Natural hazards are caused by geophysical processes: plate tectonics and volcanic processes
World at Risk is the first topic of WGE01, Global Challenges, the 1 hour 45 minute, 90-mark paper.
Its hazards half (the first three enquiry questions) is examined by Section A Question 1, a 15-mark data-response question, and it can return in Section B Question 5, where one part is always on hazards and the other on climate.
Its first enquiry question asks What are global hazards and what causes them? This first section teaches the geophysical hazards that the rest of the unit keeps using: earthquakes, volcanic eruptions and tsunami.
- Items worth 1 to 4 marks are point-marked: one mark per valid point, one more for each linked development. Every 4-mark item is marked as two developed points (2 + 2).
- The 6-mark Explain in (c) is levels-marked (1-2, 3-4, 5-6) on accuracy, range and development; examiners say named, located examples are what lift answers into the top level.
- Section B (a) is a 10-mark Suggest on a figure (levels 1-4, 5-7, 8-10; AO1 5 + AO2 5). Section B (b) is a 20-mark "To what extent do you agree?" essay on a quoted statement (four levels; AO1 5 + AO2 15). An essay with knowledge but no argument scores at most 4.
- When a stem has two halves (floods and droughts; scale and type), an answer that covers one half is capped.
- Command words on WGE01: Define, Describe, Compare, Suggest, Explain, multiple-choice questions, and "To what extent do you agree?". Nothing else.
The idea: a moving surface
At 04:17 local time on 6 February 2023, a fault ruptured near Pazarcık in Kahramanmaraş province, southern Türkiye.
The main shock was Mw 7.8; about nine hours later a second earthquake of Mw 7.5 struck near Elbistan (USGS figures).
Türkiye's Interior Ministry, which runs AFAD, the national disaster agency, put the deaths in Türkiye at 53,537 (February 2024).
In Syria about 6,000 died: 1,414 counted by the Syrian government and 4,537 by the authorities in the opposition-held north-west, with other estimates up to about 8,500.
Nothing about the weather that night caused it.
The cause lay in the crust below, where the Arabian plate is pushing north against Eurasia and squeezing the Anatolian plate westwards along the East Anatolian Fault.
That is what a geophysical hazard is: a natural event, driven by energy from inside the Earth, that threatens people and property.
The idea to learn is that plate tectonics and volcanic processes cause geophysical hazards (earthquakes, volcanic eruptions, tsunami).
You need all three, and why each happens where it does.
Why plates move, and why margins are dangerous
The lithosphere (the crust and the rigid top of the mantle) is broken into plates that move a few centimetres a year over the asthenosphere, a hot layer of mantle that is solid but able to flow very slowly.
Plates are pulled by the weight of old, cold oceanic lithosphere sinking at subduction zones (slab pull), pushed away from mid-ocean ridges (ridge push), and carried by convection in the mantle.
Where two plates meet, their motion is resisted by friction.
Strain builds in the rock for decades or centuries, then the fault slips suddenly and the stored energy is released as seismic waves.
This is why most earthquakes and volcanoes sit along plate margins (Figure 1).
Margin (Figure 1)
What happens
Main geophysical hazards
Middle East example
A Divergent
Plates move apart; the mantle below decompresses and partly melts; basaltic magma rises
Shallow earthquakes; effusive basaltic eruptions
Red Sea rift; the lava fields (harrats) of western Saudi Arabia
B Convergent (subduction)
Denser oceanic plate sinks below a less dense plate; water released from the slab lowers the melting point of the mantle above
The largest (megathrust) earthquakes; explosive andesitic eruptions; tsunami
Makran subduction zone off southern Iran and Pakistan
B Convergent (collision)
Two continental plates meet; neither sinks far
Large shallow earthquakes over a wide zone; few volcanoes
Zagros Mountains, Iran
C Transform
Plates slide past each other
Shallow, often very destructive earthquakes; no volcanoes
East Anatolian Fault, Türkiye
Earthquakes
An earthquake starts at the focus (hypocentre) underground; the point on the surface directly above is the epicentre.
The shaking is the primary hazard.
It then triggers secondary hazards: liquefaction, where shaking makes saturated sand and silt behave like a liquid so buildings tilt and sink; landslides on steep slopes (the next section); and tsunami when the seabed is moved.
The 2023 Türkiye-Syria earthquakes show a transform margin at its worst.
The ruptures were shallow (the USGS put the main shock's focus about 10 km down and the second shock's about 7 km) and very long: USGS mapping from satellite data shows the main shock broke about 345 km of the East Anatolian Fault and the second about 175 km of the Çardak fault to the north, more than 500 km of surface rupture in all.
Strong shaking therefore reached cities along the whole length, including Antakya, Kahramanmaraş and Adıyaman, and on reclaimed land along the shore at İskenderun liquefaction made buildings sink by up to about 0.7 m (GEER-EERI engineering reconnaissance, 2023).
Volcanic eruptions
Magma forms in three settings.
At divergent margins and hotspots, rising mantle decompresses and melts, giving runny basaltic magma, low in silica and gas, so eruptions are mostly effusive (lava flows, fire fountains).
At subduction zones, water driven out of the sinking plate lowers the melting point of the mantle above; the magma that rises through thick crust becomes richer in silica and gas, so it is sticky and erupts explosively.
Volcanic hazards include lava flows, pyroclastic flows (fast avalanches of hot gas and ash), tephra and ash fall, volcanic gases and lahars (volcanic mudflows).
Western Saudi Arabia shows that volcanic hazard is not only a Pacific issue.
Between April and June 2009, more than 30,000 earthquakes shook Harrat Lunayyir, a basalt lava field near Al-Ays, as a dyke of magma about 10 km long pushed up to within a few kilometres of the surface (Nature Geoscience, 2010).
The largest earthquake was M5.4 (Saudi Geological Survey) or M5.7 (USGS).
Saudi civil protection moved between about 30,000 and 40,000 people out of the area (sources differ).
The magma never erupted, but the episode showed that a "quiet" harrat can wake up.
Tsunami
A tsunami is a series of long waves caused by a sudden vertical displacement of a large body of water, usually when a megathrust earthquake lifts the seabed.
In the deep ocean a tsunami travels at about , where is the depth: in 3,000 m of water that is about 170 m s⁻¹, or roughly 620 km h⁻¹, with a wave height of under a metre.
As it reaches shallow water it slows, the wavelength shortens and the height grows sharply (shoaling), and the sea floods inland as a fast-moving surge, not a breaking surf wave.
The Makran subduction zone, where the Arabian plate dives below Eurasia off southern Iran and Pakistan, produced the Arabian Sea's deadliest recorded tsunami.
In the early hours of 28 November 1945 (local time) a magnitude 8.1 thrust earthquake off Pasni sent waves onto the Makran coast; a field survey published in Natural Hazards and Earth System Sciences (2021) put the run-up at about 14 m at Pasni and 11 m at Ormara, and floodwater reached almost 1 km inland at Pasni and 2.5 km at Ormara.
Estimates of the death toll range from about 300 to 4,000 and remain uncertain.
Oman's coast lies within a few hundred kilometres of this zone, which is why Oman's National Multi-Hazard Early Warning System, opened in 2015, covers tsunami as well as tropical cyclones and flash floods.
Connect it
These processes explain the distribution of geophysical hazards (the section on where hazards occur), feed the magnitude scales (Moment Magnitude, VEI) in the section on hazards, disasters and magnitude, and cause the mega-disasters met later in the topic.
Volcanic emissions also return in climate change (the section on solar output and volcanic emissions), where large eruptions cool the planet for a year or two.
- Geophysical hazard: a natural hazard driven by processes within the Earth, such as an earthquake, volcanic eruption or tsunami.
- Plate margin: the boundary where two tectonic plates meet: divergent, convergent or transform.
- Subduction: the sinking of a denser oceanic plate below another plate at a convergent margin.
- Liquefaction: the loss of strength in saturated, loose sediment during shaking, so that it behaves like a liquid.
- Tsunami: a series of long sea waves caused by the sudden vertical displacement of a large body of water, most often by a submarine earthquake.
Explain two reasons why earthquakes at subduction zones can cause tsunami.
(4)
Top answer (4 of 4). Subduction zones produce megathrust earthquakes in which the over-riding plate, locked against the sinking plate, suddenly springs upwards (1), so the seabed and the whole column of water above it are lifted at once, starting the wave (+1).
These earthquakes can rupture hundreds of kilometres of fault (1), so a very large volume of water is displaced along a long stretch of coast at the same time, which is why the waves carry so much energy across a whole ocean basin (+1).
Weaker answer (2 of 4). The plates move and make an earthquake under the sea (1).
The waves get bigger near the coast and flood it.
What it lacks. The second sentence describes shoaling, which is how a tsunami behaves at the coast, not why the earthquake causes one, so it earns nothing.
The first point is never developed into vertical displacement of the water.
Explain why volcanic eruptions at some plate margins are more hazardous than at others.
(6)
Level 3 answer (5-6). At divergent margins, such as the Red Sea rift, decompression of the rising mantle produces basaltic magma that is low in silica and gas, so it flows easily and eruptions are usually effusive; lava moves slowly enough for people to leave.
At subduction zones the sinking plate releases water that lowers the melting point of the mantle; the magma rising through continental crust gains silica, becomes viscous and traps gas, so pressure builds and is released explosively.
Explosive eruptions produce pyroclastic flows and ash columns, which travel too fast to escape and spread hazards tens of kilometres away.
The hazard also depends on what lies around the volcano: snow or crater lakes turn an eruption into lahars, as at Nevado del Ruiz, Colombia, in 1985, where a lahar killed more than 20,000 people in Armero.
Even a basaltic field can be hazardous where people live on it: the 2009 dyke intrusion at Harrat Lunayyir, Saudi Arabia, forced the evacuation of tens of thousands from Al-Ays.
Why Level 3. A broad range of developed reasons (magma type, explosivity, secondary hazards, exposure), each a chain from cause to hazard, set in named, located examples.
Level 2 answer (3-4). Subduction volcanoes are more dangerous because they are explosive.
They have pyroclastic flows that kill people.
Volcanoes at constructive margins have runny lava which is less dangerous.
More people live near some volcanoes.
What it lacks. Accurate but undeveloped: it never says why subduction magma is explosive (silica, gas, viscosity), and the exposure point is generic, with no named place.
Read which hazard the stem names and stay with it.
A question about tsunami deaths wants wave mechanisms (wave height, arrival time, warning time), not building collapse from shaking; a question on geophysical hazards gives no credit for floods or cyclones.
- Wrong: "Plates float on molten magma." The asthenosphere is almost entirely solid; it flows slowly because it is hot and under pressure. Magma forms only where melting conditions are met.
- Wrong: "The oceanic plate sinks because it is heavier." It sinks because it is denser; a continental plate can have more mass and still not subduct.
- Not enough: "Earthquakes happen at plate boundaries." Say what the margin does (strain builds where plates lock, then releases), and note that intraplate earthquakes also occur.
- Wrong: "A tsunami is a tidal wave." Tides play no part; the cause is displacement of the water column.
Check your understanding
1. Study Figure 1. Explain one reason why margin B in Figure 1 is more likely than margin C to cause a tsunami. (3) Total [3]
2. Explain how liquefaction can increase the damage caused by an earthquake. (2) Total [2]
3. Explain one reason why transform margins such as the East Anatolian Fault produce no volcanoes. (2) Total [2]
4. A student says: "The Harrat Lunayyir episode in 2009 was not a hazard event because the volcano did not erupt." Explain whether the student is right. (3) Total [3]
5. Explain two reasons why the earthquakes of February 2023 caused such severe damage across southern Türkiye. (4) Total [4]
6. Explain why some coastlines face a higher tsunami hazard than others. (6) Total [6]
Answers
1. At a subduction margin the over-riding plate is locked against the sinking plate and is dragged down until it springs back upwards in a megathrust earthquake (1), so the seabed is pushed up vertically and the whole column of water above it is lifted at once (+1); at the transform margin C the plates slide past each other sideways, so the seabed hardly moves up or down and little water is displaced (+1).
2. Shaking makes saturated, loose sand and silt lose strength and behave like a liquid (1), so the ground can no longer hold up foundations and buildings tilt, sink or topple even where they survived the shaking itself, as on reclaimed land at İskenderun in 2023 (+1).
3. At a transform margin the plates slide past each other, so no crust is created or subducted (1); there is no decompression of rising mantle and no water released from a sinking slab, so the mantle does not melt and no magma forms (+1).
4. Wrong (1). A hazard is a threat, not only an event that causes damage: more than 30,000 earthquakes shook the area, opened an 8 km surface fault and damaged buildings in Al-Ays (+1), and the risk of an eruption was real enough for tens of thousands of people to be evacuated, which disrupted lives and livelihoods (+1).
5. Two developed reasons, 2 + 2, for example: the foci were shallow, about 10 km down (1), so seismic energy reached the surface with little loss and shaking was very strong in the cities above (+1); the two ruptures broke a very long stretch of the fault system (1), so many cities along its length were shaken at once, which spread the damage over a wide area and stretched the rescue effort (+1). Building collapse due to poor enforcement of codes can also be credited as a human reason.
6. Level 3 (5-6) needs a range of developed points in located examples: coasts facing a subduction zone are close to the source, so waves arrive in minutes, leaving little warning time (Pasni and Ormara on the Makran coast, 1945); a broad, shallow continental shelf makes waves shoal and grow higher; low, flat coastal plains and river mouths let the surge travel kilometres inland (2.5 km at Ormara); bays and inlets funnel and amplify the wave. Coasts far from any subduction zone, or with deep water close inshore and high cliffs, face a lower hazard. Level 2 (3-4): two or three reasons, partly developed or generic. Level 1 (1-2): isolated points.