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Earthquakes and Volcanoes
Structure of the Earth
Crust: thin, solid outer layer (continental & oceanic).
Mantle: semi‑molten rock (magma), convection currents drive plate movement.
Lithosphere: crust + upper mantle; broken into tectonic plates.
Convection currents: heat from core causes mantle to circulate → plates move.
Core:
Outer core → liquid iron/nickel.
Inner core → solid iron/nickel, ~5000°C.
Distribution & Causes
Global pattern: earthquakes & volcanoes concentrated along plate boundaries.
Plate boundaries:
Constructive (divergent): plates move apart → magma rises → shield volcanoes, earthquakes.
Destructive (subduction): oceanic plate sinks under continental → explosive stratovolcanoes, earthquakes.
Collision (convergent): two continental plates collide → fold mountains, earthquakes (no volcanoes).
Conservative (transform): plates slide past → earthquakes only.
Hotspots: isolated volcanic activity (e.g., Hawaii).
Tectonic Plates
Magnitude & Richter Scale
Magnitude: energy released by earthquake.
Richter scale: logarithmic, each step = 10× increase in amplitude.
Minor (<4), Moderate (5–6), Major (7+).
Volcanic Eruptions
Shield volcanoes: gentle slopes, basaltic lava, frequent but less violent.
Stratovolcanoes: steep, alternating lava/ash layers, explosive eruptions.
VEI (Volcanic Explosivity Index): scale 0–8 measuring eruption size.
Impacts: lava flows, ash clouds, pyroclastic flows, lahars, global climate effects.
Reducing Earthquake Risk
Before: hazard maps, building codes, drills, emergency kits.
During: drop‑cover‑hold, avoid glass/buildings.
After: rescue, medical aid, rebuilding with safer structures.
Tropical Cyclones
Distribution
Form 5°–20° latitude N & S.
Ocean temp ≥27°C, depth ≥60m.
Warm moist air rises → low pressure → Coriolis effect spins system.
Causes
Warm ocean → rapid evaporation → moist air rises.
Coriolis effect spins system → spiral winds.
Low pressure center → “eye” of cyclone.
Life Cycle
Disturbance: cluster of thunderstorms over warm ocean.
Depression: organized low pressure, winds <39 mph.
Storm: stronger winds (39–73 mph), spiral bands form.
Cyclone/Hurricane/Typhoon: winds ≥74 mph, eye develops, peak intensity.
Impacts
People: deaths, injuries, homelessness, disease spread.
Environment: flooding, coastal erosion, habitat destruction.
Reducing Risk
Before: forecasting, evacuation plans, shelters.
During: stay indoors, emergency supplies.
After: relief aid, rebuilding, improved defenses.
Flooding
Causes
Natural:
Heavy rainfall: overwhelms rivers → flash floods.
Prolonged rainfall: soil saturation → river overflow.
Snowmelt: sudden thaw → rivers swell.
Land relief: steep slopes → rapid run‑off.
Saturated/compacted soil: reduces infiltration → more surface water.
Storm surges: cyclones push seawater inland.
Tsunamis: seismic waves flood coastlines.
Sea level rise (climate change): gradual permanent flooding risk.
Physical factors: steep relief, saturated/compacted soil.
Human factors: deforestation, cultivation, urbanization → reduced infiltration, increased run‑off.
Impacts
People: loss of life, property damage, displacement, disease.
Environment: soil erosion, habitat loss, pollution spread.
Reducing Risk
Before: flood defenses, afforestation, land‑use planning.
During: evacuation, emergency shelters.
After: relief aid, rebuilding, improved drainage.
Drought
Causes
Meteorological: prolonged high pressure → lack of rain.
ENSO (El Niño Southern Oscillation):
El Niño: warming of Pacific → reduced rainfall in SE Asia/Australia.
La Niña: cooling → opposite effects.
Climate change: altered rainfall patterns, higher evaporation.
Impacts
People: famine, water shortages, migration, economic loss.
Environment: desertification, crop failure, biodiversity loss.
Reducing Risk
Before: water storage, drought‑resistant crops, monitoring.
During: rationing, emergency aid.
After: recovery programs, reforestation, improved irrigation.
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