Key Concepts of Natural Hazards

Why This Matters

Natural hazards sit at the intersection of physical geography's most important systems—plate tectonics, atmospheric dynamics, hydrological cycles, and geomorphological processes. When you're tested on this material, you're not just being asked to name hazards; you're being evaluated on whether you understand what drives these events, how they connect to Earth's larger systems, and why some places are more vulnerable than others. The concepts here—energy release, threshold conditions, cascading effects—show up repeatedly across physical geography.

Don't just memorize that earthquakes happen at fault lines or that hurricanes need warm water. Know why each hazard occurs, what mechanisms control its intensity, and how hazards interact with each other. That's what separates a student who can answer multiple choice from one who nails the FRQ.

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Tectonic and Geologic Hazards

These hazards originate from Earth's internal energy—the slow convection of the mantle and the movement of lithospheric plates. They're concentrated along plate boundaries but can occur anywhere stored crustal stress exists.

Earthquakes

• Caused by sudden energy release along faults when accumulated tectonic stress exceeds rock strength—this elastic rebound generates seismic waves
• Measured by moment magnitude scale (Mw)—replaced the Richter scale for scientific use; each whole number represents roughly 32x more energy released
• Trigger cascading hazards including tsunamis, landslides, liquefaction, and infrastructure collapse—secondary effects often cause more damage than shaking itself

Volcanic Eruptions

• Driven by magma buoyancy and gas pressure—eruption style depends on magma viscosity and dissolved gas content (high silica = explosive; low silica = effusive)
• Produce multiple hazard types including lava flows, pyroclastic flows, lahars, ash fall, and volcanic gases—each with different risk zones and timescales
• Concentrated at plate boundaries—subduction zones produce explosive stratovolcanoes; divergent boundaries and hotspots produce shield volcanoes with gentler eruptions

Tsunamis

• Generated by sudden seafloor displacement—underwater earthquakes, volcanic flank collapses, or submarine landslides displace massive water volumes
• Travel at jet-plane speeds (up to 800 km/h in deep water) but slow and amplify dramatically in shallow coastal waters—wave shoaling concentrates energy
• Require integrated warning systems—Pacific Tsunami Warning Center monitors seismic activity; evacuation effectiveness determines survival rates

Landslides

• Occur when gravitational stress exceeds slope stability—triggered by heavy rainfall (adding weight, reducing friction), earthquakes, or undercutting of slopes
• Classified by movement type—falls, slides, flows, and creep behave differently and require different mitigation approaches
• Often follow other hazards—post-earthquake landslides and post-wildfire debris flows demonstrate hazard cascades that amplify total damage

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Atmospheric Hazards

These hazards derive energy from the sun—differential heating creates pressure gradients, evaporation, and atmospheric instability. They're driven by moisture, temperature contrasts, and wind shear.

Hurricanes/Tropical Cyclones

• Require warm ocean water (≥26°C) to fuel evaporation and latent heat release—this thermal engine drives the low-pressure circulation
• Classified by Saffir-Simpson scale (Categories 1-5) based on sustained wind speed—but storm surge typically causes the most deaths and damage
• Weaken over land or cold water—loss of moisture source cuts off energy supply; this is why tracking landfall location matters for impact prediction

Tornadoes

• Form from rotating updrafts in supercell thunderstorms—wind shear creates horizontal rotation that gets tilted vertical by strong updrafts
• Rated by Enhanced Fujita (EF) scale—assesses damage to estimate wind speeds retrospectively; ranges from EF0 (65-85 mph) to EF5 (200+ mph)
• Concentrated in "Tornado Alley"—where Gulf moisture, cold Canadian air, and dry air from the Rockies create ideal instability conditions

Wildfires

• Spread through the fire triangle—fuel (vegetation), oxygen, and heat must all be present; remove one and fire stops
• Intensified by weather conditions—low humidity, high temperatures, and strong winds create fire weather; climate change is extending fire seasons
• Ignition sources vary—lightning causes natural fires; human activities (power lines, campfires, arson) cause most fires in populated areas

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Hydrological Hazards

These hazards involve water in motion—the hydrological cycle concentrates or redirects water in ways that exceed normal channel capacity or soil infiltration rates.

Floods

• Result from water input exceeding drainage capacity—causes include intense rainfall, rapid snowmelt, dam failure, or storm surge
• Classified by mechanism—flash floods (rapid onset, steep terrain), riverine floods (gradual rise, floodplain inundation), and coastal floods (storm surge, tsunamis)
• Floodplain development increases risk—urbanization replaces permeable surfaces with impervious ones, accelerating runoff and peak discharge

Droughts

• Defined by prolonged precipitation deficit—meteorological drought (lack of rain) leads to agricultural drought (soil moisture deficit) and hydrological drought (low streamflow/groundwater)
• Develop slowly but persist—unlike sudden-onset hazards, droughts accumulate over months and can last years; this makes them economically devastating
• Amplified by feedback loops—dry soil reduces evapotranspiration, which reduces local precipitation recycling, intensifying the drought

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Gravitational Mass Movement Hazards

These hazards involve material moving downslope under gravity—when driving forces (weight, slope angle) exceed resisting forces (friction, cohesion), failure occurs.

Avalanches

• Triggered when snowpack stability fails—weak layers within the snowpack collapse under the weight of overlying snow, often initiated by weather changes or human activity
• Classified by snow type—loose snow avalanches start at a point; slab avalanches release as cohesive blocks and are far more dangerous
• Forecasting relies on snowpack analysis—avalanche centers assess layer structure, temperature gradients, and recent weather to issue danger ratings

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Quick Reference Table

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Self-Check Questions

1. Which two hazards are both driven by tectonic processes but differ significantly in their predictability? Explain why one can be forecasted while the other cannot.

2. Identify three hazards that commonly occur as secondary effects of other hazards. For each, name the primary hazard that triggers it.

3. Compare and contrast hurricanes and tornadoes in terms of their energy source, spatial scale, duration, and warning time.

4. A region experiences a severe drought followed by intense rainfall. Explain why this sequence might produce worse flooding than the same rainfall on non-drought-affected land.

5. FRQ-style: Choose one tectonic hazard and one atmospheric hazard. For each, explain the physical mechanism that causes it, identify where it is most likely to occur geographically, and describe one mitigation strategy humans use to reduce its impacts.