Natural Hazards
A Natural Hazard is an event caused by natural causes which imposes a danger upon human
lives. This event may cause damage to human surroundings. Examples of natural hazards are
earthquakes, tsunamis and tornadoes. Risks of natural hazards can be increased by
urbanisation, building on floodplains, climate change and poverty.
Earthquakes
Tectonic plates and the Earth’s structure
Thickness (KM) Temperature Composition
(ºC)
Lithosphere 70 Varies Basalt and granite
(Crust)
Asthenosphere 100-410 200 Semi molten rock called magma
(Upper mantle)
Mesosphere 2500 1000-3500 Magnesium and iron bearing
(Lower Mantle) silicates - it is solid due to
pressure
Outer Core 2900 3500-4000 Liquid iron and nickel
Inner Core 1200 4000-4700 Solid iron ore and nickel
The outermost layer of the earth is the solid crust, and there are two main types of crust:
oceanic and continental. The crust is broken into several major fragments called tectonic
plates. Where two plates meet is known as a plate margin.
Convection currents theory
The core’s temperature is around 6000ºC and causes magma to rise in the mantle. As it rises
further from the core, it cools and sinks back towards the core. The convection currents
flowing in the mantle carry the plates with them.
Slab pull theory
At destructive margins the denser plate sinks back into the mantle under the influence of
gravity, which pulls the rest of the plate along behind it.
Constructive plate margins
At constructive plate margins, two plates move away from each other and often form shield
volcanoes by letting magma rise and cool on the surface.
, Destructive plate margins
Destructive plate margins occur when tectonic plates move towards each other and collide.
The effect depends on the type of plate:
● If two continental plates collide, they are both buoyant and so cannot sink into the
mantle. As a result, compression forces the plates to collide and form mountains.
● If an oceanic and a continental plate collide, the denser oceanic plate is subducted and
sinks under the continental plate, into the mantle where it is recycled, causing
earthquakes, fold mountains and composite volcanoes.
Conservative plate margins
A conservative plate margin occurs when tectonic plates move parallel to each other and can
usually be found on the ocean floor. These plate margins usually cause earthquakes and an
example of these margins is the San Andreas fault line.
Volcanoes
There are three different types of volcanoes:
Active volcanoes are volcanoes which have erupted recently.
Dormant volcanoes are volcanoes which has erupted in the last 2000 years but not recently.
Extinct volcanoes are volcanoes which are unlikely to erupt ever again.
Features of a volcano
Feature Definition
Crater An opening at the top of a volcano
Magma chamber The store of hot molten rock beneath a
volcano
Layers of ash and lava They build up overtime to form the sides of
the volcano
Main vent The tunnel in which magma rises to the top
of the volcano.
Secondary vent and cone Allows magma to escape from the side of
the volcano
Lava flow Molten rock flowing down the side of the
volcano.
Volcanic cloud Gas, steam and ash escaping from the
volcano.
Volcanic bombs Large pieces of rock ejected from the
volcano.
A Natural Hazard is an event caused by natural causes which imposes a danger upon human
lives. This event may cause damage to human surroundings. Examples of natural hazards are
earthquakes, tsunamis and tornadoes. Risks of natural hazards can be increased by
urbanisation, building on floodplains, climate change and poverty.
Earthquakes
Tectonic plates and the Earth’s structure
Thickness (KM) Temperature Composition
(ºC)
Lithosphere 70 Varies Basalt and granite
(Crust)
Asthenosphere 100-410 200 Semi molten rock called magma
(Upper mantle)
Mesosphere 2500 1000-3500 Magnesium and iron bearing
(Lower Mantle) silicates - it is solid due to
pressure
Outer Core 2900 3500-4000 Liquid iron and nickel
Inner Core 1200 4000-4700 Solid iron ore and nickel
The outermost layer of the earth is the solid crust, and there are two main types of crust:
oceanic and continental. The crust is broken into several major fragments called tectonic
plates. Where two plates meet is known as a plate margin.
Convection currents theory
The core’s temperature is around 6000ºC and causes magma to rise in the mantle. As it rises
further from the core, it cools and sinks back towards the core. The convection currents
flowing in the mantle carry the plates with them.
Slab pull theory
At destructive margins the denser plate sinks back into the mantle under the influence of
gravity, which pulls the rest of the plate along behind it.
Constructive plate margins
At constructive plate margins, two plates move away from each other and often form shield
volcanoes by letting magma rise and cool on the surface.
, Destructive plate margins
Destructive plate margins occur when tectonic plates move towards each other and collide.
The effect depends on the type of plate:
● If two continental plates collide, they are both buoyant and so cannot sink into the
mantle. As a result, compression forces the plates to collide and form mountains.
● If an oceanic and a continental plate collide, the denser oceanic plate is subducted and
sinks under the continental plate, into the mantle where it is recycled, causing
earthquakes, fold mountains and composite volcanoes.
Conservative plate margins
A conservative plate margin occurs when tectonic plates move parallel to each other and can
usually be found on the ocean floor. These plate margins usually cause earthquakes and an
example of these margins is the San Andreas fault line.
Volcanoes
There are three different types of volcanoes:
Active volcanoes are volcanoes which have erupted recently.
Dormant volcanoes are volcanoes which has erupted in the last 2000 years but not recently.
Extinct volcanoes are volcanoes which are unlikely to erupt ever again.
Features of a volcano
Feature Definition
Crater An opening at the top of a volcano
Magma chamber The store of hot molten rock beneath a
volcano
Layers of ash and lava They build up overtime to form the sides of
the volcano
Main vent The tunnel in which magma rises to the top
of the volcano.
Secondary vent and cone Allows magma to escape from the side of
the volcano
Lava flow Molten rock flowing down the side of the
volcano.
Volcanic cloud Gas, steam and ash escaping from the
volcano.
Volcanic bombs Large pieces of rock ejected from the
volcano.
