Geography (9696) | Hazards resulting from tectonic processes

Welcome to Hazardous Environments: Tectonic Processes!

Hi there! This chapter might seem intimidating because it deals with massive natural forces, but don't worry. We're breaking down how the Earth’s movement (tectonics) creates some of the most dramatic and dangerous natural events—earthquakes and volcanoes—and, crucially, how humans attempt to live alongside them.
Understanding these processes is key to evaluating global risk and management strategies. Let's dive in!

1. The Global Distribution of Tectonic Hazards

Relating Hazards back to Plate Tectonics

Tectonic hazards (earthquakes and volcanoes) are not random; their distribution is directly linked to the edges of the Earth's tectonic plates.

Quick Review: Types of Plate Boundaries

• Divergent (Constructive): Plates move apart (e.g., Mid-Atlantic Ridge). Causes frequent, but usually gentle, earthquakes and quiet lava eruptions.
• Convergent (Destructive): Plates move toward each other, often resulting in subduction (one plate sinking beneath the other) (e.g., Pacific coast of South America). Causes the most powerful earthquakes and explosive volcanoes.
• Conservative: Plates slide past each other (e.g., San Andreas Fault, California). Causes powerful earthquakes but no volcanic activity.

Key Distribution Patterns

The vast majority of tectonic hazards occur along these plate boundaries.

• The Pacific Ring of Fire: This is the single most important location. It is a horseshoe shape around the Pacific Ocean basin, characterized by intense subduction zones, causing approximately 90% of the world's earthquakes and housing over 75% of the world's active volcanoes.
• The Mediterranean/Alpine-Himalayan Belt: A major convergent zone stretching from Europe through Asia.
• Mid-Ocean Ridges: Locations of divergent boundaries, generating underwater volcanic activity and shallow earthquakes.
• Note: Although rare, hazards can occur in the middle of plates (intra-plate), often linked to old fault lines or hotspots (like Hawaii).

Key Takeaway: If you can map the plate boundaries, you have mapped the hazardous zones. Convergent boundaries are generally the most hazardous.

2. Earthquake Hazards and Their Results

When plates shift suddenly, energy radiates out as seismic waves, causing an earthquake. The resulting hazards can be grouped into two categories: Primary (direct results) and Secondary (events triggered by the primary hazard).

Primary Earthquake Hazard: Shaking

Shaking (Ground Motion): The ground moves violently, causing buildings, bridges, and infrastructure to collapse. The intensity of shaking depends on the earthquake magnitude (energy released) and the distance from the epicentre.

Secondary Earthquake Hazards

These are the dangerous effects that often happen after the initial shaking stops.

1. Landslides and Mass Movement

On steep slopes, the intense shaking destabilises rock and soil. This triggers rapid downhill movement, such as rockfalls or slumps.
Example: The 1994 Northridge earthquake in California triggered thousands of landslides, blocking roads and complicating rescue efforts.

2. Soil Liquefaction

Soil Liquefaction is a massive secondary hazard that occurs when saturated (water-filled) soil behaves like a liquid when violently shaken.
Step-by-Step Explanation:

1. Loose, water-logged soil is stable because the soil grains touch each other.
2. Earthquake shaking increases the pressure in the water between the grains.
3. The water pressure forces the grains apart, making the soil lose all its strength.
4. The ground becomes fluid, causing structures to sink, tilt, or collapse intact.

Analogy: Think of a sandy beach. If you stand on wet sand, it’s firm. If you quickly stamp your feet, the water rises to the surface and the sand turns mushy—that’s liquefaction!

3. Tsunami (Seismic Sea Waves)

A Tsunami is a series of extremely large waves generated by the sudden vertical displacement of the seabed, usually caused by a large underwater earthquake in a subduction zone (oceanic plate slips beneath continental plate).

• Mechanism: The movement of the plate boundary rapidly lifts or drops the water column above it.
• In the deep ocean, the wave is long but low (hard to spot).
• As it approaches shallow coasts, the wave energy compresses, creating massive, fast-moving surges that can travel far inland.

Common Mistake to Avoid: A tsunami is not a tidal wave. It is caused by geological movement, not the tide.

3. Volcanic Hazards and Their Products

Volcanic hazards are highly diverse, depending on the type of magma (viscous vs. runny) and the eruption style (explosive vs. effusive).

The Products of Volcanic Eruptions

You need to know the specific, dangerous materials volcanoes produce.

1. Nuées Ardentes / Pyroclastic Flows

Nuées Ardentes (French for "glowing clouds") are the most lethal volcanic hazard. They are fast-moving currents of extremely hot gas and rock fragments (tephra) that rush down the volcano flank.

• Characteristics: Can travel over 700 km/h and reach temperatures up to 1000°C.
• Impact: Destroys everything in its path through heat and kinetic force. Instant death upon contact.
• Analogy: Imagine a superheated, dense avalanche.

2. Lava Flows

Streams of molten rock (magma) moving across the land surface.

• Runny (Basaltic) Lava: Found in shield volcanoes (divergent/hotspots). Moves quickly but usually follows predictable paths.
• Viscous (Andesitic/Rhyolitic) Lava: Found in composite cones (convergent zones). Moves slowly, but can block valleys and cause explosions.
• Impact: Destruction of property through burning and burying. Because it's slow, it rarely causes direct death.

3. Volcanic Mudflows (Lahars)

A Lahar is a dangerous, rapid-moving slurry (mixture) of volcanic ash, rock debris, and water.

• Cause: Eruption heat melts snow/ice on the volcano, or heavy rainfall mixes with loose ash deposits.
• Impact: They flow like wet cement, burying towns, roads, and agricultural land far from the volcano peak. They are a secondary hazard.

4. Ash Fallout (Tephra)

Solid material ejected into the atmosphere, ranging from fine dust (ash) to large rocks (bombs).

• Near-field Impact: Heavy ash can cause roof collapse and destroy crops.
• Far-field Impact: Airborne ash disrupts aviation and causes respiratory problems; it can also affect global climate temporarily.

5. Volcanic Landslides and Debris Avalanches

The steep sides of composite volcanoes are often unstable. Eruptions or earthquakes can cause large sections of the cone to collapse, creating fast-moving landslides, similar to those triggered by earthquakes (secondary hazard).

Key Takeaway: Lava flows are manageable; pyroclastic flows (nuées ardentes) and lahars are the greatest risks to human life.

4. Primary and Secondary Impacts on Lives and Property

When discussing impacts, remember to always separate the immediate consequence (primary) from the resulting chain of events (secondary).

Primary Impacts (Immediate Results)

• Lives: Instantaneous death from shaking, being hit by falling debris, or incineration by pyroclastic flows.
• Property: Collapse of buildings due to ground shaking or burial by lava/ash. Destruction of utility lines (gas, water, electricity).

Secondary Impacts (Resulting Consequences)

These impacts often cause more long-term damage and death than the primary event.

• Lives:
  • Disease and infection from contaminated water (due to broken pipes or tsunamis).
  • Starvation and injury due to difficulty accessing aid (blocked roads).
  • Long-term respiratory illness from volcanic ash inhalation.
• Property & Economy:
  • Fires sparked by broken gas lines and downed power cables (very common in earthquakes).
  • Loss of land stability due to liquefaction or landslides, making reconstruction impossible.
  • Insurance costs rising and loss of tourism/agricultural income.
  • Did you know? Tectonic events in one country can affect global supply chains (e.g., disruption to microchip production in Japan).

Key Takeaway: Primary impacts kill instantly; Secondary impacts usually cause sustained injury, economic devastation, and long-term misery.

5. Management: Prediction, Monitoring, and Risk Perception

While we cannot stop tectonic hazards, effective management significantly reduces vulnerability and risk. This involves science (monitoring) and human actions (preparedness).

A. Monitoring and Prediction (Earthquakes)

Prediction (forecasting the exact time and place) is impossible, but monitoring allows for early warning systems and long-term risk assessment.

• Seismometers: Record tiny tremors (foreshocks) which can sometimes precede a major quake.
• Radon Gas Emissions: Some scientists monitor changes in radon gas release from the ground, which may increase if rocks are fractured before a quake.
• Measuring Ground Deformation: Highly accurate satellite measurements (GPS, remote sensing) track very slow ground movement along fault lines.
• Tsunami Warning Systems: Systems using sea-floor pressure sensors (DART buoys) detect wave formation and relay warnings to coastal areas, offering crucial evacuation time.

B. Monitoring and Prediction (Volcanoes)

Volcanoes are generally easier to predict than earthquakes because they show signs over days or weeks.

• Seismicity: Monitoring small earthquakes (tremors) caused by rising magma cracking the rock.
• Ground Deformation (Tiltmeters): Instruments measure the 'swelling' or 'bulging' of the volcano flank as magma moves into the chamber.
• Gas Analysis: Increased emission of sulphur dioxide (\(\text{SO}_2\)) or carbon dioxide (\(\text{CO}_2\)) indicates magma is rising closer to the surface.
• Temperature Monitoring: Thermal cameras detect rising heat around vents.

C. Hazard Mapping and Preparedness

Hazard Mapping: This involves creating maps that show which areas are most likely to be affected by specific hazards (e.g., areas prone to liquefaction, expected lava paths, or maximum tsunami run-up heights).

• Use: Maps dictate zoning laws (where houses can be built), evacuation routes, and the placement of critical infrastructure (hospitals, fire stations).
• Preparedness (Reducing Vulnerability):
  1. Education: Holding earthquake drills ("Drop, Cover, Hold On") and teaching coastal communities tsunami warning signs.
  2. Infrastructure: Earthquake-proofing buildings (e.g., steel reinforcement, flexible foundations, cross-bracing) to withstand shaking.
  3. Emergency Kits: Ensuring communities have food, water, and first aid ready.

D. Perception of Risk

Why do people choose to live in high-risk zones, such as the slopes of Mount Fuji or the Pacific Coast of Chile?

1. Economic Benefits: Volcanic soils are incredibly fertile (high yields for farming). Geothermal energy is cheap and abundant.
2. Lack of Alternatives: People may be too poor to move, or all local job opportunities are tied to the hazardous region.
3. Fatalism: The belief that the risk is controlled by nature or fate ("If it's going to happen, it will happen," or "It won't happen in my lifetime.").
4. Low Frequency: Hazards like super-volcanic eruptions occur very rarely, making the risk seem insignificant day-to-day.

Key Takeaway: Management shifts focus from stopping the hazard to reducing the human vulnerability to the hazard. Effective strategies combine sophisticated technology (monitoring) with community resilience (preparedness).