Tectonics - Essential Notes

Tectonics - Essential Notes

1. Global Distribution of Hazards

  • Definition of Hazard:

    • A hazard is a potential threat to human life and property.

    • Natural hazards can be classified as:

    • Hydro-meteorological: Caused by climatic processes.

    • Geophysical: Caused by land processes.

  • Geophysical Hazards:

    • Typically occur near plate boundaries where tectonic plates move at different speeds and in different directions, resulting in:

    • Collisions

    • Earthquakes

    • Volcanic activity

    • Earthquakes can also occur within plates, termed intra-plate earthquakes. These are not fully understood but may arise from pre-existing weaknesses in solid crust that crack under pressure.

    • Example: An intraplate earthquake could occur due to the weakening of the crust over time, causing it to crack.

  • Volcanic Hotspots:

    • Areas like the Ring of Fire where magma rises due to upwelling hot molten material from the core, forming volcanic activity.

    • Specific Example: The Hawaii hotspot, where magma rises as plumes of hot rock.

2. Earth’s Structure and Plate Tectonics

  • Earth’s Structure:

    • Divided into four layers: crust, mantle, outer core, inner core.

    • Crust consists of tectonic plates, which are:

    • Oceanic: Thin and dense.

    • Continental: Thick.

  • Movement of Tectonic Plates:

    • Driven by convection currents in the mantle caused by radioactive reactions in the core.

    • At mid-ocean ridges, push and slab pull action occurs:

    • Subduction: Heavier oceanic plates are pushed under lighter continental plates.

    • Example: The Pacific Plate, characterized by significant subduction around its edges.

3. Evidence Supporting Plate Tectonic Theory

  • Wegener’s Continental Drift Theory:

    • Suggests that continents like South America and Africa were once part of a supercontinent, evidenced by matching fossil records found across these continents which now lie separated by oceans.

  • Seismic Waves Analysis:

    • Studying seismic waves along the Wadati-Benioff zones shows the trajectory and depth of subducting oceanic plates.

  • Sea Floor Spreading:

    • Occurs when oceanic plates move apart, allowing magma to rise and form new crust, resulting in the widening of the sea floor.

  • Palaeomagnetism Studies:

    • Magnetic patterns of cooled magma at mid-ocean ridges indicate the historical shifts in the Earth’s magnetic field, helping date the oceanic crust.

4. Plate Boundaries

A. Types of Plate Boundaries

  • Destructive Plate Boundaries:

    • Continental and Oceanic:\n - Denser oceanic plate subducts beneath continental, creating deep ocean trenches and explosive volcanoes on continental land.

    • Oceanic and Oceanic:\n - Heavier oceanic plate subducts, forming ocean trenches and underwater volcanoes which contribute to island arcs.

    • Continental and Continental:\n - Both plates exert pressure without subducting significantly, leading to the formation of fold mountains from the piled-up crust.

  • Constructive Plate Boundaries:

    • Oceanic and Oceanic:\n - Magma rises to fill the gap between separating plates, forming new land and less explosive underwater volcanoes (sea floor spreading).

    • Continental and Continental:\n - Separating land creates rift valleys where volcanoes may form, eventually filling gaps with water.

  • Further Forces Influencing Convergent Boundaries:

    • Ridge Push: Influenced by gravity acting on the elevated slopes when plates move apart.

    • Slab Pull: When an oceanic plate subducts, it pulls the rest of the plate downward, causing more subduction.

B. Conservative Plate Boundaries

  • At these boundaries, parallel plates move in different directions or at varying speeds, causing no landforms to be created as no plates are destroyed.

5. Geophysical Hazards

A. Earthquakes

  • Powerful earthquakes typically occur at destructive and conservative boundaries while constructive boundaries can also produce them.

    • Mechanism:

    • Pressure builds until the crust fails, leading to the sudden release of energy as seismic waves.

    • Earthquakes at conservative boundaries occur when locked plates suddenly slip.

  • Types of Seismic Waves:

    • Primary Waves (P-waves): Initial shock waves that travel rapidly.

    • Secondary Waves (S-waves): Arrive seconds later, have longer wavelengths.

    • Love Waves: Travel horizontally through the crust, causing lateral movement.

    • Rayleigh Waves: Displace land vertically and horizontally due to compressional effects in a rolling manner.

    • Secondary hazards from earthquakes may include landslides, avalanches, and soil liquefaction.

B. Tsunamis

  • Result from underwater earthquakes, particularly at subduction zones, causing massive water displacements and deep trough waves.

  • They compound the destructive effects of earthquakes, leading to higher death tolls and damage to coastal infrastructures, such as seen in Malibu/Santa Monica, California.

C. Volcanic Hazards

  • Active volcanoes occur at constructive and destructive plate boundaries and hotspots with various emissions:

    • Explosive eruptions occur at subduction zones due to rising magma from melting oceanic plates.

    • Various types of volcanic structures can produce differing levels of threats:

    • Super-volcanoes: Highly destructive but rare.

    • Composite cones: Generally more dangerous compared to shield volcanoes.

    • Specific Hazards:

    • Lava flows

    • Phreatic eruptions

    • Pyroclastic flows, which can travel large distances.

    • Secondary hazards: Lahars (mudflows) and jokulhlaups (glacial floods).

6. Hazards, Disasters, and Vulnerability

  • Disaster Definition:

    • A disaster occurs when a hazard impacts human wellbeing.

  • Degg’s Disaster Model:

    • A disaster happens when a hazardous event meets a vulnerable population. Vulnerability indicates susceptibility, and resilience reflects recovery capability.

  • Risk Equation:

    • Risk = (Capacity to Cope Hazard) / Vulnerability

    • Illustrates why similar hazards lead to different disaster outcomes, as exemplified by two earthquakes in Izmit (Turkey) and Kashmir (Pakistan) in 1999 and 2005, respectively.

    • Kashmir faced higher fatalities due to its remote geography and limited access to resources, highlighting how development influences disaster impacts.

7. Measuring Tectonic Hazards

  • Various scales are employed:

    • Magnitude Measurement:

    • Richter Scale: Based on seismic wave arrival times.

    • Moment Magnitude Scale: More accurate, assessing energy release and shockwave movement for magnitude.

    • Intensity Measurement:

    • Mercalli Scale: Assesses damage produced by an earthquake.

    • Volcanic Activity:

    • Volcanic Explosivity Index (VEI): Uses energy released and eruption type to determine scale.

  • Comparative Analysis of Disasters:

    • Tectonic hazards are characterized by magnitude, speed of onset, frequency, and other factors affecting their destructive capacity. The impacts can be categorized into social, economic, environmental, and classified into direct/indirect or short/long-term.

8. Governance and Developmental Aspects of Disasters

  • Vulnerability Factors:

    • Inequalities in education, healthcare, housing, and income influence disaster resilience.

    • Rapid urbanization and population growth further exacerbate vulnerability.

    • The World Risk Report (2014) emphasized the challenges for megacities like Mumbai in reducing risk through effective planning and infrastructure.

  • Preparedness in Economically Developed Countries:

    • Adequate public infrastructure can lessen disaster impacts, but corruption and poor management may exacerbate crises.

9. Management and Mitigation of Tectonic Hazards

A. Trends and Patterns

  • An observed global trend shows that despite decreased deaths, economic losses have escalated due to increased wealth and better preparedness.

  • Monitoring Tectonic Activity:

    • Earthquakes are unpredictable, while volcanoes can be monitored by changes in shape or minor eruptions.

  • Regional disasters can have global ramifications, evidenced by the 2010 Eyjafjallajökull eruption impacting airline operations worldwide.

B. Hazard Hotspot: The Philippines

  • The Philippines faces diverse hazards due to its geographic and meteorological context, including:

    • Volcanic activity (example: Mount Pinatubo eruption in 1991)

    • Landslides

    • Earthquakes

    • Typhoons (typhoon Haiyan in 2013)

    • Drought and flooding events in various years.

10. Hazard Recovery and Management Models

A. Park’s Model

  • Illustrates recovery stages post-disaster, emphasizing the variation in recovery times (longer in low-income countries).

  • Stages of Recovery:

    • Stage 1 - Relief (hours to days): Immediate response including medical aid and foreign assistance.

    • Stage 2 - Rehabilitation (days to weeks): Restoration of services and infrastructure, temporary shelters.

    • Stage 3 - Reconstruction (weeks to years): Full recovery to pre-disaster quality of life, ecosystem restoration.

B. The Hazard Management Cycle

  • Steps involve:

    • Preparedness: Community education, training, and awareness.

    • Response: Immediate actions such as evacuation and medical assistance.

    • Recovery: Long-term restoration processes.

    • Mitigation: Strategies to lessen future hazard impacts (e.g., engineering defences, hazard-resistant buildings).

11. Mitigation and Adaptation Techniques

A. Modifying Hazards

  • Strategies:

    • Land-Use Zoning: Policies to minimize construction in high-risk areas.

    • Hazard-Resistant Buildings: Designing structures to withstand hazards (e.g., aseismic architecture in earthquake-prone areas).

    • Engineering Defences: Creating barriers (e.g., stronger sea walls).

    • Lava Diversion: Using seawater to divert lava flows.

B. Modifying Vulnerability and Resilience

  • Strategies include:

    • Hi-Tech Monitoring: Utilizing satellites for earth changes monitoring.

    • Education and Preparedness: Community awareness and training measures to enhance shock resilience.

C. Strategies to Modify Loss

  • Emergency aid and insurance can promote recovery, reducing dependency on aid organizations and bolstering financial resilience in communities during disasters.

Conclusion

  • Effective management and mitigation of tectonic hazards require comprehensive approaches that incorporate community preparedness, technological monitoring, and government policy to minimize vulnerability and enhance resilience to potential disasters.