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Concept of a Natural Hazards

A natural phenomena caused by rapid or slow onset events.

Geophysical - Earthquakes, landslides, tsunamis and volcanic activity.

  • Internal - Processes of tectonic origin.
  • External - Processes of geomorphological origin involving mass movements.

Hydrological - Avalanches and floods.

Meteorological - Extreme temps, drought, wildfires, cyclones and storm/wave surges.

Hydrometeorlogical - Floods and hurricanes.

Hazards can be highly variable in nature, cause, magnitude, frequency, location and scale of impact.

6 main types of hazards - Flood, drought, volcano, earthquake, landslide/avalanche and cyclone/hurricane.

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Risk Equation

The risk equation is Risk (R) = Hazard (H) x Vunerability to the hazard (V)

                                             Capacity to cope/recover (C)

Risk - The probability of harmful consequences, or expected losses (deaths, injuries, property, etc.).

Hazard - A potentially damaging physical event, phenomenon or human activity that could cause loss of life.

Vulnerability - Conditions determined by physical, social, economic and environmental factors, which increases the susceptibility of a community to the impact of hazards.

Capacity - A combination of all the resources available within an organisation, community or society to reduce the level of risk or effects of the disaster.

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Response to Hazards

Human response - Reducing the risk, the nature of response can be influenced by:

  • Perception of hazard
  • Nature of hazard
  • Resources available
  • Technology available

Response can be by prevention, prediction and reducing the loss.


Park response model - Shows changing quality of life through stages of disaster.

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Plate Tectonics, Simple Definitions

Crust - Outer or surface layer of tectonic plate (20km in depth)

Mantle - Layer beneath crust, zone of rock that is composed mainly of nickel and iron (3000km in depth)

Core - Layer beneath mantle

  • Outer core - iron and nickel, liquid as it is so hot.
  • Inner core - centre of Earth, made of iron and nickel also, solid as rocks are in so much pressure (temp - 5000°C.

Pangea - What all of the plates together as one large continent was called.

Sea-floor Spreading - At mid-ocean ridges new oceanic crust is formed, fractures occur in the lithosphere. The lithosphere is the crust and upper mantle.

Ridge Push - Oceanic crust pushes over the lithosphere crust as it is at a higher point. The continental crust subdues underneath the oceanic crust.

Slab Pull - A plate sinks into the mantle beneath it and is subdued.

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More Plate Tectonics

(             The Oceanic crust has a density of about 2.6 and a thickness of 6-10km. The Continental crust has a density of about 3  and a thickness of 30-70km.

Lithosphere - Rigid plate of crust and upper mantle.

Asthenosphere - Dense, plastic-like in upper sections, allows plate to move on top of it.

Mantle - Dense, solid layer rich in iron and magnesium.

Beneath the mantle there is the semi-molten outer core and solid inner core.

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Plate Boundaries

Constructive - Where plates move apart from each other and often create new land or deep-sea ocean trenches. An example of oceanic constructive plate movement would be the mid-Atlantic ridge between the Eurasian plate and the North American plate. An example of continental constructive plate movement woulod be on the East coast of Africa.

Collision - This is where two continental plates collide and earthquakes are caused usually. They are not as common as the other three types but can create fold mountains, such as the Himalayas.

Destructive - This is where an oceanic plate sinks underneath a continental plate, as the continental is lighter. The friction between the two plates causes melting in the oceanic plate, causing earthquakes. Volcanoes can also be formed here as the friction in the subduction zone causes the melting of the crust to be turned to molten magma. An example would be the Nazca plate being forced under the South American plate.                           (

Conservative - This is where plates slide past each other in opposite directions, or in the same direction but at different speeds. Friction eventually is overcome and shockwaves are created from a sudden movement from the plates slipping past.

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Magma Plumes and Intraplate Tectonic Activity

Magma Plumes - Where magma breaks through the mantle and lithosphere through a column of hot rock snd stays in the same position beneath the crust. When plates move, the magma also moves to the same position, so can often lie in the middle of plates before the head breaks through the shallow part of the crust. When they have broken through previously, they have formed islands such as Hawaii that have been made from volcanic crust. The centre of where a volcano has formed on the volcanic island is above what is thought to be a hotspot. There are many examples of hotspots occuring around the world in the middle of plates such as Hawaii and Yellowstone. There are also hotspots that lie on plate boundaries such as the one where Iceland is situated. 


As the volcanoes get older, you can see that they are progressively moving further away from the hotspot.

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Volcanic Hazards and Impacts

As the population in the 20th century grew eponentially, more people died in this century more than any other from volcanic hazards.

Primary - Ash fallout (large cloud of ash ejected from volcano), pyroclastic flow (mixture of rock and gases), lava flow (hot liquid magma produced from the core), volcanic gases and tephra (rock fragments).

Secondary - Lahar (Nevado del Ruiz, Colombia), flooding (melting of glaciers/ice caps), tsunamis (giant waves after violent eruptions, created by landslides, Krakatoa), landslides (slide of mass on a slope) and climate change (gases and processes released that change the global temp).

Impacts -

  • Short term - Pyroclastic flow
  • Long term - Global warming
  • Social - Loss of life
  • Economic - Businesses destroyed
  • Environmental - Wildlife habitats destroyed/deforestation
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Volcanic Impacts Continued

Impacts whether social, environmental, economic or short/long term can be categorised into primary and secondary.

Primary - Houses/buildings destroyed, roads/railways destroyed and communication lines are cut off.

Secondary - Disease spread by people, explosions in gas pipes, shortages of drinking water and food and economic problems to many people and businesses.

Positive impacts - Creation of new land, fertilised soil, geothermal energy oppurtunities, cheaper housing, tourism and land rich in minerals.

Negative impacts - Loss of life/injuries, wildlife habitats destroyed, climate change, pollution, settlements destroyed, human/natural landscape damaged and deforestation.

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Nature of Vulcanicity and its Causes

Volcanic hazards can depend on the position of plate boundaries. 95% of volcanoes lie on plate margins, whereas 5% are intraplate. Mount Fuji is an example of a plate boundary volcano and Kīlauea in Hawaii is an example of an intraplate volcano. The nature of volcanic hazards depends on the type of eruption, this is determined by the type of lava at the volcano.

Basic lava - This type has a high temperature and is runny. It has low silica content, which means it flows easily and doesn't trap gas. This creates Hawaiin type eruptions which are characterised by shield volcanoes, dome shaped, the lava type is basaltic.

Acidic lava - This type has a lower temperature (still hot), and a higher silica content. This means that it is more viscous and builds up a higher gas content. The lava flows slower and this creates Plinian eruptions which are characterised by composite (stratovolcanoes), the lava type is andesitic, and as the lava gets more acidic it becomes rhyolitic.

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Lava Types at Different Plate Boundaries

Constructive - Basaltic magma, runny, low silica lava, gentle eruptions, gases escape easily, mainly just lava that is erupted, frequency of eruptions are regular and can be continuous, the shape of the volcanos are usually a shield. Examples include Shield, Fissure and Dome typed volcanos and Icelandic and Hawaiian type eruptions (VEI 1 + <).

Destructive - Acidic magma, very viscous, slow moving, high silica content, explosive, lava shatters, lava bombs, ash and dust, they have an irregular frequency and can go into long dormant periods, the shape of the volcanos are usually Composite. Examples include Composite, Caldera and Ash-cinder volcanos and Vulcanian, Plinian, Strombolian and Pelean type eruptions (VEI 1 + >).

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Volcanic Case Study - Iceland 2010

In April 2010, the Icelandic volcano Eyjafjallajokull erupted several times with a few of the eruptions being large enough to disturb the public, making it a volcanic hazard.

  • The volcano lies in the North of the Atlantic Ocean on the mid-Atlantic ridge, on a constructive plate margin between the North American and Eurasian plate boundaries.
  • The plates move away from each other underneath where Iceland is situated.
  • The volcano lies East of the capital of Iceland, Reykjavik and has acidic lava. It attracted tourism as previously, the volcano had been dormant for 187 years.
  • It is a Stratovolcano and had a Strombolian/Vulcanian eruption with a VEI of 4.
  • The volcano erupted in ice, over an ice cap, this changed the ash cloud, causing it to spread all over Europe. This could've lead to a secondary impact, the eruption of Katla, a much larger and dangerous dormant volcano. Underneath Iceland there is a magma plume.
  • The crater of the volcano measures 1.8 miles to 2.5 miles across and the ice depth is 500m, the volcano generated an explosive eruption with an 11km high ash plume.
  • 3000 earthquakes preceded the main eruption (harmonic tremors) on the 14th of April.
  • The ash cloud reached Germany, South England and Western Russia and farmers in these locations were warned to not let cattle drink tainted ash water.
  • The eruption led to a European flight ban that costed the airline industry millions, as it affected the whole world, and lasted from the 14th to the 21st of April.
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Can be either very deadly or very harmless, depending on how prepared people are. Caused by 12 major plates that float on magma (friction occurs between plates). The more shallower, the more dangerous they are. The most dangerous ones are caused by friction in the subduction zone. Seismic related events cause 10,000 deaths a year, when 500,000 occur and 100 of them cause damage.

Caused by plate movements, mainly triggered by destructive plate boundaries as they include subduction zones and Benioff seismic zones. Destructive cause the most friction as they are the most shallow. The driver of the movement are convection currents (pressure + heat).

Earthquakes are frequent seismic events that have shaking caused by sudden release of energy, 90% occur on plate boundaries, 10% occur intraplate. Continuation of adjustmunt in position causes aftershocks. The USGS estimates several million a year, many are too small to be felt/occur in remote locations.

Primary - Shockwaves, soil liquefaction and ground shaking/rupture.

Secondary - Landslides/avalanches, tsunamis, fires, volcanoes and environmental/social/economic impacts.

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Seismic Case Study - Nepal 2015

  • Lies on the Indian and Eurasian plates, the Himalayas are located on the boundary and were formed by the Eurasian plate pushing on top of the Indian plate.
  • The two plates create such friction that Nepal lies on top of a destructive plate boundary.
  • Nepal is a poor LIC and is very vulnerable to hazards.
  • Occured on the 25th of April 2015 at 12:58 AM Nepal time.
  • The epicentre was at Barpak, which is 80km North-West of Kathmandu, the capital.
  • 7.8 on the magnitude.
  • The focus was only 15km deep, which is shallow, so the waves didn't have to travel far.
  • The plates collide at a rate 45mm per year, (the plates had over 100 years of strain).
  • 352 aftershocks including a second quake on the 12th of May 2015, with a magnitude of 7.3.


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Nepal Impacts

Primary -

  • A total of 8841 people died, 16800 were injured and 1 million were made homeless. S
  • 26 hospitals were destroyed, many were treated outside in the streets. S, Ec
  • Historic buildings and temples in Kathmandu including the iconic Dharahara Tower and a UNESCO World heritage site were destroyed. Ec
  • Many modern buildings collapsed as there were no compulsory building standards. S, Ec
  • The district of Sindhupalchok, worst affected as 95% of the homes were destroyed. S, Ec
  • 50% schools destroyed (29000 killed if earthquake occured in school hours). S, Ec

Secondary -

  • It triggered an avalanche which swept through Everest Base Camp, killing 19. S, En
  • 2 aftershocks including a second earthquake on the 12th of May 2015 measuring 7.3 magnitude. En
  • Food shortages caused by rice being destroyed, as the earthquake occured just before the Monsoon season when rice is planted. S, Ec
  • Tourism decreased, an important industry to Nepal, 8.9% of the GDP relies on this. Ec
  • 5 day walks to some villages and many people were made to live in shelters. S
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