Causes of Tectonic Hazards (Edexcel A-Level Geography): Revision Notes

Causes of Tectonic Hazards

Earthquakes are caused by sudden movements comparatively near to the Earth's surface along a fault.

↳ Fault zones are of pre-existing weakness in the Earth's crust.

Earthquake Waves:

Types of Earthquake Wave:

Primary wave

• Arrives first

• Very fast

• Moves through solid rock and fluids, pushes and pulls in the direction of travel Secondary wave

• Slower than P wave

• Only moves through solid rock

• Up and down movement Surface (love) wave

• Only travels through the surface of the crust

• Fastest of the surface waves

• Moves from side to side (horizontally) as it moves forwards

  

Hazards caused by Earthquake Waves:

Hazard	Description	Effects
Crustal fracturing (P)	Shockwaves travel fast through solid rock & can increase stress in it, especially if rock types are different as waves can travel at diff speeds through them. Stress causes strain until the rock fractures.	Causes faults, which can rupture the surface, causing subsidence or uplift Anything on these faults can get moved and damaged.
Ground shaking (P)	Shockwave ground movements inc. (in order): 1. P-waves → Pushing & pulling 2. S-waves → Up & down 3. L-waves → Side to side Often happens in <60s	Buildings that are not earthquake proof (aseismic) may only survive some types of shaking, and will be severely damaged.
Liquefaction (S)	Occurs when shockwaves travel through loose unconsolidated material, shaking it, making it act like a fluid	Cause buildings to tilt over or collapse and the ground to crack. Underground infrastructure such as cables and pipes may be ruptured.
Landslides (S)	Occur when there are steep slopes. Shockwaves loosen rock & cause it to move downslope under the influence of gravity	Landslides may hit settlements directly or destroy/block transport and communication infrastructure.

Hazards caused by Volcanoes:

Hazard (P=Primary, S=Secondary)	Description
Pyroclastic flows (P)	• Responsible for the majority of primary volcanic-related deaths • Bubbles of magma burst explosively to eject the hot gases and pyroclastic material, which contain glass shards, pumice, crystals, and ash • Can be up to 1000°C • Most harmful when they come out sideways from the volcano, at lower levels • Can move at speeds up to 700 km/h
Tephra (P)	• When a volcano erupts, it will sometimes eject material such as rock fragments into the atmosphere (tephra) • Can vary in size • The ash and larger materials can cause building roofs to collapse as well as start fires on the ground • Dust can reduce visibility and affect air travel
Lava flows (P)	• Molten magma that is erupted from volcanoes flows down the sides until it cools • Basaltic lava flows fastest (thin) • Pose a big threat to human life if they are fast moving • The viscosity of the lava is determined by the amount of silicon dioxide it contains • Can move as fast as 15 m/s
Volcanic gases (P)	• Gases are associated with explosive eruptions and lava flows • Mix normally includes water vapour, sulfur-dioxide, hydrogen, and carbon monoxide • Most deaths are associated with CO₂ • Can cause acid rain
Lahars (S)	• Mixture of water and erupted volcanic material of various sizes, which flow very fast (60 km/h) down river valleys • Water may come from rainfall, or the melting of a snow cap on the top of a volcano
Jokulhlaups (S)	• Where a volcano erupts underneath an ice cap and heat melts some of the ice • The meltwater builds up around the top of the volcano, trapped by the surrounding ice • Eventually, the warm meltwater will melt and lift the ice away from the ground, and then the water bursts out from under the ice and flows rapidly to lower ground • When this happens, anything in its path is washed away

Tsunamis:

↳ Known as one of the most distinctive earthquake-related hazards.

• Do not resemble normal sea waves as their wavelength is much longer.
  • Out at sea not seen as a hazard as they are often low and only when it begins to reach land it grows in height as water becomes shallower.
• Caused by seabed displacement.
• Occur in waves, normally minutes apart. Sometimes can be even hours apart. This is dangerous as people return home between waves thinking its safe.

Causes of Tsunamis:

1. Undersea earthquake at subduction zone may cause a rapid movement of the seabed
2. Violent explosive eruptions of volcanic islands
3. Major landslides or volcanic cone collapse into the sea
4. Underwater landslides at the edge of continental shelves

Impacts of Tsunamis (dependent on both human & physical factors):

1. Duration of event
2. Wave amplitude, water column displacement and the distance travelled
3. The physical geography of the coast, especially water depth and gradient of the shoreline
4. The degree of coastal ecosystem buffer (mangroves)
5. The timing of the event - night vs day
6. Quality of early warning systems
7. The degree of coastal development and its proximity from the coast