Earthquakes and Volcanoes
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Plate tectonics
The Earth's crust is made up of large pieces called tectonic plates that sit on the semi-solid mantle and move slowly.
Most earthquakes and volcanoes occur at plate boundaries — where plates meet.
Plate tectonics — Key Knowledge
- Crust thin outer layer — oceanic crust is thinner and denser; continental crust is thicker and less dense
- Mantle semi-solid layer beneath the crust — convection currents in the mantle drive plate movement
- Tectonic plates large sections of the crust that move a few centimetres per year
Constructive (divergent) boundaries
Two plates move apart from each other.
New ocean floor is being created at constructive boundaries — the Atlantic is widening by about 2.5 cm per year.
Constructive (divergent) boundaries — Key Knowledge
- Plates move apart, Magma rises to fill the gap creating new crust
- Forms mid-ocean ridges, volcanic islands — e.g. mid-Atlantic ridge, Iceland
- Earthquakes mild, shallow
- Eruptions gentle, basaltic lava
Destructive (convergent) boundaries
Two plates move towards each other. The denser plate is forced under the other (subduction).
The Pacific Ring of Fire is a chain of destructive boundaries — responsible for about 75% of the world's volcanoes.
Destructive (convergent) boundaries — Key Knowledge
- Oceanic plate subducts beneath continental plate oceanic is denser
- Subduction zone where the plate is forced down and melts
- Ocean trenches form deep depressions in the ocean floor
- Fold mountains form crumpling of rock at the boundary
- Violent volcanic eruptions viscous, explosive
- Strong earthquakes friction between plates
Conservative boundaries
Two plates slide past each other horizontally.
Conservative boundaries produce earthquakes but not volcanoes — no new crust is created or destroyed.
Conservative boundaries — Key Knowledge
- Plates move past each other same or opposite direction, at different speeds
- No volcanoes no magma rises because plates are not moving apart or subducting
- Powerful earthquakes friction builds up, then plates jerk suddenly — e.g. San Andreas Fault, California
Volcano types — composite
Strato-volcanoes (composite cone) are steep, cone-shaped volcanoes built from layers of lava and ash.
Examples include Mount St Helens and Mount Pinatubo.
Volcano types — composite — Key Knowledge
- Steep sides cone shape
- Built from alternate layers of lava and ash, Viscous thick) lava (doesn't flow far before solidifying
- Violent eruptions explosive, pyroclastic flows, ash clouds
- Found at destructive boundaries
Volcano types — shield
Shield volcanoes are wide, gently sloping volcanoes formed from runny lava.
Mauna Loa in Hawaii is the world's largest shield volcano.
Volcano types — shield — Key Knowledge
- Wide base with gentle slopes, Runny basaltic) lava (flows long distances before cooling
- Gentle eruptions lava flows rather than explosions
- Built from layers of lava little ash
- Found at constructive boundaries and hotspots
Volcano features
The key parts of a volcano.
Magma becomes lava once it erupts — the terms are not interchangeable.
Volcano features — Key Knowledge
- Crater bowl-shaped opening at the top
- Vent pipe through which magma reaches the surface
- Magma chamber underground pool of molten rock beneath the volcano
- Lava magma that has reached the surface
Earthquake features
The key terms used to describe earthquakes.
Damage is usually greatest at the epicentre and decreases with distance.
Earthquake features — Key Knowledge
- Focus the point underground where the earthquake starts
- Epicentre the point on the surface directly above the focus
- Magnitude the strength of an earthquake — measured on the Richter scale or moment magnitude scale
- Seismic waves energy waves that travel outward from the focus
- Shallow-focus earthquakes cause more damage than deep-focus
Effects of earthquakes and volcanoes on people
Tectonic hazards can cause devastating effects on people and the environment.
The effects are usually worse in LEDCs — poorer building quality, less emergency planning, fewer resources for recovery.
Effects of earthquakes and volcanoes on people — Key Knowledge
- Primary effects immediate — building collapse, deaths and injuries, roads and bridges destroyed, fires
- Secondary effects follow-on — tsunamis, landslides, disease from contaminated water, homelessness, disruption to economy and services
Opportunities from volcanoes
Despite the dangers, volcanoes offer significant benefits to people.
These benefits explain why millions of people choose to live near active volcanoes.
Opportunities from volcanoes — Key Knowledge
- Fertile soil volcanic ash weathers into rich farmland
- Geothermal energy heat from underground used for electricity and heating
- Tourism visitors attracted to volcanic landscapes — jobs and income
- Minerals valuable resources found in volcanic rock
Reducing the impacts
The effects of tectonic hazards can be reduced through preparation and technology.
Earthquakes cannot be reliably predicted — preparation and building design are the most effective strategies.
Reducing the impacts — Key Knowledge
- Earthquake-resistant buildings reinforced concrete, flexible steel frames, deep foundations
- Monitoring seismometers detect tremors; tiltmeters and gas sensors on volcanoes
- Emergency planning drills, evacuation routes, emergency supplies
- Land-use zoning avoiding building on fault lines or near active volcanoes
- Education teaching people what to do during an earthquake or eruption
Case studies required
The spec requires named examples for tectonic hazards.
Compare effects and responses between an LEDC and MEDC event where possible.
Case studies required — Key Knowledge
- An earthquake named event with causes, effects and responses
- A volcano named event with causes, effects and responses
Map your gaps
Earthquakes and Volcanoes
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