Earth's Structure and Plate Tectonics

Overview

This lecture covers the structure of the Earth, tectonic processes and boundaries, causes and consequences of tectonic hazards, disaster risk models, measurement and management of tectonic disasters, and strategies to reduce vulnerability and loss. It also explains key case studies, hazard management cycles, and the relationship between development, vulnerability, and disaster impacts.

Earth's Structure

• The Earth is composed of several distinct layers, each with unique properties:
  • Lithosphere: The rigid, outermost layer, made up of the crust and the uppermost part of the mantle. It is broken into tectonic plates that move over time.
  • Asthenosphere: Located beneath the lithosphere, this is a semi-fluid, ductile region of the upper mantle. The asthenosphere allows tectonic plates to move and is the site of convection currents.
  • Mantle: A thick, mostly solid layer of rock between the crust and the core. Convection currents in the mantle, driven by heat from radioactive decay, are responsible for plate movement.
  • Mesosphere: The lower part of the mantle, where rocks are brittle and can break under stress.
  • Outer Core: A fluid layer composed mainly of iron and nickel, located between the mantle and the inner core. Its movement generates Earth's magnetic field.
  • Inner Core: The solid, dense center of the Earth, made of an iron-nickel alloy. Despite high temperatures, it remains solid due to immense pressure.
• Convection currents in the mantle are crucial for driving the movement of tectonic plates.

Types of Plate Boundaries

• There are three main types of plate boundaries, each associated with specific processes and hazards:
  • Convergent (Destructive) Boundaries: Two plates move towards each other. The denser plate is subducted beneath the other, melting into the mantle. This process causes the largest and most damaging earthquakes and creates explosive volcanoes. Examples: Japan 2011 earthquake and tsunami, Nepal 2015 earthquake, Pinatubo 1991 eruption.
  • Conservative Boundaries: Plates slide horizontally past each other. No crust is created or destroyed. These boundaries produce shallow-focus earthquakes, which can be highly destructive, but do not cause volcanic activity. Examples: Los Angeles 1994 earthquake, Christchurch 2011 earthquake.
  • Divergent (Constructive) Boundaries: Plates move apart, allowing magma to rise and form new crust. This is most evident at mid-ocean ridges. Earthquakes here are frequent but generally low-risk, and volcanoes tend to be effusive rather than explosive. Examples: 2002 Mount Nyiragongo eruption, mid-Atlantic ridge.

Plate Tectonic Theory

• Plate movement is driven by several interconnected processes:
  • Convection: Heat from radioactive decay in the mantle creates convection currents in the asthenosphere, moving tectonic plates.
  • Slab Pull: As newly formed oceanic crust cools and becomes denser, it sinks into the mantle, pulling the rest of the plate with it.
  • Subduction: One plate moves beneath another and is destroyed in the mantle, balancing the creation of new crust at divergent boundaries.
  • Seafloor Spreading: Magma rises at divergent boundaries, creating new crust and pushing plates apart.
  • Paleomagnetism: As lava cools, minerals align with Earth's magnetic field, which reverses every ~400,000 years. These patterns in rocks provide evidence for plate movement.

Detailed Plate Boundaries & Features

• Convergent Boundaries:
  • Oceanic-Continental: The denser oceanic plate subducts beneath the continental plate, forming deep ocean trenches, volcanic arcs, fold mountains, and causing earthquakes and volcanic eruptions.
  • Oceanic-Oceanic: One oceanic plate subducts under another, creating deep trenches, island arcs, submarine volcanoes, and earthquakes.
  • Continental-Continental: Two continental plates collide, neither subducts. The crust uplifts and folds, forming fold mountains and causing earthquakes, but little to no volcanism.
• Divergent Boundaries:
  • Oceanic-Oceanic: Two oceanic plates move apart, magma rises to form new crust, creating mid-ocean ridges, volcanic activity, and earthquakes.
  • Continental-Continental: Two continental plates move apart, forming rift valleys or volcanoes as the crust stretches and thins, with associated volcanic activity and earthquakes.
• Conservative Boundaries:
  • Plates slide past each other horizontally, creating strike-slip faults and earthquakes, but no volcanic activity.

Volcanic Hazards

• There are seven main volcanic hazards to be aware of:
  • Pyroclastic Flows: Fast-moving, extremely hot currents of gas, ash, and rock fragments that flow down volcano slopes, reaching temperatures up to 1,000°C. They are highly destructive at ground level and result from explosive eruptions.
  • Ash (Tephra): Material ejected into the atmosphere during eruptions, ranging from fine ash to large volcanic bombs (>32 mm). Ash can collapse roofs and disrupt air travel.
  • Lava Flows: Streams of molten rock that move away from a volcano. Their speed and danger depend on viscosity, which is influenced by silicon dioxide content.
  • Lahars: Volcanic mudflows formed when volcanic material mixes with water (often from rainfall). They can travel rapidly and cause significant destruction.
  • Volcanic Gases: Explosive eruptions release gases such as water vapor, sulfur dioxide, hydrogen, and carbon monoxide. These can cause acid rain and have environmental, social, and economic impacts.
  • Tsunamis: Large, destructive waves generated when volcanic activity displaces a large volume of water.
  • Jökulhlaups: Sudden glacial outburst floods caused when a lake fed by glacial meltwater breaches its dam, releasing water catastrophically.

Hotspot Volcanoes & Mantle Plumes

• Hotspot volcanoes form away from plate boundaries due to stationary mantle plumes—columns of hot material rising from deep within the Earth.
• As a tectonic plate moves over a hotspot, magma rises through the crust, forming a chain of volcanoes. The hotspot remains stationary, so older volcanoes become extinct as the plate moves. Example: the Hawaiian Islands.

Earthquakes & Tsunamis

• Earthquakes occur when tectonic strain builds up in the crust, storing elastic energy. When the stress exceeds the strength of rocks, they fracture, releasing energy as seismic waves:
  • P-waves (Primary): Fastest, compressional waves that travel through solids and liquids (8 km/s).
  • S-waves (Secondary): Slower, move at right angles to direction of travel, cannot pass through liquids (4 km/s).
  • L-waves (Love): Surface waves with high amplitude, causing the most damage.
• Secondary hazards:
  • Soil liquefaction: Waterlogged, loosely packed sediments lose strength during shaking, causing buildings to sink or collapse (e.g., 1989 Loma Prieta earthquake).
  • Landslides: Common in mountainous areas, can travel long distances and cause extensive damage (e.g., Nepal).
• Tsunamis:
  • Formed by sudden displacement of water due to undersea earthquakes or volcanic activity.
  • Large waves travel across the ocean, increasing in height as they approach shallow coastal areas.
  • Impact depends on factors such as wave amplitude, water depth, beach gradient, time of day, level of development, infrastructure strength, warning systems, and notice given.

Natural Hazards, Disasters & Vulnerability

• Hazard: A perceived natural event with the potential to threaten life and property.
• Disaster: The realization of a hazard causing significant impact on a vulnerable population.
• Vulnerability: The ability to anticipate, cope with, resist, and recover from a disaster.
• Resilience: The capacity to protect lives, livelihoods, and infrastructure, and to restore areas after a disaster.
• Disaster models:
  • DEG’s Model: Visualizes that a disaster only occurs when a vulnerable population is exposed to a hazard (similar to a Venn diagram).
  • Pressure and Release Model: Shows vulnerability as dynamic and progressive, with root causes, dynamic pressures, and unsafe conditions all contributing to disaster risk.
• Hazard risk equation: Disaster risk = hazard x vulnerability / capacity to cope. Capacity refers to a country’s ability to withstand and manage hazard events.

Measuring Hazards

• Earthquake measurement:
  • Richter Scale: Measures the amplitude of seismic waves (0–9), absolute and location-independent.
  • Moment Magnitude Scale: Modern scale measuring the energy released by an earthquake, based on seismic moment.
  • Mercalli Scale: Assesses the observed impacts and damage, subjective and based on human experience.
• Volcanic Explosivity Index (VEI): Measures the size of explosive volcanic eruptions using volume of products, eruption cloud height, and qualitative observations.

Development, Vulnerability & Disaster Impact

• The relationship between development and disaster risk is complex:
  • Development can reduce risk: Improved infrastructure, access to clean water, high-quality buildings, and poverty reduction increase resilience.
  • Development can increase risk: Unsustainable development can create unsafe conditions, environmental degradation, and social inequality, increasing vulnerability.
  • Disasters can destroy development: Physical assets, infrastructure, and livelihoods are lost, reducing production capacity and market access. Workforce losses can have long-term effects.
  • Disasters can create development opportunities: Post-disaster, there may be more willingness to invest in risk reduction and resilience.
  • Risk-poverty nexus: Poverty and disaster impacts are closely linked. Poor populations are more vulnerable, suffer greater losses, and have less capacity to recover, perpetuating the cycle of poverty.
  • Governance: Effective governance reduces disaster impacts. Poor governance, as seen in the Haiti 2010 earthquake, leads to inadequate preparation, weak infrastructure, and slow response, worsening outcomes.

Trends, Statistics & Mega Disasters

• Disaster trends:
  • The number of recorded disasters has increased since 1960 due to climate change, population growth, urbanization, environmental degradation, and improved detection/reporting.
  • Human activities (e.g., fossil fuel burning, deforestation) intensify hazards.
  • Advanced technology allows for better disaster tracking, but data quality varies by region and is affected by resources, infrastructure, and politics.
• Mega disasters:
  • Characterized by massive physical, social, or economic impacts.
  • Require immediate international aid and can affect multiple countries.
  • Infrequent but severe, and cannot be fully mitigated.
• Disaster hotspots:
  • Regions highly vulnerable to multiple hazards due to environmental and human factors (e.g., Bangladesh: frequent flooding, cyclones, river erosion).

Hazard Management Models

• Hazard management cycle: Involves four stages—preparation, response, recovery, and mitigation—to reduce impacts and improve resilience.
• Park’s Model (Disaster Response Curve): Illustrates the phases of recovery after a disaster, showing initial disruption, immediate impact, recovery, and potential improvement beyond pre-disaster conditions. Highlights differences in recovery time and resilience among communities.

Prediction and Forecasting

• Earthquakes:
  • Cannot be predicted precisely in terms of time, location, or magnitude.
  • Probability estimates are made using:
    • Ground deformation monitoring: GPS and radar detect crustal changes indicating stress buildup.
    • Early detection systems: Detect seismic waves and provide short-term alerts.
    • Historical data analysis: Identifies patterns, but no definitive predictive links.
• Volcanic eruptions:
  • Can be forecasted in advance, allowing for risk mitigation.
  • Key methods:
    • Increased seismic activity: Detected by seismometers as magma rises.
    • Gas emissions: Changes in sulfur dioxide and carbon dioxide levels indicate rising magma.
    • Thermal imaging: Satellites and infrared sensors detect surface temperature changes.
  • Other methods exist, but these are the most important for exam preparation.

Modifying Hazards, Vulnerability & Loss

• Modifying the event:
  • Hazard avoidance: Land zoning to prevent development in high-risk areas.
  • Engineering defenses: Building barriers, retrofitting homes, and designing hazard-resistant structures (e.g., bracing walls, reinforcing pillars, installing window shutters).
  • Lava diversion: Using barriers or trenches to redirect lava flows (difficult and costly).
  • Ashfall management: Techniques like water cannons to prevent ash buildup on roofs and roads.
  • Safe zones and evacuation plans: Ensuring people can quickly move away from danger.
• Modifying vulnerability:
  • Prediction and warning: Early warning systems for hazards.
  • Community preparedness: Education, emergency kits, evacuation plans, and understanding warning systems.
  • Planning and zoning: Regulating construction in high-risk areas.
  • Stronger infrastructure: Earthquake-resistant buildings, flood barriers.
  • Economic development: Wealthier communities have more resources for preparation and recovery.
• Modifying loss:
  • Aid: Providing support to those in poverty after disasters.
  • Insurance: Helps manage property loss, more accessible to wealthier populations.
  • Emergency relief: Reduces losses from secondary hazards.

Key Terms & Definitions

• Lithosphere: Rigid outer layer (crust and upper mantle).
• Asthenosphere: Semi-fluid mantle layer beneath the lithosphere, enables plate movement.
• Subduction: Process where one plate moves beneath another into the mantle.
• Pyroclastic flow: Fast, hot avalanche of volcanic gas, ash, and debris.
• Lahar: Volcanic mudflow triggered by mixing volcanic material and water.
• Jökulhlaup: Sudden glacial outburst flood.
• Disaster hotspot: Region exposed to multiple frequent natural hazards.
• Hazard risk equation: Risk = hazard x vulnerability / capacity.
• Retrofitting: Strengthening existing structures for hazard resistance.
• Resilience: Ability to protect and restore after a disaster.
• Vulnerability: Susceptibility to harm from hazards.

Action Items / Next Steps

• Review case study summaries for Japan 2011, Nepal 2015, Pinatubo 1991, Haiti 2010, and Bangladesh.
• Watch the 5-minute summary of the Mount Pinatubo eruption.
• Study diagrams and models: DEG, Pressure & Release, Park’s model, hazard management cycle.
• Revise key terms and hazard management strategies for exams.
• Familiarize yourself with the features and impacts of different plate boundaries and volcanic hazards.
• Understand the links between development, vulnerability, and disaster impacts for use in exam answers.