes are found above “hot spots” or earthquakes can occur wherever the
crust moves.
Plate boundaries
Constructive (divergent):
Two oceanic plates pulling away from each other, magma rises to the surface because of
convection currents. The magma solidifies when in touch with the cold ocean water, turns
into lava, and forms a new basaltic ocean crust (sea-floor spreading/ridge push). Small
earthquakes are triggered, the new ocean crust builds up to form mid-ocean ridges and can
also form submarine volcanoes (shield or basic volcanoes) usually have non-explosive
eruptions because there is little pressure build up.
If two continental plates move away from each other, a rift valley is formed as the central
block of land drops between the faults.
Deconstructive (converging):
Two tectonic plates moving towards each other.
Continental and oceanic plates: They move towards each other because of convection
currents. The denser, oceanic plate is forced down under the lighter, continental plate. This
is called subduction, and takes place in the subduction zone. An ocean trench is formed
where the oceanic plate is subducted.
As the oceanic plates move downwards, pressure and friction trigger severe earthquakes in
the benioff zone. The heat produced by friction turns the descending plate into magma and
then disintegrates. The magma then starts to rise and it can erupt through a weakness in the
crust as an explosive composite (strato) or acidic volcano. Fold mountains are formed on the
continental plate as the sediments get pushed upwards and folded.
Two oceanic plates: The older and denser plate is subducted, the magma that rises upwards
and erupts at the surface forms a chain of volcanic islands (island arc)
Página 2 | 8
