Hazardous Earth

What Hazardous Earth covers

Hazardous Earth is examined in Component 03, Geographical debates. It covers the Earth's internal structure and the global pattern of plates; the evidence for continental drift and sea-floor spreading; the processes and landforms of divergent, subduction, collision and conservative boundaries; how magma chemistry shapes explosive and effusive eruptions, hot-spot volcanism and super-volcanoes; and the hazards of lava, pyroclastic flows, volcanic gases, ashfall, lahars and volcanic tsunamis. It also covers earthquake waves, depth and measurement; the landscapes faulting creates; ground shaking, liquefaction, earthquake-triggered landslides and tsunamis; why people live in tectonically active places; the impacts of eruptions and earthquakes in countries at contrasting levels of development; predicting, monitoring and managing volcanic and earthquake hazards; changing risk and the disaster risk equation; and the Park model of disaster response.

The 24 subtopics

Each one has its own page.

• The Earth's internal structure and the global pattern of platesRecall questions on the layers of the Earth, the lithosphere and asthenosphere, oceanic and continental crust, the solid inner core, the Moho, radioactive heat and mantle convection, ridge push and slab pull, plate speeds, the seven major plates, the distribution of earthquakes and volcanoes, the Pacific Ring of Fire, mantle plumes, and GPS measurement.Open →
• Continental drift and the evidence from fossils, rocks and ancient glaciationsRecall questions on Wegener and the theory of continental drift, Pangaea, Laurasia and Gondwanaland, and the fossil, rock and glacial evidence for the theory.Open →
• Sea-floor spreading, palaeomagnetism and the age of the ocean floorRecall questions on sea-floor spreading and Harry Hess, palaeomagnetism and the Curie point, geomagnetic reversals and symmetrical magnetic stripes explained by Vine and Matthews, the age of oceanic crust and why it is young, sediment thickness and heat flow across ocean basins, and confirmation by ocean drilling.Open →
• Divergent boundaries and the landforms of the mid-ocean ridgesRecall questions on divergent boundaries and decompression melting, the Mid-Atlantic Ridge and the form of mid-ocean ridges, transform faults, shallow earthquakes, pillow lava and black smokers, Iceland, effusive eruptions, continental rifting and the Red Sea, and a worked spreading-rate calculation.Open →
• Subduction at oceanic-continental and oceanic-oceanic boundariesRecall questions on subduction, ocean trenches and the Challenger Deep, Benioff zones, how magma forms above a subducting slab and why it is silica-rich, accretionary prisms, the Andes, island arcs such as the Marianas and Aleutians, back-arc basins, great and deep-focus earthquakes, and violent eruptions.Open →
• Continental collision and conservative boundariesRecall questions on continental collision, the Himalayas and the Tethys Sea with a worked convergence calculation, why collision zones lack volcanoes but have destructive earthquakes, thick Himalayan crust, conservative boundaries such as the San Andreas and North Anatolian faults, stick-slip movement, shallow earthquakes, seismic gaps, and offset landscapes.Open →
• Magma chemistry, viscosity and explosive volcanism at subduction zonesRecall questions on viscosity and what controls it, the silica content of basaltic, andesitic and rhyolitic magma, temperature, why viscous magma erupts explosively, water from the subducting plate, composite volcanoes, Plinian, Peléean and Strombolian eruptions, Mount St Helens in 1980, and lava domes.Open →
• Effusive eruptions, shield volcanoes and hot-spot volcanismRecall questions on effusive eruptions and fluid basaltic lava, shield volcanoes and Mauna Loa, fissure eruptions and Laki in 1783, lava plateaus and basalt columns, pahoehoe and aa lava, cinder cones, mantle plumes and the Hawaiian chain with a worked age calculation, Yellowstone, and the volcanoes of the East African Rift.Open →
• Volcano size and shape, super-volcanoes and the Volcanic Explosivity IndexRecall questions on the slopes and volumes of shield, composite and cinder cone volcanoes, calderas and how they form, super-volcanoes, the Toba and Yellowstone super-eruptions and their climatic effects, the Volcanic Explosivity Index, its range, tenfold steps and a worked comparison, Pinatubo's rating, and the limits of the index.Open →
• Lava flows, pyroclastic flows and volcanic gasesRecall questions on the damage and reach of lava flows, pyroclastic flows, their speed, temperature, lethality and origins, Mount Pelée and Saint-Pierre in 1902, volcanic gases, carbon dioxide, sulphur dioxide and acid rain, cooling after Pinatubo, fluorine poisoning, volcanic smog, and the deadly gases of long effusive eruptions.Open →
• Tephra, ashfall, lahars and volcanic tsunamisRecall questions on tephra, ash, lapilli and volcanic bombs, ashfall damage to buildings, farming, water, aircraft and health, the 2010 Eyjafjallajökull ash cloud, lahars and their water sources, Armero and Nevado del Ruiz in 1985, lasting lahar risk, jökulhlaups, volcanic tsunamis, and Krakatoa in 1883.Open →
• Earthquake focus, depth and the seismic waves releasedRecall questions on the focus and epicentre, elastic rebound, stress and strain, P, S, Love and Rayleigh waves and how they move rock, the S-wave shadow zone, locating an epicentre, shallow-focus and deep-focus earthquakes and why depth matters, the 700 km limit, Benioff zones, and aftershocks.Open →
• Measuring earthquakes: Richter, moment magnitude and Mercalli intensityRecall questions on magnitude and intensity, the Richter scale, amplitude and energy steps with a worked comparison, its saturation, seismic moment and the moment magnitude scale, the 1960 Chile earthquake, the Modified Mercalli scale and its range, why intensity varies, the drawbacks of intensity scales, and isoseismal lines.Open →
• Fault scarps, rift valleys and the landscapes faulting createsRecall questions on faults, normal, reverse and strike-slip movement, fault scarps and escarpments, grabens and horsts, how rift valleys form, the East African Rift system and the Ruwenzori block mountains, offset streams and ridges along strike-slip faults, and coastal uplift in earthquakes.Open →
• Ground shaking, ground displacement and liquefactionRecall questions on ground shaking as the primary hazard, peak ground acceleration, duration and resonance, amplification on soft ground in Mexico City in 1985, ground displacement and infrastructure, the mechanism and conditions of liquefaction, its effects on buildings, sand boils, and Christchurch in 2011.Open →
• Earthquake-triggered landslides, avalanches and floodingRecall questions on why shaking triggers landslides and which slopes fail, landslides in the 2008 Sichuan earthquake, landslide dams and how they are made safe, avalanches at Yungay in 1970 and after the 2015 Nepal earthquake, flooding from failed dams, levees and subsidence, and what makes a landslide a disaster.Open →
• Tsunamis from sea-bed uplift and underwater landslidesRecall questions on how subduction earthquakes and underwater landslides generate tsunamis, why strike-slip faults rarely do, open-ocean wavelength and speed with a worked calculation, shoaling, run-up and drawback, successive waves, the 2004 Indian Ocean and 2011 Japanese tsunamis, sea-bed pressure sensors, and warning times.Open →
• Why people choose to live in tectonically active placesRecall questions on fertile volcanic soils on Java, geothermal energy in Iceland, minerals and tourism in tectonic regions, inertia and poverty as reasons to stay, the probability of events, risk perception and past experience, why large cities lie in active areas, and confidence in technology and government.Open →
• The impacts of volcanic eruptions in countries at contrasting levels of developmentRecall questions on primary and secondary impacts and the social, economic, environmental and political effects of eruptions, with Eyjafjallajökull in 2010, Pinatubo in 1991, Nyiragongo at Goma in 2002 and Soufrière Hills on Montserrat, and why deaths, costs and recovery differ between lower-income and higher-income countries.Open →
• The impacts of earthquakes in countries at contrasting levels of developmentRecall questions on why buildings rather than earthquakes kill, the social, economic, environmental and political impacts of earthquakes, the 2010 Haiti earthquake and the cholera that followed, the 2011 Tōhoku earthquake, tsunami and Fukushima, and why death tolls, costs, recovery and development setbacks differ with national wealth.Open →
• Predicting, monitoring and managing volcanic hazardsRecall questions on why eruptions can be forecast, earthquake swarms, harmonic tremor, tiltmeters, satellite radar and gas monitoring, hazard maps and exclusion zones, mitigating the event by diverting lava at Heimaey and on Mount Etna, sabo dams, mitigating vulnerability and losses, community preparedness, and careful evacuation warnings.Open →
• Land-use zoning, building design and managing earthquake riskRecall questions on why earthquakes cannot be predicted but can be forecast, land-use zoning along California's faults and in fast-growing cities, early warning systems, base isolation, cross-bracing, tuned mass dampers and deep foundations, retrofitting, simple house reinforcement, earthquake insurance and its uneven uptake, and public drills.Open →
• Changing risk over time and the disaster risk equationRecall questions on the disaster risk equation, vulnerability, capacity to cope and exposure, why tectonic events are not becoming more frequent while people affected and economic costs rise, how urbanisation and development raise or reduce risk, probability and low-probability, high-magnitude events, and future risk reduction strategies.Open →
• The Park model of disaster responseRecall questions on what the Park model shows, its stages from steady state through relief, rehabilitation and reconstruction, what sets the steepness of its downward limb, its three possible outcomes, how curves differ between high-income and low-income countries, why a community can end up better off, and the model's criticisms and uses.Open →

What you need to know

Answer each one before you reveal it.

Take Hazardous Earth further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Focus vs Epicentre

  Focus: the point within the Earth at which rock first ruptures and from which seismic energy begins to radiate. Epicentre: The point on the Earth's surface lying directly above the place where the rock ruptured.

• Magnitude vs Intensity

  Magnitude: the amount of energy released at the source of an earthquake, expressed as a single value for the whole event. Intensity: The strength of shaking and the damage felt at a particular place, which varies from point to point across the affected area.

• P wave vs S wave

  P wave: a compressional pulse that pushes and pulls rock back and forth along its direction of travel, arrives first, and passes through liquids. S wave: A shear pulse that shakes rock from side to side at right angles to its direction of travel, arrives later, and is stopped by the liquid outer core.

Questions from this topic

Each one with the model answer underneath.

1. 1

   Name the four main layers of the Earth in order from the surface to the centre.

   Model answer

   The crust, the mantle, the outer core and the inner core.

2. 2

   What is the lithosphere?

   Model answer

   The rigid outer shell of the Earth, made up of the crust and the solid uppermost mantle, broken into tectonic plates and roughly 100 km thick.

3. 3

   What is the asthenosphere, and why does it matter for plate movement?

   Model answer

   It is the weak, partly molten layer of upper mantle beneath the lithosphere, roughly 100 to 300 km deep, and its ductile behaviour allows the rigid plates above to slide over it.

4. 4

   Give two ways in which oceanic crust differs from continental crust.

   Model answer

   Oceanic crust is thinner, at about 5 to 10 km against 30 to 70 km, and denser, at around 3.0 g cm⁻³ against 2.7 g cm⁻³.

5. 5

   Which rock type dominates oceanic crust, and which dominates continental crust?

   Model answer

   Oceanic crust is dominated by basalt, and continental crust by granite.

6. 6

   Why is the inner core solid when it is hotter than the liquid outer core?

   Model answer

   The pressure at the centre of the Earth is so great that it raises the melting point of iron above the temperature there, so the inner core stays solid.

What is inside Levo

Six ways to work the same material.

Hazardous Earth revision questions

What does Hazardous Earth cover in OCR A-Level Geography?

24 subtopics: the earth's internal structure and the global pattern of plates, continental drift and the evidence from fossils, rocks and ancient glaciations, sea-floor spreading, palaeomagnetism and the age of the ocean floor, divergent boundaries and the landforms of the mid-ocean ridges, subduction at oceanic-continental and oceanic-oceanic boundaries, continental collision and conservative boundaries, magma chemistry, viscosity and explosive volcanism at subduction zones, effusive eruptions, shield volcanoes and hot-spot volcanism, volcano size and shape, super-volcanoes and the volcanic explosivity index, lava flows, pyroclastic flows and volcanic gases, tephra, ashfall, lahars and volcanic tsunamis, earthquake focus, depth and the seismic waves released, measuring earthquakes: richter, moment magnitude and mercalli intensity, fault scarps, rift valleys and the landscapes faulting creates, ground shaking, ground displacement and liquefaction, earthquake-triggered landslides, avalanches and flooding, tsunamis from sea-bed uplift and underwater landslides, why people choose to live in tectonically active places, the impacts of volcanic eruptions in countries at contrasting levels of development, the impacts of earthquakes in countries at contrasting levels of development, predicting, monitoring and managing volcanic hazards, land-use zoning, building design and managing earthquake risk, changing risk over time and the disaster risk equation and the park model of disaster response. Each one has its own page with questions and model answers.

How is Hazardous Earth organised?

By the OCR H481 specification, subtopic by subtopic, so it matches the order you are taught it in.