Tectonic hazards

Cambridge IGCSE Geography 0460 · Paper 1 Physical Geography Syllabus: 2027, 2028 and 2029 Official syllabus points: 4.1.1 to 4.4.4

Topic 4 is one of the five physical topics examined on Paper 1. It is the most heavily illustrated topic on the paper: past questions are built on cross sections through volcanoes, world maps of plates, photographs of craters, isoseismal maps and scatter graphs, so you are expected to read a diagram as fluently as you write.

The extended response at the end of each structured question is where the topic is won or lost. Every levelled mark scheme in this topic prints the same rule, "Max 5 if no named or inappropriate example", and several add a second rule that is far less well known: for volcanoes, a country name is not a good enough example. Section 17 exists to fix that.

1. The structure of the Earth

Syllabus point 4.1.1 names five layers. You can be asked to label them on a cross section or to give a characteristic of any one, so learn a property of each, not just the order.

Why the density contrast matters. Oceanic crust is denser than continental crust. That single fact decides which plate goes down when two plates meet, and it is the difference between a Level 1 and a Level 2 answer on a subduction question.

Exam trap: a plate is not the same thing as the crust. Mark schemes accept "a large section of the Earth's crust", but the accurate answer, and the one the 2027 syllabus is pointing at with the word lithosphere, is that a plate is a slab of crust and rigid upper mantle moving as one piece.

2. The tectonic plates and how they move

Syllabus point 4.1.2 requires the names and location of the main plates. Past papers ask you to pick plate names from a list against letters on a world map, so learn them on a map rather than as a list.

The seven largest plates

• Pacific, under most of the Pacific Ocean, the only large plate that is almost entirely oceanic.
• North American, North America and the western half of the North Atlantic.
• South American, South America and the western half of the South Atlantic.
• Eurasian, Europe and most of Asia.
• African, Africa and the eastern Atlantic.
• Indo-Australian, India, Australia and the Indian Ocean.
• Antarctic, Antarctica and the surrounding ocean.

Smaller plates that appear constantly in exam resources: Nazca (off western South America), Cocos (off Central America), Caribbean, Philippine, Arabian, Juan de Fuca and Scotia.

How the plates move

Heat from radioactive decay deep inside the Earth heats the base of the mantle. The heated rock becomes less dense and rises, spreads sideways below the lithosphere, cools, becomes denser and sinks again, forming a convection current. The drag of that current, together with the weight of a cold subducting slab pulling the rest of the plate behind it, moves the plate.

1. heat from the core
2. mantle rock is heated and becomes less dense
3. it rises
4. it spreads sideways beneath the lithosphere
5. it drags the plate above it
6. cooled rock sinks back down
7. the cycle repeats and the plate keeps moving.

Plates move at roughly the speed a fingernail grows, about 2 cm to 10 cm a year. The Mid Atlantic Ridge is opening at about 2.5 cm a year, while the Nazca Plate is pushing into South America at roughly 7 cm a year.

Marks note: every earthquake and volcano mark scheme in the bank lists convection currents as a creditable point, and several award it as the opening mark. Write it in every process answer. One 2024 mark scheme, however, refuses it as an answer to a question specifically about a conservative boundary, because there the examiner wants the sideways movement, the friction and the release, not the driving force.

3. Types of plate boundary

Syllabus point 4.1.3 names four, using both the newer and the older term for each. Learn both words in each pair, because resources and questions use them interchangeably.

The two "no volcano" boundaries are the most examined fact in this section. No crust is destroyed at a conservative boundary and no gap opens, so there is no route for magma. At a collision boundary neither continental plate is dense enough to sink far enough to melt.

Exam trap: "convergent" alone is ambiguous in the 2027 syllabus, because both the destructive and the collision boundary are convergent. If the question gives you a boundary described only as convergent, look at the resource: ocean floor on one side means destructive and subduction, two land masses means collision and fold mountains.

4. The distribution of earthquakes and volcanoes

"Describe the distribution" is asked almost every year and is marked one mark per idea, so give several different kinds of statement, not the same idea reworded.

Four kinds of statement that mark schemes credit:

• The overall pattern: uneven, clustered, linear, in lines, in a curve or crescent.
• The relationship: on or very close to plate boundaries, and close to each other.
• Named regions: around the edge of the Pacific Ocean, the Pacific Ring of Fire; down the centre of the Atlantic Ocean; south east and eastern Asia; western North, Central and South America; southern Europe and the Mediterranean; the East African Rift.
• The anomalies: a few scattered points away from any boundary.

Marks note: a 2022 mark scheme caps named plates and named regions at 2 of the 3 marks, so you must include at least one general pattern word such as "linear" or "clustered" to score full marks. A 2023 mark scheme rewards spotting anomalies separately.

Not every earthquake is on a boundary. Australia's largest earthquake, at Newcastle in New South Wales in 1989, measured 5.6 and killed 13 people, and a 2021 exam question asked what was unusual about it. The answer printed in the mark scheme is simply "it is not on a plate boundary". Intraplate earthquakes happen along ancient faults in the middle of plates. Similarly, the Hawaiian volcanoes sit in the middle of the Pacific Plate over a hot spot, a fixed plume of rising magma. Hot spots are not named anywhere in the 2027 syllabus, but they appear in exam resources and one mark scheme accepts them, so know the word.

5. Processes at each type of plate boundary

Syllabus point 4.2.1 wants the process, not the label. Mark schemes are explicit about this: naming the margin type twice still only earns Level 1, and the word "subduction" on its own is only Level 1. It becomes Level 2 when you say which plate goes under which, and why.

Divergent boundaries: why volcanoes form

1. convection currents pull the plates apart
2. a gap or fissure opens in the crust
3. pressure on the mantle below is released
4. the rock melts and magma rises to fill the gap
5. magma reaches the surface as lava
6. the lava cools and solidifies as new crust
7. repeated eruptions build a shield volcano or a mid ocean ridge.

Iceland is the standard exam resource for this, and the mark scheme for a 2021 question asks for exactly four things: divergent margin, the Mid Atlantic Ridge, melting in the mantle, and magma rising.

Destructive boundaries: why volcanoes and earthquakes both form

1. the denser oceanic plate moves towards the less dense continental plate
2. the oceanic plate is forced down beneath it, which is subduction
3. friction and the heat of the mantle melt the descending plate
4. the molten rock is less dense than the surrounding rock so it rises
5. pressure builds up in the magma chamber
6. magma is forced out through cracks and lines of weakness
7. a violent eruption builds a strato-volcano.

The earthquake at the same boundary comes from the same movement.

1. the subducting plate does not slide smoothly
2. friction makes the two plates stick
3. the plates keep moving so pressure and stress build up
4. eventually the friction is overcome and the rock fractures
5. the plates jerk and the stored energy is released as seismic waves
6. the ground shakes.

Collision boundaries

1. two continental plates move towards each other
2. neither is dense enough to subduct
3. the sediment and crust between them is compressed, crumpled and folded upwards
4. fold mountains are built
5. the plates stick, pressure builds and is released as a shallow, powerful earthquake, but no magma is created so there are no volcanoes.

Conservative boundaries

1. plates slide past each other in opposite directions, or in the same direction at different speeds
2. friction makes them lock together
3. the plates keep being pushed so pressure and tension build up
4. the rock fractures along the fault and the plates jolt past each other
5. energy is released as seismic waves
6. a shallow, violent earthquake occurs, but no crust is created or destroyed so there are no volcanoes.

Marks note, and it is worth several marks a year. Mark schemes for "explain the causes of an earthquake" state that answers may refer to a conservative or destructive margin, and one adds "do not credit constructive boundary". Mark schemes for "explain the causes of an eruption" state that answers may refer to a constructive or destructive margin. In other words, destructive works for both, and the other two are one each. Earthquakes do genuinely occur at divergent boundaries, but they are weak, and the examiner will not credit that route.

6. The main characteristics of earthquakes

Syllabus point 4.2.2 names three, and past papers label all three on the same cross section diagram.

Marks note: a 2024 mark scheme awards the definition of epicentre only if you give both halves, the surface and the link to the focus below. "The centre of the earthquake" scores nothing.

The three types of seismic wave

The syllabus asks only for "seismic waves", but knowing the three types explains why some earthquakes do more damage than others.

• P waves (primary) are the fastest, arrive first, push and pull the rock in the direction of travel, and pass through both solid and liquid. Least damaging.
• S waves (secondary) are slower, arrive second, shake the rock at right angles to the direction of travel, and cannot pass through liquid.
• Surface waves are the slowest, arrive last, travel along the ground surface and roll or shear it. They cause most of the damage to buildings.

Focal depth

The depth of the focus is the single most useful control on damage after magnitude, and mark schemes credit it repeatedly.

1. the focus is shallow
2. the seismic waves travel only a short distance to the surface
3. they lose little energy on the way
4. the shaking at the epicentre is far more violent
5. more buildings collapse and more people die.

A 2021 question gave two earthquakes of similar magnitude in the same year and expected candidates to notice that the more deadly one had the shallower focus.

7. Types of volcano

Syllabus point 4.2.3 names three. Cinder cone is new for 2027, so learn it deliberately.

Why the lava decides everything. Runny basaltic lava lets gas escape easily, so the eruption is gentle and the lava spreads far before cooling, giving a wide, low cone. Viscous andesitic lava traps the gas until the pressure is enormous, so the eruption is explosive, and the lava cools close to the vent, giving a tall, steep cone.

1. viscous, high silica lava
2. gas cannot escape
3. pressure builds inside the volcano
4. an explosive eruption throws out ash and tephra
5. the lava that does escape cools quickly near the vent
6. alternating layers of ash and lava build a steep, conical strato-volcano.

Marks note: for "describe the features of a strato-volcano" a mark scheme credits conical, layers of lava and ash, steep, concave, crater, secondary or parasitic cones, and viscous lava. A 2022 mark scheme adds that describing the eruption is not credited when the question says "do not write about its eruptions", so read the instruction.

8. Active, dormant and extinct

Syllabus point 4.2.4 is new as an explicit requirement for 2027, though "active" appeared in earlier exam resources.

These labels are not permanent. Chaitén in Chile was widely treated as dormant, having not erupted for thousands of years, and then erupted violently in 2008. That is a genuinely useful line in an evaluation answer, because it is the reason exclusion zones and monitoring are kept in place around volcanoes that appear quiet.

Exam trap: a 2022 question asked candidates to tick two boxes for what type of volcano was shown, and the answer was "strato-volcano" and "active". The two classifications are independent. A volcano is one of the three shapes and one of the three activity states.

9. The main features of volcanoes

Syllabus point 4.2.5 names five. Questions label them with letters on a cross section, so be able to place each one.

Also worth being able to label: the layers of lava and ash, a fissure or side vent, the ash cloud, and a caldera, which is the very large basin left when the summit of a volcano collapses into an emptied magma chamber.

Marks note: mark schemes accept "parasitic cone" and "secondary cone" for the same feature. They also accept "vent", "pipe" and "conduit". For the exam use the syllabus words, secondary cone and vent.

10. Volcanic hazards

Syllabus point 4.2.6 is the single largest addition for 2027. It names seven hazards and then asks for the significance of speed, size, frequency and spread, which is a way of asking why one hazard kills people and another only destroys property.

Two figures worth carrying into an answer. The lahars generated by the eruption of Nevado del Ruiz in Colombia in November 1985 melted the summit ice cap and buried the town of Armero, killing around 23 000 people, from an eruption that was otherwise small. The ash cloud from Eyjafjallajökull in Iceland in April 2010 killed nobody at all, yet closed most of European airspace for six days and cancelled roughly 100 000 flights, stranding millions of passengers.

1. a small eruption melts a summit ice cap
2. meltwater mixes with loose ash on the slopes
3. a lahar forms and is funnelled into a river valley
4. it travels tens of km at speed
5. it buries a town that is far from the crater and felt safe.

How to answer the "speed, size, frequency and spread" idea. The hazard that kills is not the largest one, it is the one that arrives fastest and reaches furthest. Lava is slow, so it destroys property but takes few lives. Pyroclastic flows are fast and give no time to evacuate, so they take lives. Ash spreads furthest, so it does the widest economic damage. Lahars strike unexpectedly and long afterwards, so they defeat the exclusion zone.

11. Why people live in areas at risk

Syllabus point 4.3.1. This has been examined as a 4 mark list, a 5 mark explanation and a 7 mark levelled question in the space of four years, so it is worth learning properly. Sort your reasons into three groups so that you never dry up.

Resources and opportunity

• Fertile soils. Weathered volcanic ash and lava release minerals, giving very high crop yields. This is why the slopes of Merapi in Java support extraordinarily dense farming populations.
• Geothermal energy. Iceland heats around 90% of its homes with geothermal water. Indonesia, Kenya, New Zealand and the Philippines all run geothermal power stations.
• Mining and quarrying. Sulphur, pumice, obsidian, building stone and, in some volcanic pipes, diamonds.
• Tourism. Volcanoes and hot springs attract visitors, giving work as guides, in hotels and in transport.
• Employment generally. People live where the work is.

Social and personal ties

• They were born there, their family and friends live there, and they feel attached to the place, sometimes for religious reasons.
• Their children's school and their whole community are there.
• They cannot afford to move, and there is nowhere else to go because of pressure on land.

Perception of the risk

• The volcano is classed as dormant, or has not erupted for a very long time, so people believe it will not erupt again, and some are simply unaware of the danger.
• They have confidence in the protection: monitoring, warning systems, drills, evacuation routes, earthquake resistant buildings.
• They accept the risk knowingly, because an eruption is rare and the benefits are daily.
• Scientists live there deliberately, in order to study the volcano.

Marks note: a 2023 mark scheme rejects "tourist attraction" on its own, "cultural attraction", "feel safe" and "difficult to move" as too vague. Each of those needs one more clause: not "tourist attraction" but "jobs guiding tourists up the volcano".

12. The impacts of earthquakes

Syllabus point 4.3.2. Impacts are social, economic and environmental, and they may be positive or negative, though for earthquakes the positives are rare and mostly economic recovery.

Primary and secondary impacts

Keep these separate from primary and secondary responses in section 15, because it is easy to confuse the two.

• Primary impacts are caused directly by the ground shaking: buildings collapse, people are crushed, roads crack.
• Secondary impacts follow on from those: tsunami, fire, landslides, disease from contaminated water, homelessness, economic decline.

Marks note: a mark scheme for a 4 mark "reasons why deaths vary" question caps secondary effects such as tsunami and disease at 1 mark, so lead with the primary reasons.

Why the same magnitude produces very different death tolls

This comparison is asked in some form nearly every year and the mark scheme demands that you either state the variation or clearly imply it. "Buildings collapse" is worth nothing. "Buildings in lower income countries are more likely to lack reinforcement, so they collapse" is worth the mark.

The controls are:

• Building quality and enforcement of building codes.
• Focal depth, shallow being worse, and distance from the epicentre.
• Population density, and whether the area is urban or rural.
• Time of day, since people in bed or in school are at different risk.
• Ground conditions, since soft sediment, reclaimed land and former lake beds amplify the shaking, and steep slopes fail.
• Emergency services, health care and their speed of arrival, and access, because if the roads and airport are destroyed the aid cannot reach people.
• Education, drills and evacuation planning, and whether people know what to do.
• Whether a tsunami followed, and whether there was a warning system.
• Level of development and the availability of international aid.

The clearest real comparison is 2010. Haiti was struck on 12 January by a magnitude 7.0 earthquake with a shallow focus close to the crowded capital, Port-au-Prince, in a country where more than 70% of people lived on under US$2 a day and building codes were barely enforced. Cambridge's own exam resource records 1.2 million people made homeless, and the death toll ran into the tens of thousands at the very least. Six weeks later Chile was struck by a magnitude 8.8 earthquake, releasing several hundred times more energy, in a country with strict, enforced seismic building codes. Around 500 people died. The energy was vastly greater and the death toll was vastly smaller. Wealth, enforcement and preparation, not magnitude, decided the outcome.

13. The impacts of volcanic eruptions

Syllabus point 4.3.3. Mark schemes for this topic state explicitly that impacts can be positive, so an answer that is entirely negative has thrown away marks.

Exam trap: "the volcano destroyed everything" earns one mark at most. Say what was destroyed and how the hazard did it. Ash buried the crops, the lahar filled the river channel and flooded the town, the pyroclastic flow burnt the forest on the southern flank.

14. Measuring the magnitude of a tectonic event

Syllabus point 4.3.4 names four scales. This is a new explicit requirement for 2027, and the distinction between measuring the event and measuring the effects is exactly what an exam question will test.

Magnitude against intensity is the point of this section. Magnitude is a single number describing the earthquake itself. Intensity describes what happened at one place, and it falls as you move away from the epicentre. That is why exam resources draw isoseismal lines, lines joining places of equal intensity, and expect you to place the epicentre inside the innermost ring. A 2021 question did exactly that with the Newcastle, Australia earthquake and asked candidates to mark the epicentre in the highest intensity zone.

For reference points: the strongest earthquake ever recorded was at Valdivia, in the Bio-Bio region of Chile, in 1960, at magnitude 9.5, which appears in an exam resource. Mount Pinatubo in 1991 was VEI 6. Merapi in 2010 was VEI 4.

15. Primary and secondary responses

Syllabus point 4.4.1 is new as a named requirement for 2027. Sort responses by when they happen.

Primary responses, in the first hours and days, aimed at saving life:

• Search and rescue, digging survivors out of collapsed buildings, and evacuating people out of the danger or exclusion zone.
• Treating the injured, and setting up field hospitals when the real ones are damaged.
• Emergency shelter, tents, blankets, food and clean water.
• Restoring power, water and communications enough to run the rescue.
• Recovering and safely burying the dead, to prevent disease.
• Issuing warnings about aftershocks or a continuing eruption.
• Requesting and receiving international aid, foreign rescue teams and helicopters.

Secondary responses, over the following weeks, months and years, aimed at recovery:

• Rebuilding homes, schools, hospitals, roads and bridges, ideally to a higher standard than before, and repairing water, sewerage and power networks.
• Permanently rehousing people whose land is now unusable.
• Restoring the economy: reopening businesses, replacing lost livestock and seed, retraining people.
• Providing counselling for survivors.
• Revising and enforcing building codes, redrawing hazard maps and land use zones.
• Establishing a reconstruction authority and a fund to manage the money, and improving the warning system so the next event is handled better.

Exam trap: a response is what people do about the event. An impact is what the event does to people. "Buildings collapsed" is an impact. "The army was sent in to search the collapsed buildings" is a primary response. "Building codes were tightened" is a secondary response.

16. Evaluating the management of tectonic hazards

Syllabus point 4.4.2 asks for an evaluation, under five headings the syllabus names itself: monitoring, prediction, protection, planning and technology. Use those five as your structure, because the examiner has published them.

Monitoring

Prediction

This is the hinge of the whole evaluation, and it earns the highest marks because it is a genuine judgement rather than a list.

Volcanoes can be predicted usefully. They give warning signs over days or weeks, so monitoring translates directly into an evacuation that saves lives. Earthquakes cannot be predicted reliably. No one can say where and when the next one will strike. What is possible is forecasting: using past records and fault movement to state the probability that an area will be struck in a given period, which is what a seismic hazard map shows. An exam resource on the San Andreas Fault gave exactly this, a probability by section of the fault, and asked which section was most likely to be struck.

1. a volcano can be monitored
2. the warning signs build over days
3. an alert level is raised
4. people are evacuated before the eruption
5. deaths are prevented. An earthquake gives no such warning, so management must protect and plan instead of predict.

Protection

For earthquakes, the aim is buildings that do not kill their occupants: deep reinforced foundations anchored into bedrock; base isolation, where the building sits on rubber and steel bearings that absorb the movement; cross bracing and steel frames that flex instead of snapping; a tapered or pyramid profile, wider at the base; shatterproof glass; automatic shut off valves on gas mains to stop fire; low rise construction where enforcement is weak; and retrofitting older buildings, which is far cheaper than replacing them.

For volcanoes, the aim is to keep the hazard away from people: steeply pitched, reinforced roofs that shed ash before its weight collapses them; lava diversion channels and barriers, and spraying lava with seawater to chill and halt it, which was done successfully at Heimaey in Iceland in 1973 to save the harbour; sabo dams, concrete check dams built across valleys to trap lahars; and purpose built shelters and masks against ash.

Planning

• Hazard mapping and land use zoning, so that the most dangerous ground is left as parkland rather than housing.
• Exclusion zones around an active volcano, adjusted as the risk changes.
• Marked evacuation routes, published plans and regular drills. Japan holds a national disaster prevention day every year.
• Emergency supply kits in every household, and stockpiles of food, water and medicine.
• Search and rescue teams trained and equipped before they are needed, and funds set aside for reconstruction.

Technology

• Earthquake early warning systems, which detect the fast P waves and send an alert seconds before the damaging surface waves arrive, long enough to stop trains, close gas valves and get people under cover.
• Tsunami warning systems using seabed pressure sensors, buoys and coastal sirens. The Indian Ocean system was built after 2004, when there was none.
• Alerts pushed directly to mobile phones, and satellites and drones to survey damage and direct the rescue.
• Remote sensing of ash clouds to reroute aircraft safely.

How to write the judgement

The evaluation the examiner is looking for runs roughly like this. Monitoring and prediction work well for volcanoes and badly for earthquakes, so for earthquakes the money is better spent on protection and planning. Protection saves the most lives of any strategy, but it is expensive, it only helps in buildings that are actually built to the code, and enforcement is exactly what is missing where the deaths are highest. Planning and education are the cheapest and most sustainable measures and they work at every income level, but they depend on people acting on the warning, and they cannot protect property. Technology gives seconds or minutes of warning, which is enough to save lives but not enough to save buildings. No strategy prevents a tectonic event. Management reduces the risk, and the strongest answers say so.

17. Detailed specific examples

Syllabus points 4.4.3 and 4.4.4 require two separate detailed specific examples, one earthquake and one volcanic eruption, and each must cover causes, impacts, responses and management. Cambridge recommends choosing examples from CE 2000 onwards.

Read this before you learn either one. The mark schemes in this topic are unusually strict about what counts as a named example, and it is not what most students assume.

• Every levelled question prints "Max 5 if no named or inappropriate example". Without a named place you are capped at 5 out of 7 by rule, however good the geography is.
• For volcanoes, several mark schemes go further: "case study must be a named volcano or small place", "example to be smaller in size than a country", and "credit country names to max 5". Writing "Indonesia" is not enough. Name the volcano.
• For earthquakes, one mark scheme states "country = 0 unless very small". Name the epicentre, the district or the city, not just the country.
• Most importantly: "do not accept plate names as place specific". Naming the two plates earns you Level 2 credit as a development point, but it does not count towards the place specific detail that Level 3 requires. Level 3 needs settlement names, dates, times, magnitudes and figures.

17a. Earthquake: the Gorkha earthquake, Nepal, 25 April 2015

Location and cause

The earthquake struck at 11:56 local time on 25 April 2015. Its epicentre was near the village of Barpak in Gorkha district, about 80 km north west of the capital, Kathmandu, and its focus was very shallow, only around 8 km deep. It measured magnitude 7.8.

Nepal sits on a convergent collision boundary. The Indo-Australian Plate is driving northwards into the Eurasian Plate at roughly 4 cm to 5 cm a year. Neither plate is dense enough to subduct, so the crust between them is crumpled and forced upwards, which is how the Himalayas were built and are still rising.

1. the Indo-Australian Plate pushes north into the Eurasian Plate
2. neither continental plate is dense enough to subduct
3. friction locks the two plates together along the fault
4. the plates keep converging so stress builds for decades
5. the fault finally slips
6. energy is released as seismic waves from a focus only about 8 km down
7. violent shaking reaches the surface almost undamped.

Impacts

• Around 9000 people were killed and roughly 22 000 injured, making it Nepal's deadliest earthquake in over 80 years.
• Around 600 000 houses were destroyed and hundreds of thousands more damaged. Millions of people were left needing shelter, in a country where the monsoon was only weeks away.
• In Kathmandu the historic Dharahara Tower collapsed, killing dozens of people inside it, and UNESCO listed temples in the Durbar Squares of Kathmandu, Patan and Bhaktapur were destroyed.
• The shaking triggered avalanches and landslides in the mountains. An avalanche struck Everest Base Camp, killing around 19 climbers and staff, the deadliest day ever recorded on the mountain. The village of Langtang was buried almost completely by a landslide.
• Roads through the mountains were blocked by landslides, which cut off rural districts for weeks and made the rescue far slower than in the capital.
• Economic losses were estimated at around US$7 billion, close to a third of Nepal's annual GDP. Tourism and trekking, a major source of income, collapsed for a season.
• A powerful aftershock of magnitude 7.3 on 12 May near Dolakha killed further people and brought down buildings already weakened.

Responses

• Primary: the Nepalese army and police led search and rescue in the first hours. Nepal declared a state of emergency and appealed for international help. Foreign rescue teams, medical teams and helicopters arrived from India, China, the United Kingdom and elsewhere. Tents, tarpaulins, food and clean water were distributed, and helicopters reached the cut off mountain villages that the roads could not.
• Secondary: a National Reconstruction Authority was created to manage the rebuilding and to distribute government grants to households rebuilding their homes. Rubble was cleared, roads reopened, and reconstruction of the damaged temples was begun to bring tourists back. Building codes were revised and rebuilding was supposed to follow earthquake resistant designs.

Management, and how it worked

• Nepal had adopted a national building code in 1994, but enforcement in Kathmandu's rapidly built neighbourhoods was weak. Buildings that did follow it survived far better than those that did not, which is the clearest lesson of the event.
• Schools and hospitals were retrofitted before 2015 under a school safety programme, and buildings that had been strengthened performed much better.
• The reconstruction grants were tied to inspection, so that money was only released as each earthquake resistant stage of a house was completed. This linked recovery to protection.
• Evaluation: the earthquake could not be predicted, so prediction was never the answer. What could have been done was protection and planning, and Nepal had the right policy but not the enforcement or the money to apply it to hundreds of thousands of existing homes. The relief effort saved lives but was slowed badly by mountainous terrain and blocked roads, showing that in a country with this relief, access matters as much as resources. The reconstruction authority made the rebuilt housing far safer than the housing it replaced, which is the genuine long term gain.

17b. Volcano: Mount Merapi, Java, Indonesia, October to November 2010

Location and cause

Mount Merapi is a strato-volcano, 2930 m high, in Central Java, Indonesia, immediately north of the city of Yogyakarta. It is one of the most active volcanoes in the world. The 2010 eruption began on 26 October 2010 and continued through November. It was the largest eruption of Merapi since 1872 and is rated VEI 4.

Merapi sits on a convergent destructive boundary, part of the Pacific Ring of Fire, where the Indo-Australian Plate is subducting beneath the Eurasian Plate.

1. the denser oceanic Indo-Australian Plate is forced beneath the Eurasian Plate
2. friction and mantle heat melt the descending plate
3. magma forms and rises because it is less dense
4. viscous, gas rich magma collects in the chamber beneath Merapi
5. a lava dome builds at the summit and seals the vent
6. pressure rises until the dome fails
7. the volcano erupts explosively, generating pyroclastic flows down the flanks.

Impacts

• Around 350 people were killed, most of them by pyroclastic flows which travelled up to about 15 km down the southern flank, further than any recent eruption and beyond the original danger zone.
• Roughly 350 000 people were evacuated from the slopes, the largest evacuation in the volcano's modern history.
• Villages, farmland and livestock on the southern flank were destroyed or buried in ash. Many farming families lost both their homes and their income at once.
• Ash reached great altitude and closed airports, including Yogyakarta's, disrupting flights across the region.
• Lahars continued for months afterwards. During the following rainy season, heavy rain mobilised the loose ash and sent mudflows down the river valleys, including the Code River which runs through Yogyakarta itself, damaging bridges and homes far from the crater.
• One death became internationally known: Mbah Maridjan, the elderly spiritual gatekeeper of the mountain, refused to leave his home in accordance with tradition and was killed by a pyroclastic flow. His example illustrates precisely why evacuation orders fail even when the science is right.
• Positively, the ash that buried the fields in 2010 weathered into the exceptionally fertile soil that is the reason a million people live on Merapi's slopes at all, and sand and gravel from the lahar deposits became a major local mining industry afterwards.

Responses

• Primary: Indonesia's volcanology agency had been monitoring Merapi closely and raised the alert to its highest level on 25 October, the day before the eruption. Evacuation was ordered and the danger zone was progressively widened from 10 km to 20 km as the eruption escalated. People were moved into emergency shelters, camps and public buildings in Yogyakarta, where they were provided with food, water, masks and medical care. Ash was cleared from roads and roofs.
• Secondary: displaced families were rehoused, in some cases permanently relocated out of the highest risk valleys. Farmland was cleared and replanted, livestock replaced, and compensation paid. Hazard maps were redrawn to reflect the new, longer reach of the pyroclastic flows. Sabo dams in the river valleys were repaired and extended to trap future lahars.

Management, and how it worked

• Monitoring at Merapi is continuous, using seismometers to record the earthquake swarms caused by rising magma, tiltmeters and GPS to detect the summit swelling, and gas and thermal measurement. It is one of the best monitored volcanoes anywhere.
• Prediction and warning worked. The alert was raised before the eruption, which is why hundreds of thousands of people were off the mountain when the pyroclastic flows came.
• Planning worked partly. The exclusion zone existed and was enforced, but the 2010 flows travelled further than the zone had originally allowed for, so the zone had to be extended while the eruption was in progress.
• Protection in the form of sabo dams reduced the lahar damage in the valleys, though it could not stop it entirely.
• Evaluation: this eruption is the strongest evidence available that volcanic hazards can be managed. Merapi 2010 was several times larger than the 1994 and 2006 eruptions, yet the death toll was a small fraction of what it would have been without monitoring and evacuation, and disaster agencies credit the evacuation with saving many thousands of lives. The management failed in two specific ways worth naming: the hazard zone was drawn from past eruptions and this eruption was bigger than the past, and cultural and economic attachment to the land meant some people refused to leave or returned too early to tend livestock. Technology solved the science. It did not solve the reasons people stay.

18. Fieldwork links for Paper 4

Tectonic settings appear on Paper 4 as the location for a fieldwork investigation rather than as content. Both recent tectonic Paper 4 questions in the bank were built this way: a study of vegetation cover and infiltration on the slopes of Cotopaxi in Ecuador, and a questionnaire survey in villages in eastern Nepal affected by an earthquake, landslides and flooding.

Hypotheses that fit a tectonic setting

• Vegetation cover decreases as altitude increases on a volcano's slopes.
• The rate of infiltration varies with distance from the crater, because of the depth of ash and the amount of vegetation.
• People living closer to the volcano are better informed about evacuation procedures.

Methods to be able to describe

• Vegetation cover: place a quadrat on the ground at each site using a random or systematic system, count or estimate the percentage of squares covered by vegetation and by bare ground, and repeat at each site.
• Infiltration: push an open pipe into the ground, pour in a measured volume of water, and use a ruler and a stopwatch to record the fall in water level over a fixed time such as ten minutes.
• Perception and awareness: a questionnaire or a structured interview, using closed questions with tick boxes so the results can be counted and graphed.
• Sampling: random removes bias but may give an unrepresentative sample; systematic selects at regular intervals or every nth person; stratified deliberately matches the age and gender balance of the population.

Reliability points that earn marks

• Take each measurement at least twice at each site and use the average, and treat a large gap between two readings at the same site as an anomaly rather than data. Say by how much it differed, because mark schemes credit the figure, not just the word.
• Have a second student read the measurement independently to remove one person's error and subjective judgement.
• Use a transect or evenly spaced sites rather than randomly chosen ones when you are testing change with distance or altitude.
• For questionnaires, name a difficulty that is genuinely practical: language, literacy, people refusing or being too busy, or people giving answers they think you want.

Risk assessment. A fieldwork question in a volcanic setting asked candidates to reduce the risk from hypothermia, uneven ground and getting separated. The credited answers were entirely practical: waterproof layers and spare clothing, boots and walking poles, and staying in groups with a phone, a whistle, a meeting point and regular headcounts.

19. Common exam mistakes

• Writing "subduction" and stopping. On its own it is Level 1. Say which plate subducts beneath which, and that it is the denser oceanic one.
• Naming the boundary type twice and expecting Level 2. Mark schemes state that naming the margin is Level 1 no matter how often you do it. The movement, the friction, the pressure and the release are what lift you.
• Using a constructive boundary to explain an earthquake, or a conservative or collision boundary to explain a volcano. Mark schemes will not credit them.
• Giving plate names and thinking that is place specific detail. It is Level 2 development. Level 3 needs settlements, dates, times, magnitudes and figures.
• Answering a volcano question with a country. Several mark schemes cap a country name at 5 out of 7. Name the volcano.
• Confusing magma and lava. Magma is underground, lava is at the surface.
• Confusing focus and epicentre, or defining epicentre without mentioning the focus below it, which loses the mark outright.
• Confusing abrasion of the load with tectonic vocabulary from other topics. Keep tephra (all ejected fragments), ash (the fine fraction) and pyroclastic flow (the moving cloud of gas and rock) distinct.
• Writing only negative impacts of an eruption. Mark schemes state that impacts can be positive, and fertile soils, geothermal energy and tourism are all creditable.
• Confusing primary and secondary impacts with primary and secondary responses.
• Saying earthquakes can be predicted. They can be forecast probabilistically and mapped. They cannot be predicted.
• Describing a distribution using only named regions. One mark scheme caps that at 2 of 3 marks, so include a pattern word such as clustered or linear.
• Giving vague reasons why people live near volcanoes. "Tourist attraction", "cultural attraction" and "feel safe" are all rejected as too vague unless you add the clause that explains them.
• Listing management methods on a question that says evaluate. Say how each works, give a drawback, and reach a judgement.

20. Quick revision

• Layers: inner core solid, outer core liquid, mantle largest and convecting, crust thinnest. The lithosphere is crust plus rigid upper mantle, and it is what the plates are made of.
• Oceanic crust is thin, basaltic and dense. Continental crust is thick, granitic and less dense. The dense one subducts.
• Plates move on convection currents in the mantle, at a few centimetres a year.
• Four boundaries: divergent creates crust, destructive destroys it, collision crumples it, conservative neither.
• No volcanoes at collision or conservative boundaries. Volcanoes need constructive or destructive, earthquakes are credited at conservative or destructive.
• Earthquake mechanism: plates stick → friction → pressure builds → fracture → energy released as seismic waves.
• Focus is underground, epicentre is directly above it on the surface. A shallow focus does more damage.
• Volcano types: shield low, wide, runny lava, frequent gentle eruptions; strato-volcano tall, steep, layers of lava and ash, violent; cinder cone small, steep, loose tephra.
• Active, dormant, extinct, and the labels can change, as Chaitén showed in 2008.
• Features: magma chamber, vent, crater, secondary cone, layers.
• Seven hazards: lava flows, ash falls, lahars, pyroclastic flows, tephra, volcanic rocks, toxic gases. Pyroclastic flows kill fastest, ash spreads furthest, lahars arrive latest.
• Scales: Mw for energy, Richter for amplitude and it saturates above about 7, Mercalli for observed intensity which varies with distance, VEI 0 to 8 for eruption size.
• Responses: primary saves life in hours, secondary rebuilds over years.
• Management under five headings: monitoring, prediction, protection, planning, technology. Volcanoes can be predicted, earthquakes cannot.
• Two detailed specific examples required, one earthquake and one eruption, each with causes, impacts, responses and management.
• A named example is worth two marks. A country name is not a named example for a volcano. Place specific detail is worth the top mark.

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