Geography (9635) | Volcanic hazards

Welcome to Volcanic Hazards: Living with Earth's Power!

Hi Geographers! This chapter is all about understanding one of nature's most dramatic shows: volcanoes. Studying volcanic hazards is crucial because these events, while rare in some places, can be absolutely catastrophic where they occur. We will explore how volcanoes work, what dangers they pose, and how humans try to minimize the devastating impacts.

Don't worry if the hazard names sound complicated—we'll break them down into simple, memorable concepts! Let's dive into how the Earth's deep processes affect our lives on the surface.

1. Vulcanicity and Plate Tectonics

To understand volcanic hazards, we first need to know why and where volcanoes exist.

What is Vulcanicity?

Vulcanicity (or Volcanism) refers to all the processes associated with the movement of magma and gases from the Earth's interior out onto the surface, leading to the formation of volcanoes and associated landforms.

The Tectonic Connection

Almost all volcanic activity is linked directly to the movement of the Earth’s tectonic plates. Remember from your Plate Tectonics section (3.1.1.2) that there are three main types of plate boundaries:

• Constructive (Divergent) Margins: Plates move apart (e.g., Mid-Atlantic Ridge). Magma rises easily, creating gentle, effusive volcanoes (like shield volcanoes). Hazards here are often less explosive (mainly lava flows).
• Destructive (Convergent) Margins: Plates move towards each other, and one subducts beneath the other (e.g., Pacific Ring of Fire). As the subducting plate melts, magma rises, often mixing with ocean sediment, making it thick and sticky (viscous). This leads to highly explosive volcanoes (like composite cones). Hazards here are often the most dangerous (pyroclastic flows, ash).
• Magma Plumes (Hotspots): These are areas of intense heat far from plate boundaries (e.g., Hawaii). Magma rises directly from the mantle. These volcanoes tend to be effusive (runny lava) and long-lived.

2. The Forms of Volcanic Hazard

The specification requires you to know several specific types of hazards. It’s important to distinguish between those that are hot and fast, and those that are slower or travel further.

2.1 Nuées Ardentes (Pyroclastic Flows)

Key Term: Nuées ardentes (French for "glowing clouds") are better known as pyroclastic flows.

These are the most dangerous volcanic hazard. They are fast-moving clouds of superheated gas, ash, and volcanic rock fragments (tephra).

• Speed: Can travel at over 700 km/h.
• Temperature: Often exceed 1,000°C.
• Impact: Completely incinerates everything in its path. A famous example is the flow that destroyed the town of St. Pierre near Mount Pelée in 1902.

2.2 Lava Flows

Lava flows are streams of molten rock. The viscosity (stickiness) of the lava dictates the hazard level.

• Basaltic Lava (Runny): Common at constructive margins (e.g., Iceland). Flows quickly (up to 50 km/h) but is relatively easy to predict and avoid. It destroys infrastructure and land, but rarely causes deaths directly.
• Andesitic/Rhyolitic Lava (Sticky): Common at destructive margins. Flows slowly but cools and solidifies faster, leading to blockages and more explosive pressure build-up.

2.3 Mudflows (Lahars)

A mudflow, or lahar, is a devastating mix of volcanic ash, rock fragments (tephra), and water.

How do they form?

1. A volcano erupts, covering surrounding slopes in ash (tephra).
2. The heat melts snow/glaciers rapidly (on snow-capped volcanoes) OR heavy rainfall occurs.
3. The water mixes with the ash, creating a cement-like slurry.
4. This dense, fast-moving "volcanic river" rushes down river valleys, capable of burying entire towns far from the volcano itself. (Example: The Nevado del Ruiz eruption in Colombia in 1985, where a lahar killed over 23,000 people in Armero.)

Analogy: Think of mixing dry flour (ash) with water. If you mix enough, you get a thick, heavy pancake batter (lahar) that flows quickly and sets hard once stopped.

2.4 Pyroclastic and Ash Fallout / Tephra

Tephra is the general term for all airborne volcanic rock fragments, regardless of size. Ash fallout is the fine material that settles from the air.

• Close to the vent: Large rocks (bombs) cause immediate damage.
• Further away: Fine ash causes major problems:
  - Roof collapse (ash is heavy when wet).
  - Respiratory illnesses.
  - Engine failure in aircraft (a huge secondary impact).
  - Crop failure and livestock deaths.

2.5 Gases / Acid Rain

Volcanoes release dangerous gases like carbon dioxide (CO₂), sulphur dioxide (SO₂), and carbon monoxide (CO).

• SO₂ mixes with atmospheric moisture to produce acid rain, which damages crops, pollutes water supplies, and corrodes buildings.
• In extreme cases, CO₂ can collect in low-lying areas, suffocating people and animals (as seen at Lake Nyos, Cameroon, in 1986, though this was a limnic eruption, not strictly vulcanicity).

3. Characteristics of Volcanic Events

When we "live with hazards," we must understand their typical behaviour, distribution, and how likely they are to occur.

3.1 Spatial Distribution

Volcanoes are not spread randomly. They follow the patterns of plate boundaries and hotspots.

• The largest concentration is the Pacific Ring of Fire, encircling the Pacific Plate, characterized by destructive margins and highly explosive volcanoes.
• Other areas include divergent boundaries (e.g., Iceland, East African Rift Valley) and isolated hotspots (e.g., Hawaii).

3.2 Magnitude, Frequency, and Regularity

These three concepts help us measure and compare eruptions:

• Magnitude: This refers to the size of the eruption. It is usually measured using the Volcanic Explosivity Index (VEI). The VEI is logarithmic (like the Richter scale), running from 0 (non-explosive, like Hawaiian lava flows) up to 8 (mega-colossal).
• Frequency and Regularity:
  • A volcano with high frequency erupts often (e.g., Kilauea, Hawaii). These are usually small, gentle eruptions (low VEI).
  • A volcano with low frequency might erupt only every few hundred or thousand years (e.g., Yellowstone Supervolcano). These are often extremely high magnitude (high VEI).

  Regularity simply means how predictable the timing is. High-frequency volcanoes are often more regular (predictable intervals). Low-frequency volcanoes are highly irregular and thus more surprising.

3.3 Predictability of Volcanic Events

Unlike earthquakes, volcanoes often give clear warning signs, making them relatively more predictable. This allows authorities to implement effective risk management strategies.

Monitoring signs include:

• Seismicity: Monitoring small earthquakes caused by magma moving under the ground.
• Ground Deformation: Measuring how the volcano's slopes swell or tilt as magma pushes upwards (often using tiltmeters or GPS).
• Gas Emissions: Measuring the increase in sulphur dioxide (SO₂) and carbon dioxide (CO₂) released, indicating magma is rising closer to the surface.
• Thermal Changes: Using satellite imagery to detect rising temperatures on the volcano's surface.

4. Impacts of Volcanic Hazards

Volcanic impacts are wide-ranging and can be categorised in two main ways: by timing (primary/secondary) and by sector (PESTLE).

4.1 Primary vs. Secondary Impacts

Primary Impacts occur immediately as a result of the eruption.

• Examples: Death and injury from pyroclastic flows, destruction of infrastructure by lava flows, suffocation by volcanic gas.

Secondary Impacts occur later, often as a result of the primary impacts.

• Examples: Lahars (mudflows) forming hours or days later; widespread crop failure leading to famine; mental health issues and displacement; long-term economic collapse due to loss of tourism.

Did You Know? Volcanic ash, despite being destructive in the short term, eventually breaks down to release minerals, making the soil around older volcanoes incredibly fertile—a long-term positive environmental impact!

4.2 PESTLE Analysis of Impacts

Environmental Impacts

• Short-term: Acid rain damages forests and surface water; immediate loss of habitat; burial of land by ash/lava.
• Long-term: Increased soil fertility; large eruptions can cool the global climate temporarily (as sulphur aerosols block solar radiation).

Social Impacts

• Short-term: Loss of life and injury; mass displacement (people become refugees or internally displaced); disruption to communication networks.
• Long-term: Loss of livelihoods; psychological trauma; relocation required for communities.

Economic Impacts

• Short-term: Destruction of property, factories, and farmlands; huge costs associated with emergency response and aid; closure of airports and airspace (affecting global travel and trade).
• Long-term: High reconstruction costs; potential boost to tourism (volcano tourism) or loss of key economic sectors (like agriculture).

Political Impacts

• Short-term: Need for national governments to declare states of emergency; managing international aid and cooperation.
• Long-term: Disputes over land ownership after major landscape change; review and enforcement of building codes and hazard zoning.

5. Responses and Risk Management

The goal of human response is risk management—reducing vulnerability and increasing resilience. The responses fall into two categories: short-term and long-term.

5.1 Risk Management Strategies

Risk management involves four core elements:

1. Preparedness: Getting ready for an event.
(E.g., Creating evacuation routes, stocking emergency supplies, public education campaigns.)

2. Mitigation: Reducing the severity of the hazard's effects.
(E.g., Building strong houses to withstand ash weight, constructing barriers to divert lava flows—though these are rarely successful.)

3. Prevention: Although impossible to "stop" a volcano, this refers to preventing the *impact*.
(E.g., Zoning laws preventing settlement in high-risk areas, closing airspace.)

4. Adaptation: Changing behaviour or infrastructure to cope with the hazard.
(E.g., Developing ash-resistant farming techniques, establishing early warning systems.)

5.2 Short-term vs. Long-term Responses

• Short-term (Immediate): Search and rescue, emergency aid, temporary shelter, evacuation orders (usually based on monitoring predictions).
• Long-term (Sustainable): Investment in monitoring equipment, infrastructure reconstruction (often in safer areas), insurance schemes (risk-sharing), long-term land-use planning (prevention).

6. Concluding Thoughts

Volcanic hazards present a complex challenge in the "Living with hazards" theme. While we cannot prevent the Earth's processes, modern technology allows us to monitor, predict, and ultimately manage the risk through strategic long-term planning (mitigation and preparedness), helping human communities live safer, more resilient lives near these powerful natural phenomena.

Keep practicing those key terms—especially the differences between the major hazards (nuées ardentes vs. lahars) and the four pillars of risk management! You've got this!