What is the difference between hazard, vulnerability, exposure and risk?

The hazard is the natural process or event with the potential to cause harm (the earthquake, eruption or landslide). Exposure is the people, property and infrastructure located in the hazard zone. Vulnerability is how susceptible those people and structures are to harm (poor buildings, an unprepared or poor population). Risk is the likelihood of harmful consequences, combining the hazard with exposure and vulnerability. The hazard itself cannot usually be removed, so risk is reduced by lowering exposure (land-use planning, hazard mapping) or vulnerability (resistant design, preparedness).

How does magma composition control volcanic hazard?

Silica content controls viscosity and gas escape, and so eruption style and hazard. Low-silica basaltic magma is runny, gas escapes easily, and eruptions are effusive, so the main hazard is escapable lava and casualties are low (shield volcanoes). High-silica andesitic and rhyolitic magma is viscous and traps gas, so eruptions are explosive, producing the lethal hazards: pyroclastic flows (fast hot density currents), heavy tephra and ash fall, and lahars (volcanic mudflows). The same factors that build a steep stratovolcano make it a mass-casualty hazard.

How do you calculate the metal contained in an orebody, and what is the cut-off grade?

The contained metal equals the tonnage multiplied by the grade expressed as a fraction, so 50 million tonnes at 0.8 percent copper contains 50,000,000 times 0.008 = 400,000 tonnes of copper. The common trap is forgetting to convert the percentage to a fraction. The cut-off grade is the lowest grade that can be mined and processed at a profit at current prices and costs: rock above it is ore, rock below it is waste. Because the cut-off depends on price and costs, the same rock can be ore or waste, and reserves (the economically extractable part) change with the economics.

What must coincide for an oil or gas field to exist?

Six elements must all be present and correctly timed: a source rock (organic-rich shale), maturation (burial to the oil window, about 60 to 120 degrees Celsius, or hotter for gas), a migration path, a reservoir rock with both porosity (storage) and permeability (connected pores allowing flow), an impermeable seal or cap rock, and a trap (anticline, fault, stratigraphic or salt dome). Crucially the trap must have formed before the hydrocarbons migrated, or they would have escaped. In a trap the fluids stack by density: gas on top, oil beneath, water below.

What is Darcy's law and why does groundwater move slowly?

Darcy's law gives the groundwater flow velocity as v = K times i, where K is the hydraulic conductivity (permeability) and i is the hydraulic gradient (the head difference divided by the distance). The volume flow rate is Q = K times i times A. Groundwater moves slowly (often centimetres to a metre per day) because it must thread through small, tortuous connected pores, so aquifers recharge slowly and any pollution, though slow to spread, is persistent and costly to clean up. The common slip is forgetting to divide the head difference by the distance to get the gradient.

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Eduqas A-Level Geology Geohazards and economic geology: earthquakes, volcanoes, ore, oil and water

A deep-dive Eduqas A-Level Geology guide to the Geohazards and economic geology module that drives Component 3. Covers earthquake, volcanic and landslide hazards and their mitigation, ore deposits, grade and reserves, the petroleum system, and groundwater and Darcy's law, with the exam patterns Eduqas repeats.

Generated by Claude Opus 4.819 min readEduqas-A220-Geohazards-EconomicUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this module actually demands
Earthquake hazards
Volcanic hazards
Mass movement and landslides
Ore deposits
Hydrocarbons
Groundwater
How this module is examined
Check your knowledge

What this module actually demands

Geohazards and economic geology is the applied module that drives Component 3 (Geological Applications). It runs from the hazards that threaten people (earthquakes, volcanoes, landslides) and how to reduce them, to the resources society extracts from the crust (metal ores, oil and gas, water). The examiners test two linked skills: explaining the processes and controls (what makes a hazard severe, how an ore or oilfield forms), and the quantitative and decision tasks (contained metal, Darcy's law, judging a slope or a prospect).

This guide walks through the six clusters in a sensible order, then sets out the exam patterns Eduqas repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Earthquake hazards

The primary hazard is ground shaking; the secondary hazards it triggers (liquefaction, landslides, tsunami, fire) are often the bigger killers. Hazard is the event, exposure the people and assets in the zone, vulnerability their susceptibility, and risk the combined likelihood of harm. Damage for a given magnitude depends on focal depth, distance, ground conditions (soft sediment amplifies and liquefies), building quality and preparedness. Earthquakes cannot be precisely predicted, so mitigation relies on monitoring, hazard mapping, resistant design, early warning and education.

Volcanic hazards

The hazards (lava, pyroclastic flows, tephra, lahars, gases, sector collapse) follow from magma composition: basaltic eruptions are effusive (lava, low casualties), andesitic and rhyolitic eruptions explosive (the lethal pyroclastic flows and lahars). Unlike earthquakes, volcanoes give warning signs, so eruptions can be forecast by monitoring rising seismicity, ground inflation, gas emissions and thermal changes, allowing evacuation, the most effective mitigation.

Mass movement and landslides

Slopes fail as rockfalls, slides, slumps, flows and creep when the driving force (gravity on the slope) exceeds the resisting friction and cohesion. The controls are slope angle and undercutting, water (raising pore pressure, the master control), rock strength, and bedding or joints dipping out of the slope (a slip surface). Failure is triggered by heavy rain, earthquakes or loading. The hazard is reduced by regrading, drainage (the most effective measure), retaining structures and anchors, toe protection and planning.

Ore deposits

Metals are concentrated into ore by magmatic segregation, hydrothermal vein deposits (precipitation from hot fluids), placer deposits (density sorting of gold and cassiterite), secondary enrichment and sedimentary processes (banded iron, bauxite). Grade is the metal concentration, the cut-off grade the lowest profitable grade (separating ore from waste, changing with price), and reserves the economically extractable part. Contained metal equals tonnage times grade as a fraction.

Hydrocarbons

A petroleum accumulation needs six elements correctly timed: a source rock, maturation (the oil window), migration, a reservoir with porosity and permeability, a seal, and a trap (anticline, fault, stratigraphic, salt) that predates migration. In a trap, gas, oil and water stack by density. Porosity is storage; permeability is the ability to flow (connected pores); a reservoir needs both.

Groundwater

Water is stored in porosity and moves through permeability. An aquifer is porous and permeable; an aquiclude is impermeable; the water table is the top of the saturated zone; artesian conditions arise where a confined aquifer is recharged at height. Flow follows Darcy's law, $v = K i$ . Over-abstraction lowers the water table, causes subsidence and draws in seawater (saline intrusion), and aquifers are vulnerable to persistent pollution.

How this module is examined

A typical Eduqas profile for Geohazards and economic geology (Component 3):

Calculation questions. Contained metal from grade and tonnage, and groundwater velocity from Darcy's law, the predictable quantitative tasks here.
Process and control questions. What makes an earthquake or eruption severe, how an ore deposit or oilfield forms, and what controls slope stability.
Decision and evaluation questions. Judging a slope, a prospect or a mitigation strategy, and distinguishing hazard, vulnerability and risk.
Levels-of-response extended answers. Comparing volcano types and their hazards, explaining over-abstraction, and evaluating mitigation are predictable extended questions.

Eduqas Geohazards and economic geology drives Component 3. The geohazards: earthquakes (primary shaking plus secondary liquefaction, landslides, tsunami and fire; mitigated by monitoring, resistant design, hazard mapping and early warning), volcanoes (lava versus the lethal pyroclastic flows and lahars, set by magma composition; forecast from seismicity, inflation and gas), and mass movement (slides, slumps, flows and creep, controlled by slope angle, water and out-of-slope bedding, stabilised by drainage, regrading and retaining structures). The resources: ore deposits (magmatic, hydrothermal, placer, secondary and sedimentary; grade, cut-off grade and reserves; contained metal = tonnage times grade), hydrocarbons (source, maturation, migration, reservoir, seal and trap, all timed), and groundwater (aquifers, the water table, artesian conditions and Darcy's law $v = K i$ , with over-abstraction causing falling tables, subsidence and saline intrusion). Master the contained-metal and Darcy's-law calculations and the hazard-vulnerability-risk framework.

Check your knowledge

A mix of recall and application questions covering the whole module. Attempt them under timed conditions, then check against the solutions.

State one primary and two secondary hazards of an earthquake. (3 marks)
Explain the difference between hazard and risk. (2 marks)
Explain why an andesitic volcano is more hazardous than a basaltic one. (2 marks)
Explain why water reduces slope stability. (2 marks)
An orebody is 40 million tonnes at 1.2 percent copper. Calculate the mass of copper. (2 marks)
State the six elements needed for an oil or gas accumulation. (3 marks)
State the difference between porosity and permeability. (2 marks)
An aquifer has a hydraulic conductivity of 10 m/day and a hydraulic gradient of 0.03. Calculate the flow velocity. (2 marks)
State two problems caused by over-abstraction of groundwater. (2 marks)

Solutions

Q1: Primary: ground shaking. Secondary (any two): liquefaction, landslides, tsunami, fire.
Q2: The hazard is the natural event with the potential to cause harm; risk is the likelihood of harmful consequences, which also depends on the exposure and vulnerability of the people and structures.
Q3: Andesitic magma is high in silica, so it is viscous and traps gas, erupting explosively with lethal pyroclastic flows and lahars; basaltic magma is runny and erupts effusively, mainly producing escapable lava.
Q4: Water raises the pore pressure between grains, reducing the effective stress and so the friction holding the slope together, and it adds weight; saturation can turn soil into a flow.
Q5: 1.2 percent = 0.012; copper = 40,000,000 times 0.012 = 480,000 tonnes.
Q6: A source rock, maturation, migration, a reservoir, a seal (cap rock) and a trap, all correctly timed (the trap predating migration).
Q7: Porosity is the percentage of pore space (how much fluid the rock can store); permeability is the ability to transmit fluid through connected pores (how easily it flows). A reservoir or aquifer needs both.
Q8: v = K times i = 10 times 0.03 = 0.3 metres per day.
Q9: Any two of: a falling water table (drying wells and springs); land subsidence as pore pressure falls; saline intrusion of seawater at the coast.