Natural Disasters

Tectonic Plates and Natural Disasters

Earth's crust is broken into large sections called tectonic plates that float on the semi-molten mantle and move very slowly (a few centimetres per year). Most natural geological disasters occur at or near plate boundaries where plates interact.

Three types of plate boundary:

• Convergent — plates move towards each other; one subducts beneath the other
• Divergent — plates move apart; magma rises to fill the gap
• Transform (conservative) — plates slide past each other horizontally

Earthquakes

Cause: Tectonic plates lock together at fault lines due to friction. Stress builds until the plates suddenly slip, releasing stored energy as seismic waves.

Key terms:

• Focus (hypocenter): the point underground where the earthquake originates
• Epicentre: the point on Earth's surface directly above the focus — this experiences the strongest shaking
• Fault: a fracture in rock along which movement occurs

Seismic waves:

• P-waves (primary/compressional): fastest; travel through solids and liquids; arrive first
• S-waves (secondary/shear): slower; travel through solids only; cannot pass through Earth's liquid outer core
• Surface waves: slowest; travel along the surface; cause the most destruction

Measurement: Earthquake magnitude is measured on the Richter scale (logarithmic — each whole number represents a tenfold increase in amplitude). Modern seismologists also use the moment magnitude scale (Mw).

Volcanoes

Cause: Magma (molten rock) rises through weaknesses in the crust and erupts at the surface.

Volcanoes form at:

• Convergent boundaries — subducting plate melts and magma rises (e.g., Ring of Fire, Mount Pinatubo)
• Divergent boundaries — magma fills the gap as plates separate (e.g., Mid-Atlantic Ridge, Iceland)
• Hotspots — plumes of magma rise through the middle of a plate (e.g., Hawaiian Islands)

Volcanic hazards:

• Lava flows — molten rock; usually slow enough to escape but destroys everything in its path
• Pyroclastic flows — fast-moving (up to 700 km/h) mixture of hot gas, ash, and rock; extremely deadly
• Ash clouds — disrupt aviation and agriculture; can cause respiratory problems
• Lahars — volcanic mudflows triggered by melting snow or rain mixing with ash

Tsunamis

Cause: A sudden vertical displacement of the ocean floor — usually caused by an underwater earthquake, volcanic eruption, or submarine landslide. This displaces a huge volume of water, generating waves.

Characteristics:

• In the open ocean: fast (up to 900 km/h) but only 1–2 m high — barely noticeable
• Near the coast: waves slow down but increase in height dramatically (sometimes 30+ m)
• The sea often withdraws before a tsunami arrives — a warning sign

Impact: 2004 Indian Ocean tsunami killed over 230,000 people; 2011 Japan tsunami triggered a nuclear disaster.

Early warning systems using seismometers and ocean buoys can detect tsunamis and give coastal communities time to evacuate.

Hurricanes (Cyclones / Typhoons)

Cause: Form over warm tropical oceans (water temperature above 26°C). Warm, moist air rises rapidly, cools, and condenses — releasing latent heat that drives further rising and rotation due to Earth's spin (Coriolis effect).

Characteristics:

• Circular storm with an eye (calm centre) surrounded by the eyewall (most violent zone)
• Winds exceeding 119 km/h (74 mph)
• Heavy rainfall and storm surges (sea level rise due to low pressure and wind)
• Weaken when they move over land or cooler water

Regional names: Hurricane (Atlantic), Typhoon (Pacific), Cyclone (Indian Ocean)

Why Impacts Vary

The same natural hazard can have very different impacts depending on:

The 2010 Haiti earthquake (magnitude 7.0) killed over 200,000 people. A similar earthquake in New Zealand in 2011 (magnitude 6.3) killed 185. The difference was largely due to building standards and emergency response capacity.

Key Terms

• Tectonic plate — a large section of Earth's crust that moves slowly on the mantle
• Focus — the point underground where an earthquake originates
• Epicentre — point on Earth's surface directly above the focus
• Seismic wave — energy wave produced by an earthquake
• Magma — molten rock beneath Earth's surface; called lava once it erupts
• Pyroclastic flow — fast-moving avalanche of hot gas and volcanic debris
• Storm surge — abnormal rise in sea level caused by a hurricane

Common Mistakes

• Confusing the focus (underground origin) with the epicentre (surface point above it)
• Saying tsunamis are caused by storms — they are caused by displacement of water, usually by earthquakes
• Thinking all volcanoes are at convergent boundaries — they also form at divergent boundaries and hotspots
• Assuming wealthier countries have fewer natural disasters — they have fewer deaths due to better preparation

Tips and Tricks

• Remember: epicentre is on the surface (like epi = upon); focus is underground
• P-waves travel through Plasma (liquid) and solids; S-waves only through Solids
• Hurricanes need warm water to form — they weaken over land or cold water
• The Richter scale is logarithmic: magnitude 7 is 10x stronger than magnitude 6