Earthquakes and volcanoes Revision Notes

Volcanoes are openings in the Earth's crust through which molten rock, gases and ash erupt. There are two primary types: strato-volcanoes (composite cones) and shield volcanoes, each with distinct shapes and eruption styles.

Strato-volcanoes have steep, conical profiles formed by alternating layers of lava and ash. Their eruptions tend to be explosive due to viscous magma trapping gases, producing pyroclastic flows and ash clouds. Examples include Mount Fuji in Japan.

Shield volcanoes have broad, gentle slopes created by low-viscosity lava that flows easily and spreads widely. Their eruptions are less violent, producing extensive lava flows. Mauna Loa in Hawaii is a classic shield volcano example. Key features common to volcanoes include: craters (the summit depression where eruptions occur), vents (openings through which magma escapes), and magma chambers (large underground pools of molten rock feeding eruptions).

• Strato-volcanoes: steep slopes, explosive eruptions, layered lava and ash.
• Shield volcanoes: gentle slopes, effusive eruptions, fluid lava flows.
• Common features: crater, vent, magma chamber.

Earthquakes result from sudden movements along faults in the Earth's crust, releasing energy as seismic waves. The focus (or hypocentre) is the point underground where the rupture starts. The epicentre is the point on the surface directly above the focus.

Magnitude measures the energy released, commonly using the Richter scale or moment magnitude scale. Larger magnitudes indicate stronger earthquakes with greater potential damage. The intensity felt at locations depends on magnitude, depth, distance from epicentre and local geology.

Seismic waves radiate from the focus, causing ground shaking that can damage buildings, trigger landslides or tsunamis. Understanding these characteristics helps assess earthquake hazards and plan responses.

• Focus: underground origin point of earthquake.
• Epicentre: surface point above focus.
• Magnitude: energy released, measured by Richter or moment magnitude scale.
• Intensity varies with distance and local conditions.

Earthquakes and volcanoes are not randomly distributed but mainly occur along tectonic plate boundaries where plates interact.

There are three main types of plate boundaries: constructive (divergent), destructive (convergent), and conservative (transform).

Constructive boundaries occur where plates move apart, allowing magma to rise and form new crust. This creates volcanic activity, such as along the Mid-Atlantic Ridge. Earthquakes here tend to be shallow and less severe than at other boundaries. Destructive boundaries happen where an oceanic plate subducts beneath a continental or another oceanic plate. This subduction causes intense pressure, melting and magma formation, leading to explosive volcanoes and powerful earthquakes. The Pacific Ring of Fire is a prime example. Conservative boundaries involve plates sliding past each other horizontally, causing friction and stress build-up that releases as earthquakes, like along the San Andreas Fault in California. Volcanoes are rare at these boundaries. Understanding these processes explains why earthquakes and volcanoes cluster in certain regions and vary in intensity and type.

• Constructive boundaries: plates move apart, new crust forms, volcanoes and shallow earthquakes.
• Destructive boundaries: subduction causes explosive volcanoes and strong earthquakes.
• Conservative boundaries: plates slide past, causing earthquakes but no volcanoes.

Earthquakes are caused by sudden release of stress along faults due to plate movements. Volcanic eruptions occur when pressure from magma forces its way to the surface.

Both hazards can cause loss of life, injury, destruction of homes and infrastructure, disruption of services, and environmental damage such as landslides, tsunamis and ash fall.

However, volcanoes also provide benefits: fertile soils from volcanic ash support agriculture, geothermal energy can be harnessed for power, and volcanic landscapes attract tourism, creating economic opportunities. For example, the volcanic soils around Mount Etna in Italy are highly productive for vineyards.

• Earthquakes caused by fault rupture; volcanoes by magma pressure.
• Impacts include casualties, damage, environmental hazards.
• Volcanoes offer fertile soils, geothermal energy, tourism opportunities.

Reducing the effects of earthquakes and volcanoes involves preparedness, monitoring and planning.

Earthquake mitigation includes enforcing building codes to withstand shaking, educating the public on safety drills, and installing early warning systems where possible.

Volcanic hazards can be managed by monitoring volcanic activity (seismic activity, gas emissions), establishing exclusion zones, and planning evacuation routes. Land-use planning avoids building in high-risk zones. Community awareness and emergency response plans are essential to reduce casualties and damage. These strategies do not prevent disasters but reduce their impact on human populations.

• Earthquake mitigation: strong buildings, drills, early warnings.
• Volcano mitigation: monitoring, exclusion zones, evacuation plans.
• Community education and land-use planning critical.