Hazardous Earth


What is a hazard?

  • A hazard is an event (natural or human) that has the potential to cause loss to life, injury, property damage, socio-economic disruption or environmental degradation.
  • Natural Hazards lie at the interface between human and physical geography.
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The Structure of The Earth

The Structure of the Earth:

  • Core = the centre of the earth, an iron-nickel mass that gives the Earth its magnetic field. The inner core is 1250km thick. The outer core is liquid and is 2200km thick.
  • Mantle = accounts for more than 80% of the volume of the earth. It consists of semi-solid rock containing silicon and oxygen. 2900km deep. The upper part of the mantle consists of two layers: a layer extending from 100km-300km - the asthenosphere. This has plastic properties that allow it to flow under pressure. The layer above this is the lithosphere, a rigid layer between the crust and the asthenosphere.
  • Crust = the outer shell consisting of oceanic crust (solid) composed of dense basalt rock, average 5km deep, and the continental crust (solid), mainly granite which can be less dense than basalt, averaging 30km deep.

Together the lithosphere and crust make up oceanic and continental plates. Within the asthenosphere convection currents, caused by the intense heat generated in the mantle, mean that this semi-molten layer 'flows', carrying with it the solid lithosphere and crust.

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Continental Drift and Plate Tectonics

In 1912, Alfred Wegener put forward his theory for continental drift. He proposed that 250 million years ago, in the Carboniferous period, a large single tectonic plate- Pangaea - existed. Initially, it broke apart into two land masses to the north and south; this spread continued to give the present-day land masses.

Geological Evidence for this theory: the fit of continents such as South America and Africa on either side of the Atlantic. Evidence from 290 million years ago of the effects of contemporaneous glaciation in southern Africa, Australia, South America, India and Antarctica: suggesting that these land masses were joined at this time, located close to the South Pole. Mountain chains and some rock sequences on either side of oceans show great similarity, e.g. northeast Canada and northern Scotland.

Biological Evidence: similar fossil brachipods (marine shellfish) found in Australian and Indian limestones. Similar fossil reptiles found in South America and South Africa. Fossils from rocks younger than the Carboniferous period, in places such as Antarctica and India, showing fewer similarities, suggesting that they followed different evolutionary paths.

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Evidence of Sea-Floor Spreading

In the 1960s, magnetic field data showed that fresh molten rock from the asthenosphere reached the sea bed and older rock was pushed away from the ridge.  The theory of sea-floor spreading linked to the theory of continental drift. It became clear that plates were being moved by sea-floor spreading.

Paleomagnetism- as lava erupts, it cools and the magnetic orientation of iron particles within the lava is 'locked' into the rock. The direction of the Earth's magnetic field changes every 400,000-500,000 years (paleomagnetism).

The Age of Sea-Floor Rocks- in the 1960s, an ocean drilling programme showed that the thickest and oldest sediments were near the continents and the younger deposits were further out in the oceans, giving support for sea-floor spreading.

Evidence from Ancient Glaciations- in the present day, glacial deposits formed during the Permo- Carboniferous glaciation are found in Antarctica, South America, India and Australia. If the continents in the Southern Hemisphere are re-assembled near the South Pole, then the Permo-Coniferous ice sheet is a realistic size. If the continents had not moved then there woould be an ice sheet extended from the South Pole to the Equator- unlikely.

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More Evidence for Sea-Floor Spreading

  • Fossil Records= there are many examples of similar fossils found on separate continents, suggesting the continents were once joined. If continental shift had not occured, then either the species evolved independently on different continents, which contradicts Darwin's theory, or they swam to other continents to establish new populations, which is also thought to be unlikely. When the continents of the Southern Hemisphere are re-assembled into a single land mass, the distribution of the fossil types in question forms a more realistic continuous pattern of distribution.
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Earth's Crustal Features and Processes

The global patterns of plates and plate boundaries:

  • The lithosphere is divided into seven large and three smaller tectonic plates.
  • The plates are moved by convection currents, operating as convection cells within the asthenosphere.
  • They can move towards each other (destructive plate margins), move away from each other (constructive plate margins), or slip alongside each other (conservative plate margins).
  • Most plate movement is slow and continuous but sudden movements produce earthquakes.
  • It is at the plate boundaries/margins that most landforms (e.g. fold mountains and volcanoes) are formed.
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Divergent (constructive) Plate Boundaries

New crust forms at constructive plate margins where rising plumes of magma from the upper mantle stretch the crust and lithosphere. The resulting intense volcanic activity builds submarine mountain ranges; mid-ocean ridges; parallel faults produce rift valleys.

  • Ocean Ridges= formed when plates move apart in oceanic areas. The space between the plate is filled with basaltic lava from below to form a ridge. Volcanoes also exist along this ridge and may rise above sea level. e.g. Surtsey, south of Iceland.
  • Rift Valleys= formed when plates move apart from continental areas. Sometimes the brittle crust fractures as sections of it move and areas of crust drop down between parallel faults to form a valley, e.g. East African Rift Valley.
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Convergent (destructive) Plate Boundaries

Oceanic-Continental Plate Margins= different densities of plates. Denser oceanic plate subducts under the continental plate. A deep ocean trench is formed at the plate boundary. Sediments and rocks fold and are uplifted along the leading edge of the continental plate. Continental crust buckles and mountain chains form. e.g. the Andes. The angle of subduction on the oceanic plate is between 30- 70 degrees: faulting occurs in the Benioff zone, releasing energy in the form of earthquakes.

Oceanic-Oceanic Plate Margins= the slightly denser plate will subduct under the other, creating a trench. Descending plate melts, magma rises and chains of volcanoes- island arcs- form, e.g. the Antilles.

Continental-Continental Plate Margins= little, if any subduction because of similar densities. Impact and pressure tends to form fold mountains, e.g. the Himalayas.

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Conservative Plate Boundaries

At a conservative plate margin, two plates slide past each other. The movement may be violent and an additional build-up of pressure which eventually gives way results in powerful earthquakes. There is no volcanic activity, e.g. San Andreas fault between the Pacific and North American Plates.

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Different Types of Volcanoes

The range of landforms produced by volcanic eruptions is related to: where and how the eruption takes place; types of lava (viscosity affects how lava flows); materials (ash and lava); and gases produced.

Explosive Eruptions and Effusive Eruptions: explosive= violent because of build-up of pressure, with viscous magma (e.g. andesite) which prevents escape of gases; and effusive= a gentle, free-flowing basic eruption of lava (e.g. basalt).

Explosive Eruptions:

  • Location= convergent plate boundaries.
  • Types of Lava= rhyolite (more acid) and andesite (less acid).
  • Style of Eruption= violent bursting of gas bubbles when magma reaches surface; highly explosive; vent and top of cone often shattered.
  • Materials Erupted= gas, dust, ash, lava bombs, tephra.
  • Frequency of Eruption= tend to have long periods with no activity.
  • Shape of Volcano= steep-sided strato-volcanoes; caldera.
  • Products= composite cone volcanoes are made up of layers of ash and acidic lava. Internal lava flows sills and dykes. The acidic magma does not flow easily and solidified magma plugs the vents. Calderas (deep craters) form when the cone is destroyed by an explosive eruption.
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Different Types of Volcanoes 2

Effusive Eruptions:

  • Location= divergent plate boundaries.
  • Types of Lava= basalt.
  • Lava Characteristics= basic (low % of silica), low viscosity, higher temperature at eruption.
  • Style of Eruption= gas bubbles expand freely; limited explosive force.
  • Materials Erupted= gas, lava flows.
  • Frequency of Eruption= tend to be more frequent; an eruption can continue for months.
  • Shape of Volcano= gently sloping sides, shield volcanoes; lava plateaux when eruption from multiple fissures.
  • Products= basic magma flows freely and covers large areas: flood basalts.

Eruptions not at plate boundaries- these eruptions are associated with hot spots. Hot spots are places where a plume of magma rises from the mantle and erupts at the surface. They are usually associated with intense volcanic activity and eruptions of balsatic lava. The Hawaiian chain of volcanic islands is an example.

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Size and Shape of Volcanoes

1. Icelandic Lava Eruptions- are characterised by persistent fissure eruption. Large quantities of balsatic lava build up vast horizontal plains. On a large scale, they have formed the Deccan Plateau and the Columbia Plateau.

2. Hawaiian Eruptions- runny, balsatic lava travels down the sides of the volcano in lava flows. Gases escape easily. Occasional pyroclastic activity occurs but this is less important than the lava eruption.

3. Strombolian Eruptions- are characterised by frequent gas explosions which blast fragments of runny lava into the air to form cones. They are very explosive eruptions with large quantities of pyroclastic rock thrown out. Eruptions are commonly marked with a white cloud of steam emitted from the crater.

4. Vulcanian Eruptions- violant gas explosions blast out plugs of sticky or cooled lava. Fragments build up into cones of ash and pumice. These eruptions occur when there is very viscous lava which solidifies rapidly after an explosion. Often the eruption clears a blocked vent and spews large quantities of volcanic ash into the atmosphere.

5. Vesuvian Eruptions- are characterised by vey powerful blasts of gas pushing ash clouds high into the sky. They are more violent than Vulcanian eruptions. Lava flows also occur. Ash falls to cover surrounding areas.

6. Plinian Eruptions- gas rushes up through sticky lava and blast ash and fragments into the sky in a huge explosion. The violent eruptions create immense clouds of gas and volcanic debris several metres thickGas clouds and lava can also rush down the slopes. Part of the volcano may be blasted away during the eruption.

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The Volcanic Explosive Index

Super-Volcanoes= are volcanoes that erupt more than 1000km cubed of material in a single event. An example is the Yellowstone super-volcano in Wyoming, which has a caldera measuring 75km in diameter.

The Volcanic Explosive Index= Magnitude (the amount of material erupted) and intensity (the speed at which material is erupted) can be used to compare different eruptions. The Volcanic Explosivity Index (VEI) combines these two factors into a single figure on a scale of 0 (least explosive) to 8 (most explosive). Each increase in number represents a ten-fold increase in explosivity.

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Hazards Generated by Volcanic Eruptions

  • Lava Flows - flows or streams of molten rock that pour from an erupting vent. The speed at which lava moves depends on the types if lava (balsatic lava is free flowing and runs considerable distances, while acidic lava is thick and does not flow easily), its viscosity, the steepness of the ground and whether the lava flows as a broad sheet, through a confined channel, or down a lava tube. Lava of any type is destructive and will burn, bury or bulldoze infrastructure, property, natural vegetation and agricultural land.
  • Pyroclastic Flows - nuees and ardente - flows of gas and tephra which are extremely hot (over 500 degrees celsius) flow down the side of the volcano at speeds of over 100km/h.
  • Gas Emissions - carbon dioxide, carbon monoxide, sulphur dioxide and chlorine escape through fumaroles (openings in or near volcano, through which hot sulphurous gases escape). Sulphur dioxide + water = acid rain (weathering and pollution). 
  • Tephra - volcanic bombs and ash ejected into the atmosphere. Size ranges from ash to larger bombs of >6cm in diameter.
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Hazards Generated by Volcanic Eruptions 2

  • Lahars and flooding associated with melting ice - lahars are a type of mud flow. Snow and ice on a volcano summit melt during the eruption. Rock, ash and soil mix together to destory and bury anything in the path of the rapid flow of material as they follow valleys. The melting of the snow and ice associated with volcanic eruptions can also lead to flooding as large volumes of water are released. In Iceland, these are know as jokulhlaups.
  • Tsunamis - violent eruption of an island volcano can displace oceanic water and lead to a tsunami- large wave travelling at speeds up to 600km/h.
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Earthquake - is a release of stress in the Earth's crust. A series of shock waves originate from the earthquake focus (the location where the stress is released) and on the Earth's surface this point is known as the epicentre. Fore-shocks can be released before the main earthquake event. The main locations for earthquake activity are mid-ocean ridges, ocean trenches and island arcs, collision zones and conservative plate margins.

Earthquake Characteristics:

  • Shallow-Focus Earthquakes= surface down to approximately 70km, often occur in brittle rocks, generally release low levels of energy but high-energy shallow quakes can cause severe impacts.
  • Deep-Focus Earthquakes= 70-700km, increasing depth leads to high pressure and temperature, less frequent but very powerful, full understanding of deep- focus earthquakes is evolving; water and change in minerals may be contributing factors.
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Measuring Earthquake Magnitude

  • Richter Scale - developed in 1935; uses the amplitude of seismic waves to measure magnitude. Scale is logarithmic from 1-9 (although there is no upper limit); each whole number increase is a ten-fold increase.
  • Movement Magnitude Scale - scale of 1.0-9.0, measuring energy release as related to geology, the area of the fault surface and the movement on the fault. Accurate for large earthquakes as it uses the physical movement caused by the earthquake. It is not used for small earthquakes.
  • Modified Mercalli Scale - measures earthquake intensity and impact. It relates to impacts felt and seen by those affected- is qualitative not quantitative.
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Effects of Earthquakes

Mountain ranges have been created due to earthquakes (e.g. the Himalaya- Karakoram Range in Asia), as well as major fault systems, rift valleys (e.g. East Africa) and escarpments.

Rift Valley =  a valley formed by downfaulting between parallel faults.

Escarpment = is a tilt block forming an extensive upland area, with a short, steep, scarp slope on the other side.

Hazards Generated by Earthquakes:

  • Ground shaking and ground displacement= this is the vertical and horizontal moving of the ground. Severity depends on the earthquake magnitude, distance from epicentre and geology.
  • Liquefaction= whem violently shaken, soils with a high water content lose their mechanical strength and become fluid.
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Hazards Generated by Earthquakes

  • Landslides and Avalanches= slope failure as a result of ground shaking.
  • Tsunamis= a tsunami is a giant sea wave generated by shallow-focus underwater earthquakes. Tsunamis have long wavelength (often over 100km) and low was height (around 1m) in the open ocean. They travel quickly (speed over 700km/h) but on reachingshallow water bordering they increase in height. A wave trough forms in front of the tsunami where sea level is reduced: this is called drawdown. Behind this comes the tsunami itself, sometimes as high as 25m or more.
  • Flooding= earthquakes can indirectly cause flooding in ways such as: tsunamis, destabilising/ destroying dams, destroying and/or lowering protective levees.
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Living in Tectonically Active Places

  • Weathered lava produces fertile soils in countries such as Japan and Indonesia.
  • Volcanoes provide opportunities for economic activity such as tourism, e.g. in Iceland and Italy.
  • Tectonically active areas produce geothermal power, e.g. Iceland and Indonesia.
  • Volcanic eruptions supply minerals such as sulphur, used in a variety of industries.
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Strategies for Managing Tectonic Hazards

  • Modify The Event - not possible for the vast majority of volcanic eruptions. However the following have been tried with some success: lava-diversion channels, spraying lava to cool so it solidifies and slowing lava flows by dropping concrete blocks. Nothing can be done to modify an earthquake event.
  • Modify People's Loss - emergency aid, e.g. bottled water, medical supplies, tents, food packs. Disaster- response teams and equipment, e.g. helicopters and heavy lifting machinery. Search and rescue strategies. Insurance for buildings and businesses. Resources for rebuilding public servies, e.g. schools and hospitals, and help for individuals to rebuild homes and businesses.
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Strategies 2

  • Modify Peoples Vulnerability:

1. Education - recognise signs of possible eruption; what to do when an eruption occurs, e.g. evacuation routes; drills to pracrice what to do when a tectonic event strikes, e.g. in an earthquake, get to open space away from buildings or shelter under a table in a doorway.

2. Community Preparedness - increasing use of technology to monitor particularly active locations, e.g. individual volcanoes.

3. Hazard-Resistant Building Design - e.g. cross-bracing of buildings to support them during an earthquake; steep sloping roofs to prevent ash building up.

4. Hazard Mapping - e.g. predicted lahar routes; ground likely to liquefy in an earthquake.

5. Land-Use Zoning - to avoid building in locations identified by hazard mapping.

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Aseismic Design Features

  • Stepped building profile.
  • Varied building height.
  • Angled windows.
  • Soft storey.
  • Reinforcements, e.g. bracing, steel frams and deep foundations.
  • Consideration of building site to avoid difficulties such as fault lines, soft soil and slopes.
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Tectonic Hazards Changing Overtime

Natural disasters have increased over time, volcanic eruptions are less pronounced. However, on average there are now 30 earthquakes a year and have a greater impact in terms of number of deaths. 

Generally, the greater the magnitude, the less frequently it occurs. Recurrence intervals indicate that high magnitude events recur over long periods of time.

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Disaster Risk Equation

Gives an idea of the hazard vulnerability of a location, it is expressed as:

Risk (R)Frequency of magnitude of hazard (H) x Level of vulnerability (V)


                    Capacity of population to cope and adapt (C)

The relationship between the magnitude and impact of a tectonic event can be influenced by how often an event occurs and the time interval between events. The most vulnerable are those who experience a wide- ranging impact from a small- scale event.

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Possible Future Strategies

  • It is not currently possible to make predictions of when and where earthquakes will happen. A 'precursor' (a characteristic pattern of seismic activity or a physical, chemical or biological change) is needed in order to make these predictions, The creation of this has been unsuccessful it is believed will remain this way for the forseeable future.
  • Future research will focus on improving the forecasting of earthquakes.
  • The UN identifies the alleviation of poverty as a priority in reducing the effects of earthquakes in future.
  • Buildings need to be designed to withstand tectonic events.
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Characteristic Human Responses

  • Responses occur at different levels: individual, communty, national government and international.
  • Resilience is the sustained efforts of communities to respond to and withstand the effects of hazards.
  • Hazards are also managed by the integration of prediction, prevention and protection plans.
  • Prediction - not always possible but warnings can be issued from monitoring.
  • Prevention - cannot be prevented but secondary impacts can be controlled.
  • Protection - minimise impact of event.
  • Risk sharing involves education and awareness of measures available to reduce impact.
  • HAZARD MANAGEMENT CYCLE. (all of the above)
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The Park Model

The Park Model shows that hazards have varying impacts overtime: before the disaster; when the event happens; and post-event relief (rescue), rehabilitation and reconstruction. The disaster-response curve, shows the effect of a disaster on people can be generalised.

(look at image of park model)

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