AQA A2 Geography
1 (a) Earthquakes are mainly concentrated at constructive and destructive plate boundaries. At constructive boundaries, earthquakes are shallow and result from the stretching and tension of the crust associated with sea-floor spreading.
Tension causes rifting and the displacement of crust and lithosphere along fault lines. It is this sudden movement that gives rise to earthquakes. At destructive plate boundaries, earthquakes occur at greater depth, often with foci 30 km or more below the surface. These earthquakes are linked to subduction and the descent of oceanic crust into the upper mantle where it is destroyed. Subduction, however, is not a smooth process: frictional resistance to the subducting plate generates huge pressures causing rocks to fracture and sudden movements
(i.e. earthquakes) along fault lines.
(b) Convection currents in the upper mantle or asthenosphere cause the slow divergence of tectonic plates at constructive plate boundaries. This divergence
(initially induced by convection in the upper mantle) reduces pressure so that hot rocks melt to form magma which rises to the surface. Volcanic activity then adds new igneous rock to the oceanic plate, forcing it laterally, away from the plate boundary at rates of just 2 or 3 cm a year. This conveyor-like process is called sea-floor spreading. Continents, embedded in the oceanic plate, like logs in an ice flow, move with sea-floor spreading. This movement is called continental drift.
(c) The 2005 Kashmir earthquake had a magnitude 7.6 quake and was centred in northern Pakistan. Its impact was devastating, destroying infrastructure over a wide area and causing 87,000 deaths. Earthquakes are common in this part of
Pakistan, situated in the foothills of the Himalayas where the Indo-Australian and
Eurasian plates collide. The areas most badly affected were the Jhelum and
Neelum valleys. The scale of the disaster points to poor hazard management and a lack of hazard planning. Pakist an’s preparations for a disaster on this scale were inadequate, partly because the epicentre was in a remote district where access is poor. Many roads were blocked by landslides and helicopters, needed to transport international emergency aid such as tents, blankets, clean water and food, were not available in sufficient numbers. But the real reason for Pakistan’s inadequate response is poverty: Pakistan is one of the poorest countries in Asia and lacks the resources to build a modern and earthquake-proof infrastructure. Most deaths were the result of building collapse; poverty forcing cheap construction with little attention to building codes. Many of the buildings which collapsed (e.g. those made of reinforced concrete or rubble held together with clay and weak mortar) were vulnerable to even moderate earthquakes.
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The 1994 Northridge earthquake in the suburbs of Los Angeles was comparable in magnitude to the Kashmir quake. Although some infrastructure (especially freeways and bridges) were badly damaged, the overall death toll was 57 — a tiny fraction of the lives lost in Kashmir. These differences are stark and reflect the much higher level of planning and preparedness for earthquake hazards in Los
Angeles and in California in general. California is one of the most seismically active regions in the world. Yet the response in the form of disaster planning and stringent building codes (and their enforcement) makes it one of the safest. This level of management and preparedness is possible because California is the richest state in one of the richest countries (the USA) in the world. The outcome is that California is far less vulnerable to earthquake hazards than countries which face similar risks in the developing world (e.g. Pakistan, China, Iran, Haiti).
2 (a) Strato-volcanoes, the most common type of volcano (also known as composite volcanoes), have a classic, steep-sided cone and a summit crater. Well-known examples include Mount Fuji, Mount Vesuvius and Mount St Helens. The volcanic cone consists of layers of ash, tephra and lava, built up by successive eruptions.
Highly viscous andesite magma is responsible for the steep profile of the cone.
Following an eruption, sticky andestic lava flows relatively short distances from the crater before solidifying. Strato-volcanoes often erupt explosively (e.g. Mount
St Helens in 1980). This is due to the viscous magma inside the volcano which prevents the escape of gases, such as steam, and causes the build-up of extreme pressure.
(b) The global distribution of volcanoes closely follows tectonic plate boundaries. This is most obvious around the Pacific Ocean in the socalled ‘Pacific Ring of Fire’.
There, hundreds of active volcanoes encircle the Pacific Basin, stretching from
Japan, through Indonesia and southeast Asia to New Zealand, and then on the eastern side of the Pacific, in South, Central and North America (including Alaska).
Volcanoes in the ‘Ring of Fire’ are the result of subduction. Slabs of oceanic crust and lithosphere dive into the upper mantle where they melt. The melt, which is lighter than the surrounding rocks, slowly rises towards the surface to form volcanoes.
Volcanoes are also found at constructive plate boundaries. Rifting, caused by convection and stretching of the crust, brings magma to the surface on the ocean floor. Continual eruptions over millennia add to these submarine volcanoes which eventually break the ocean surface to form volcanic islands such as Iceland,
St Helena and the Canaries.
There is one other location where volcanoes occur: hot spots. Hot spots are places like Hawaii and Yellowstone, where rising mantle plumes punch holes through the crust. The result is intense volcanic activity (the Kilauea volcano on Hawaii has been in continuous eruption since 1983).
(c) In April 2
010, the Eyjafjallajökull volcano in southern Iceland erupted. The main hazards from the eruption were volcanic ash and floods caused by the melting of glaciers around the volcano’s summit (jökulhlaups). In the vicinity of the volcano around 800 people wer e evacuated. Although there were no fatalities, jökulhlaups destroyed bridges, damaged roads, farm buildings and farmland. Ash clouds
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closed Iceland’s air space, causing a temporary drop in the number of visitors and badly affecting tourism. However, the economic impact of the eruption extended well beyond Iceland. European air traffic was shut down for 6 days in mid-April, and was closed intermittently until mid-May. Airport closures across Europe disrupted 10 million passengers at a cost of between £1.3–2.2 billion. Air freight was also badly affected. The environmental impact was small and mainly limited to ash deposits on farmland and settlements around the volcano. Centuries of eruptions and jökulhlaups in southern Iceland have created a stark, lunar-like landscape, with large areas devoid of any vegetation and soil. Thus the environment was little altered by the eruption.
In the mid1990s, a series of violent eruptions from the Soufrière Hills volcano devastated large parts of the island of Montserrat in the Lesser Antilles. In 1997, pyroclastic flows destroyed the capital, Plymouth, and led to the abandonment of the southern half of the island. Altogether 19 people were killed. Between 1990 and 2000, Montserrat’s population fell from 11,000 to 6,500. People emigrated because of: (1) continuing eruptions; (2) the destruction of most the island’s best farmland; and (3) the loss of tourism, the former mainstay of the island’s economy.
3 Countries are exposed to seismic hazards regardless of their levels of development.
However, MEDCs are far less vulnerable than LEDCs. Evidence of this is provided by the death tolls from recent earthquakes — Haiti over 300,000, Kashmir (Pakistan) and
Sichuan (China) nearly 90,000, compared to 57 at Northridge (USA) and around 5,000 in Kobe (Japan). The 2004 tsunami in Indonesia killed at least 250,000, ten times more than the Japan tsunami in 2011. Even so, the economic costs of earthquake and tsunami disasters are much higher in MEDCs. This is explained by the greater investment in infrastructure and the huge insurance liabilities rather than poor management.
MEDCs have the resources to manage seismic hazards. In LEDCs, governments faced with more urgent economic problems, are unable or unwilling to devote sufficient resources to seismic hazard management. Actions taken by MEDCs to reduce their vulnerability to seismic hazards include: implementing strict building codes to mitigate the impact of earthquakes on housing, offices, bridges and other infrastructure; constructing seawalls to protect lowland areas from tsunamis; early warning of potential tsunamis to allow evacuation; disaster planning and education to organise society in the aftermath of a seismic disaster.
4 The principal hazards caused by volcanic eruptions are: lava flows, pyroclastic flows, ashfalls and lahars. The impact of these hazards depends on their power, speed and scale, and on society’s preparedness. In most eruptions, lava flows pose only a limited threat to life. However, they are difficult to divert or halt and are extremely damaging to property and infrastructure (e.g. Nyiragongo and Heimaey eruptions). Pyroclastic flows are the most violent and deadly volcanic hazard, destroying everything in their path (e.g. Montserrat eruptions). Lahars are similarly powerful, fast-moving and extremely destructive (e.g. Nevado del Ruiz eruption). Ashfalls can blanket
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landscapes with thick layers of ash, damaging buildings, destroying farmland and during periods of heavy rainfall, creating lahars (e.g. Mount Pinatubo eruption).
Human factors have an important influence on volcanic hazards. Volcanic eruptions only become hazards when they affect people, so the density of population and type of economic activity in an area exposed to an eruption are important in determining its impact. Apart from some limited success in diverting lava flows, direct intervention is not usually an option in volcanic hazard mitigation. Indirect action is more successful.
Monitoring volcanoes (e.g. seismic activity, ground inflation, gaseous emissions) prior to an eruption provides early warning of impending eruptions, giving time for evacuation. Similarly, lahar warning systems allow evacuation of vulnerable areas.
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