Geography IGCSE - Plate tectonics

Earth structure

What makes plates move

Magma (semi-molten rock) near outer core is heated → as magma warms → it expands + becomes less dense → less dense magma starts to rise towards crust → as magma nears crust → it begins to cool → cooling magma becomes denser → begins to sink → rising + falling magma creates circular currents within mantle → these currents create friction with crust above → causes crust to move → process known as convection currents

Continental crust VS Oceanic crust

Continental crust	Oceanic crust
Older and lighter Cannot be destroyed 30 - 50 km thick	Younger and denser Constantly destroyed and replaced 5 - 10 km thick

Constructive margin

Constructive margin example

Destructive margin

Destructive margin example

Collision boundary

Collision boundary example

Conservative boundary

Conservative boundary example

What are volcanoes

An opening in the earth’s crust through which lava, molten rocks and ash are erupted onto land

Where can volcanoes be found

found at edges of tectonic plate boundaries → often formed in long linear patterns → especially along destructive + constructive boundaries → majority occur along coastlines → particularly around Pacific Ocean → Ring of Fire → also found at hotspots → where magma rises through a weak point in crust

What are hotspots + how do they work

Areas in middle of tectonic plates where heat from mantle → causes fixed magma plume to rise → magma breaks through crust under pressure → forms volcanoes away from plate boundaries → as tectonic plate moves over hotspot → chain of volcanoes forms → e.g. Hawaiian Islands → older islands lie further from active hotspot

What causes volcanoes

Convection currents in mantle drives movement of tectonic plates → at destructive plate boundary → denser oceanic plate subducts beneath less dense continental plate → as it descends → oceanic plate melts due to heat + friction → forms magma → magma is less dense than surrounding rock → magma rises through cracks in crust → eventually, erupts through a vent → forms volcano

Stratovolcano (shape)

• Tall, steep-sided, cone shaped volcanoes

• Built in layers of ash + lava → from many eruption

Stratovolcano (location)

• Found at destructive plate boundaries → e.g. Andes, Japan, Philippines

• Where oceanic plate subducts beneath a continental plate

Stratovolcano (eruption style)

• Explosive + violent → due to thick, high viscosity lava → acidic and high in silica →

• Lava cools quickly + blocks vents → builds pressure

• Often produces pyroclastic flows, ash clouds + lahars

Pyroclastic flows

a fast-moving, hot current of gas + volcanic debris → flows along ground during some volcanic eruptions

Volcanic bombs

a mass of partially molten rock larger than 64 mm in diameter → formed when volcano ejects viscous fragments of lava during eruption

Lahars

mudflows created when water → from rain or meltwater from glaciers → + volcanic ash mix

Shield volcano (shape)

Wide, low, gently sloping sides → like warrior’s shield

Shield volcano (location)

Found at constructive plate boundaries + hotspots → e.g. Iceland (Mid-Atlantic Ridge) + Hawaii

Shield volcano (eruption style)

• Frequent but gentle eruption

• Less pressure builds → less explosive

• Runny (low viscosity) + basaltic lava

• Flows easily → spreads over large areas

• Lava flows can cover wide areas → but usually slow

• Less ash + gas than stratovolcanoes

Volcanic eruption monitoring

• Seismometers → detect small earthquakes → caused by rising magma

• Gas sensors → measure increases in gases → e.g. sulfur dioxide

• Tiltmeters + GPS → detect swelling of volcano → ground deformation

• Thermal imaging → spots rising temperatures at surface

• Satellite imagery → shows changes in volcano’s shape or heat

How volcanic eruptions are measured

• Volcanic Explosivity Index (VEI).

• Scale from 0 to 8 → compares eruptions based on:

  • Height of eruption column

  • Duration of eruption

  • Amount of material ejected → ash, lava, rock

• Logarithmic scale → each number is 10× stronger than the one before

Effects of volcanic eruptions on people

• Death + injury → from pyroclastic flows, lava, ash +

• Homes destroyed or buried under ash/lava

• Crops and livestock killed → food shortages

• Airports closed, roads blocked → transport + trade disrupted

• Job loss → in farming, tourism + industry

• Respiratory problems from ash inhalation

• Evacuations + long-term displacement

Volcanic eruption effects on environment

• Forests + habitats → B by lava, ash, or gas

• Wildlife killed or displaced

• Rivers + lakes polluted → by ash and acidic rain

• Soil initially damaged → long-term fertility improves from mineral rich ash

• Climate impact → ash clouds can block sunlight → cooling effect (short-term)

Volcano’s opportunities to people

• Beauty → beautiful to look at + enjoy

• Minerals → possible to mine minerals → e.g.  sulphur from volcanoes

• Geothermal potential → cheap + clean renewable energy → e.g. Iceland + El Salvador

• Tourism → tourists like to view + walk up volcanoes → e.g. Mt Bromo 

• Tourism → often hot springs near volcanoes → tourists + locals can enjoy → e.g. onsens in Japan

• Land around volcanoes very fertile → all minerals → many people choose to farm land

• Poverty → people can't afford to live anywhere else apart from marginal land on volcanoes

• Complacency or naivety → since volcano has not erupted for a long time

• Confidence → be given adequate warning to evacuate

• Family home → always lived in area → don't want to leave

• Shortage of space + high population density

Volcano’s hazards to people

• Pyroclastic flows → fast-moving, superheated gas + ash → can destroy everything quickly

• Lava flows → slow but destructive → burn homes, roads + farmland

• Ash fall → causes breathing problems, roof collapse + poor visibility

• Lahars → mudflows of ash + water → deadly + can bury entire villages

• Toxic gases → sulphur dioxide + carbon dioxide → can poison air + water

• Earthquakes + landslides → triggered by volcanic activity → damages homes + infrastructure

• Fires → caused by lava igniting vegetation or buildings

• Evacuation + displacement → people may lose homes, jobs + access to services

How to reduce impacts of volcanic eruptions

• Monitoring → use seismometers, gas sensors + satellites to detect signs

• Early warning systems → alert people to evacuate in time

• Evacuation plans → pre-planned routes + drills to move people to safety

• Public education → teach communities what to do before, during + after eruptions

• Land-use planning → avoid building in high risk zones

• Emergency services → have rescue teams + supplies ready

What are earthquakes

a sudden, violent shaking of the ground caused by tectonic plate movements

Pattern of earthquakes

Pattern of earthquakes occur along tectonic plate boundaries → in linear patterns → especially the Pacific Ring of Fire + Mid Atlantic Ridge → vast majority of earthquakes occur at or near plate boundaries → where force of tectonic plate movement is most intense

How earthquakes occur

Mostly at plate boundaries → e.g. collision or destructive margins → but can happen in all 4 plate boundaries → heat from the core moves plates → causes pressure + friction where they meet → plates get stuck → builds stress + pressure → when pressure overcomes the friction → fault gives way → releases energy as seismic waves → sudden release causes ground to shake → leads to destructive earthquakes → will occur in primary + secondary waves

Focus

the exact point at which the earthquake has occured (deep in the ground)

Epicentre

point on surface of the earth directly above the focus

Seismic waves

the shockwaves that move outwards from the focus

Richter Scale

• Measures magnitude → how strong the earthquake is

• Logarithmic scale → each number is 10× more powerful than one before

• Scale goes from 1 to 10+

• Uses a seismometer to record ground shaking

Mercalli scale

• Measures impacts + damage caused

• Based on observations → what people saw or felt

• Scale goes from I (not felt) to XII (total destruction)

Earthquakes effect on people

• Deaths + injuries f→ rom falling buildings, collapsing roads + landslides

• Homes destroyed → people made homeless or displaced

• Water pipes + power lines damaged → lack of clean water + electricity

• Hospitals + schools destroyed → reduced access to healthcare + education

• Job loss → as shops, factories + offices damaged

• Transport disrupted → roads, railways + airports shut down

• Emotional trauma + mental health issues → from losing family or property

Earthquakes effect on environment

• Landslides + rockfalls → triggered especially in mountainous areas

• Ground cracks + surface rupture → damage natural landscapes

• Rivers blocked or diverted → can cause flooding

• Soil liquefaction → ground behaves like liquid → damages ecosystems

• Fires → from ruptured gas lines → spread into forests

• Coastal earthquakes → can cause tsunamis → affect marine + coastal habitats

• Air + water pollution → from damaged infrastructure → sewage + chemicals

How to reduce impacts of earthquakes

• Monitoring → use seismometers to detect tremors → issue early warnings

• Emergency plans → practice drills + evacuation routes

• Education → teach people what to do before, during + after

• Earthquake-resistant buildings → use shock absorbers, deep foundations + flexible materials

• Land-use planning → avoid building on fault lines or unstable slopes

• Quick emergency response → rescue teams, medical aid + supplies ready to deploy

Features for earthquake resistant buildings

• Flexible materials → absorb shaking

• Shock absorbers

• Cross-bracing → strengthen walls

• Deep, strong foundations

• Automatic shut-off valves → for gas lines → prevent fires