AQA A2 Geography

Updating Unit 3

Plate tectonics and associated hazards

David Redfern

Professor David Petley

November 2012

Programme

9.45am Registration and coffee

10.00am: Requirements of the specification

  • Plate Tectonics theory and the evidence used to support it
  • Volcanic events: impact and management
  • Seismic events: impact and management
  • Case studies – depth and detail

11.00am: Morning break

11.15am

Keynote speaker: Professor David Petley (University of Durham).

The Christchurch Earthquake sequence: managing the aftermath of a series of unexpected seismic events.

12.30pm Lunch

1.30 pm Assessment strategies

  • Structured questions – their nature and demands
  • Exemplar answers of the above, and the marking thereof
  • Synoptic essays – what type of essays can be set
  • Exemplar answers of the above, and the marking thereof

3.15pm: Associated skills activities

  • Exemplars of materials that can be used to reinforce skills
  • Attitudes and values exercises

3.45pm: Day ends

Plate Tectonics and Associated Hazards

Things to learn

  • The structure of the earth. You should know the meaning of the following terms: core, crust (continental and oceanic), mantle, lithosphere, asthenosphere. Thermal convection currents operating within the asthenosphere and sea-floor spreading
  • Features of constructive (divergent) margins. You should be able to describe and know the formation of oceanic ridges, submarine volcanic activity and rift valleys
  • Features of destructive (convergent) margins. You should be able to know what is happening at oceanic/continental convergence, oceanic/oceanic convergence and continental/continental convergence. Features you should be able to describe and know the formation of include: ocean trenches, fold mountains, island arcs, explosive volcanic activity
  • Conservative margins
  • Hot spots
  • The distribution of volcanic activity
  • Intrusive volcanic landforms to include: batholiths, laccoliths, dykes, sills and metamorphic aureoles
  • Extrusive volcanic landforms to include the main forms of lava: basaltic, andesitic and rhyolitic. Features to include: lava plateaux, basic/shield volcanoes, acid/dome volcanoes/ ash and cinder cones, composite cones and calderas.
  • Minor volcanic forms to include; solfatera, geysers, hot springs/boiling mud
  • The impact of volcanic activity. You should be able to differentiate between primary effects (tephra, pyroclastic flows, lava flows, volcanic gases) and secondary effects (lahars, flooding, tsunamis, volcanic landslides, climate change)
  • The focus and epicentre of earthquakes
  • Distribution of earthquakes
  • Magnitude and frequency of earthquakes
  • The impact of earthquakes. You should be able to differentiate between the primary effect of ground shaking and the secondary effects of soil liquefaction, landslides/avalanches, effects on people and the built environment
  • The nature and effects of tsunamis

Things to understand

The Theory of Plate Tectonics – you should understand how this theory developed and the evidence that supports it its development and the evidence that supports it

Palaeomagnetism and sea-floor spreading – you should understand how magnetic striping occurs, the importance of this palaeomagnetic evidence and how this indicates the process of sea-floor spreading

The process of subduction- you should understand how this process occurs and

and its effects on the edges of both continental and oceanic plates

Vulcanicity - it is important to understand both the causes and nature of volcanic activity

Managing volcanic activity – you should understand howpeople and authorities are able to manage volcanic activity. This could come about through your case studies (see below) as response to an event is a detailed part of your investigation

Causes of earthquakes – you should understand what causes an earthquake and how seismic waves travelling through the earth give information on the internal structure of the planet

Measurement of earthquakes – you should understand how earthquakes are measured both in terms of the instrument used and the scales on which they are recorded (Richter, Mercalli)

Causes of tsunamis – you should understand how such waves are generated

Managing earthquake activity – as with volcanic activity you should understand how people and authorities are able to manage earthquake activity. This, again can come through case studies

Case studies that need to be covered

  • You are required to make two case studies of recent volcanic events. Recent, in this case, means ideally within the last 30 years. The two events should be taken from contrasting areas of the world. The best solution to these instructions is to choose one from a developing country and one from a developed area. In this way you can best bring out the differences in impact and the way in which the people/authorities coped with volcanic events. In each case the following should be examined:

The nature of the volcanic hazard, i.e. the way in which the eruption took place

The impact of the event

How people and the authorities (local and external) responded to the hazard

  • You are also required to make two case studies of recent seismic events, i.e. earthquakes. Recent, in this case, means ideally within the last thirty years. The instructions are exactly the same as those for the volcanic events above. In each case, though, the following should be examined:

The nature of the seismic hazard, i.e. the strength of the earthquakes and any particular features of it such as depth, ground acceleration, etc.

The impact of the event

The preparation within the area, and how people and authorities (local and external) responded to the event

Good examples of events on which you could base your case studies include: Volcanic activity – Montserrat (1995-96), Mt Etna (1991-93), Nyiragongo (2002) and Mount Merapi (2010)

Earthquakes – Kobe (1995), Gujurat (2001), Sumatra (2004), Kashmir (2005) and Sichuan (China) (2008). These events happen all the time and so it is very important to keep up with the information. Case studies in text books and other publications will be the best the authors can find for you up to the moment that everything goes to print. As geography students it is very important to search out new events and use the information that you have collected, in the examinations. Examiners will always credit at a high level, in this context, students who make attempts to keep their work as up to date as possible. In recent times, there were two very large earthquakes which were very well documented, those in Haiti, Chile, Christchurch, Lorca and the Japanese tsunami. Try to find as much information as you can about these events and their impact on the peoples of both countries. Do the same for any volcanic or earthquake events that happen during the time before your exam.

Questions

Here are a number of short to medium questions which can help in making sure that you have understood the subject content of this section.

1. Explain the terms “lithosphere” and “asthenosphere”.

2. What are the differences between oceanic and continental plates?

3. When Alfred Wegener published his theory of continental drift, how did he describe the distribution of land and ocean? Why did his theories fail to gain any real acceptance before the 1950s?

4. What was the original geological/biological evidence that Alfred Wegener used in his attempt to show that the continents had drifted apart?

5. What is sea-floor spreading?

6. How does the study of palaeomagnetism show the evidence for sea-floor spreading?

7 Transform faults are associated with sea-floor spreading. How are transform faults created?

8. Study Figure A which shows what happens in continental areas when plates move apart. Complete the diagram by adding labels to show what is taking place.

9. Explain the process of subduction.

10. Study Figure B which shows what happens when an oceanic plate meets a continental plate at a point of convergence. Describe and comment on what is taking place.

11. What happens when two continental plates meet?

12. What happens on conservative margins?

13. How do hot spots form?

14. Describe the distribution of volcanic activity on a global scale.

15. Describe the appearance and formation of shield volcanoes.

16. Draw a cross-section of a composite volcano and describe what happens during an eruption.

17. Describe the minor features that volcanic activity can produce.

18. What are the primary effects of a volcanic event?

19. What is a lahar?

20. How can tsunamis be generated by volcanic eruptions?

21. What causes an earthquake?

22. What information do seismic waves give us that helps in an understanding of the Earth’s interior?

23. Explain the global distribution of earthquakes.

24. What is the Richter scale?

25. Explain the process of soil liquefaction.

26. Describe the effects that earthquakes may have upon people and the built environment.

27. What determines the impact of a tsunami?

28. How have people sought to manage the volcanic hazard through prediction?

29. How can people and authorities attempt to lessen the impact of earthquake events?

30. Does your chance of surviving a volcanic eruption or an earthquake depend on your level of wealth?

Variations in the type of volcanic activity in relation to types of plate margin and types of lava

Plate margin / Destructive / Hot spot / Constructive
Magma source / A chaotic mix of old oceanic plate, ocean sediments, continental fragments, often weathered by water / Deep in the asthenosphere (mantle ) / Deep in the asthenosphere (mantle )
Rock name / Andesite / Rhyolite / Basalt / Gabbro / Basalt / Gabbro
Magma chemistry / Medium to high Acid, greater than 63% SiO2 (examples granite and rhyolite) / Quite basic(alkali), sometimes relatively rich in sodium and potassium ( alkaline) low silica ( 50% ) / Very basic (alkali) Basalt low silica 45 - 52% and typically high iron - magnesium
Magma’s physical character / Viscous, (solidifies quickly) flows over short distances , solidifies even on steep slopes, allows gases to build up pressure – can violently explode. / Quite non-viscous (fairly runny), flows over low angled slopes or can erupt as an ash. / Very non-viscous (runny), flows long distances over very low angled slopes or can create a black ash (tephra) when exploding with water vapour (steam)

The Sichuan earthquake and its aftershocks

The Sichuan quake of May 2008 was one of the most destructive on record. Although Sichuan province is densely populated, tectonically unstable and earthquake-prone, the scale of the destruction suggests other reasons for the disaster. Contrary to the Chinese government’s view, there is strong evidence that infringements of China’s seismic building code made the region especially vulnerable to a major earthquake. If this were the main cause of a disaster that killed nearly 90,000 people and cost was $US150 billion, it suggests that the Sichuan quake was as much the result of human factors as tectonic forces.

Tectonic background

On May 12th 2008 the province of Sichuan in central China was struck by a 7.9Mw earthquake. This powerful quake had its focus just 19km below the surface, and was one of the most destructive in recent history. It devastated a large area around the epicentre causing up to 90,000 deaths, injuring 375,000 people and making five million homeless.

Sichuan province is tectonically unstable and has a long history of earthquake activity: in 1933 a 7.5Mw quake killed more than 9,300 people. At the macro-scale, tectonic instability results from the collision between the Indo-Australian plate and the Eurasian plate. In Sichuan this caused local convergence between the Tibet Plateau and the Sichuan Basin. On May 12th stresses in the crust triggered a sudden movement along a thrust fault on the northwest margin of the Sichuan Basin, releasing powerful earth tremors across the region.

Earthquake impact

Severe ground shaking was the direct cause of death, injury and the catastrophic destruction of housing, schools, hospitals, dams, power lines, roads and other infrastructure. An estimated 5.4 million buildings collapsed, and further 21 million were damaged. School buildings suffered massive damage. In total, 7,000 classrooms were destroyed, killing 10,000 students. Chengdu, the capital of Sichuan province was badly shaken, though compared to Dujiangyan City, close to the epicentre, it escaped lightly.The scene in Dujiangyan was one of total devastation: most buildings were reduced to rubble and bodies lay in the streets. Fewer than 60 out of 900 children survived when a three storey school building collapsed. In the mountains the quake triggered mass movement and slope failure. The town of Beichuan was partly buried by landslides and at least 700 were killed by a landslide at Qingchuan. Landslides also blocked river valleys, creating 34 temporary barrier lakes. Rising water levels on the largest barrier lake (Tanigiashan Lake on the Jian River) threatened to breach the temporary earth dam forcing the authorities to evacuate 250,000 people downstream to higher ground. Eventually an artificial channel, completed on June 7th, drained the lake.

The aftershocks – who or what was to blame?

Two things can explain the impact of earthquakes and other natural hazards on society: exposure and vulnerability. In the context of earthquake hazards, exposure comprises the magnitude of the quake, and the number of people living around the epicentre. Overall, exposure in Sichuan is high. We know that the region is tectonically active and that major earthquakes have occurred in the past. Add to this the 15 million people that live close to the epicentre, (including 4 million in the regional capital, Chengdhu) and it’s hardly surprising that the 2008 quake was a major disaster.

But the region’s high exposure is not the whole story. Other parts of the world such as southern California and Japan have equally high exposure, yet it’s unlikely that even a 7.9Mw quake could cause so much death and destruction. For instance, the 1994 quake at Northridge (6.7Mw) in the outer suburbs of Los Angeles caused just 57 deaths. Even the Kobe quake in Japan (6.9Mw) in 1995 had a death toll only a fraction of that in Sichuan.

The fact is that despite the magnitude of Sichuan quake, the ensuing disaster was exceptional in its severity. Any inquest into why the quake was so deadly leads to one conclusion: the people of Sichuan were extremely vulnerable to earthquake hazards. Vulnerability concerns preparedness and the human response to hazards. The risks increase significantly in societies that are inadequately prepared for natural hazards.

So why was Sichuan so vulnerable? The evidence seems to point to the poor design and shoddy construction of so many buildings. True, since 1976 China has had a stringent earthquake building code. Under China’s seismic intensity scale (1-12) the Sichuan region was classed as 7 – a high enough risk to make the code mandatory for all new buildings. Essentially the code requires builders to add steel to concrete and brick structures. Steel strengthens buildings and makes them ductile, allowing them sway with ground shaking. Without it, buildings are brittle and easily snap and collapse. Of course, before the quake, millions of buildings in Sichuan pre-dated the 1976 code. These older buildings were mainly low-rise masonry constructions without steel reinforcement. Concentrated mainly in poorer rural areas, these buildings were the most vulnerable. In the quake hundreds of thousands simply collapsed, killing their occupants.

Even so, thousands of modern buildings, constructed under the 1976 regulations also failed, suggesting widespread violation of the building code. Building failure was due to a combination of severe ground shaking, poor design (especially lack of steel reinforcement) and the use of inadequate construction materials, including inferior concrete. To save money, builders often failed to use the appropriate amount of steel. Evidence of the laxness of building regulations was most evident in the collapse of so many schools. This caused outrage among bereaved parents because while schools collapsed, adjacent buildings often remained intact.

Chinese officials blamed the magnitude of the earthquake for the disaster. But local people claimed the disaster was largely man-made: the result of substandard building work, corruption and lax enforcement of building regulations. Their anger led to numerous public protests. According to the New York Times parents campaigning for justice have been harassed by police, detained and threatened with imprisonment. Others have been silenced by government promises of financial compensation, including one-off payments and pensions.

Nor does discussion of human responsibility for the disaster stop with the quake’s impact. New research suggests that human activity could even have contributed to the quake’s cause. It is well documented that large dams can trigger earthquakes. Chinese and overseas experts have pointed to the Zipingpu Dam, completed in 2006 and only 5km from the epicentre of the quake. They argue that the sheer weight of water in the dam – 315 million tonnes – could have weakened the thrust fault, increasing the stresses and causing it to rupture.