Causes of Earthquakes & Volcanoes | Geography for GCSE/IGCSE - Year 11 PDF Download

Earth's Structure

The Earth is composed of several layers, each with unique characteristics and compositions.

• Inner Core: The inner core is a solid and dense layer at the center of the Earth, approximately 1400km in diameter. It consists mainly of iron and nickel and reaches temperatures as high as 5500°C.
• Outer Core: Surrounding the inner core, the outer core is a semi-molten metal layer, about 2100km thick. Temperatures in the outer core range between 5000-5500°C.
• Mantle: Beneath the outer core lies the mantle, a semi-molten layer approximately 2900km thick. It is less dense than the outer core and plays a crucial role in the Earth's geological processes.
• Crust: The Earth's crust varies in thickness and is made up of two main types: continental crust and oceanic crust.

Crust

• There are two types of crust:
  • Continental Crust: The continental crust is thicker, ranging from 25-90km. It is older and less dense compared to oceanic crust.
  • Oceanic Crust: On the other hand, oceanic crust is thinner, about 5-10km in thickness. It is heavier and denser than continental crust.
• Oceanic crust is continually formed and destroyed due to plate tectonics. It is denser, leading it to subduct beneath the less dense continental crust.

Plate tectonics

• The Earth's crust is divided into several tectonic plates.
• These plates travel atop the partially molten mantle beneath them.
• The motion of the plates is influenced by the convection currents occurring within the mantle.
• A plate boundary or margin refers to the area where two plates come into contact.

Types of plate boundary

• Volcanic eruptions and earthquakes often occur near plate boundaries.
• There are four main types of plate boundaries:
  • Divergent Boundaries (Constructive): Plates move apart, creating new crust.
  • Convergent Boundaries (Destructive): Plates collide, leading to subduction or mountain formation.
  • Transform Boundaries (Conservative): Plates slide past each other horizontally.
  • Collision Boundaries: Plates crash into each other, forming extensive mountain ranges.

Divergent (constructive) plate boundary

• At a divergent boundary, the plates are separating from each other.
• The Mid-Atlantic Ridge serves as an illustration of a divergent plate boundary.
• This type of plate boundary is associated with both volcanic eruptions and earthquakes.

Convergent Plate Boundary

• At a convergent boundary, tectonic plates move towards each other.
• The denser oceanic plate subducts beneath the lighter continental plate.
• An example of a convergent boundary is the interaction between the Nazca plate and the South American plate.
• Both volcanic eruptions and earthquakes are common at convergent boundaries.

Collision Boundary

• When plates of similar density collide at a boundary, they push against each other without subducting.
• This compression forces the land upwards, creating majestic fold mountains like the Himalayas.
• Earthquakes represent the primary hazard at this type of plate boundary.

Transform (Conservative) Boundary

• At a transform boundary, plates slide past each other in opposite or different speeds.
• Earthquakes are the primary hazard at this boundary.

Causes of Earthquakes & Volcanic Eruptions

• Volcanoes are found at divergent (constructive), convergent (destructive) plate boundaries, and hot spots due to the movement of tectonic plates.
• Volcanoes do not form at collision boundaries or transform (conservative) boundaries.

Volcanoes at constructive boundaries

• At a divergent (also known as constructive) boundary, the tectonic plates are separating from one another.
  • Divergent plate boundaries frequently occur beneath the ocean.
  • Lava emerges through the fissure created as the plates diverge.
  • The lava cools and solidifies, resulting in the formation of fresh crust.
• At divergent plate boundaries, the lava typically has a low viscosity, and eruptions are less violent.
• These eruptions give rise to shield volcanoes, characterized by gently sloping sides.

Volcanoes at destructive boundaries

• At a destructive (also known as convergent) boundary, the tectonic plates are converging towards one another.
  • The denser oceanic plate descends beneath the lighter continental plate through a process called subduction.
  • In the subduction zone where the two plates collide, friction occurs.
  • The friction generates heat, causing the plate material to melt and form magma.
  • This magma ascends to the surface through fissures in the crust.
  • Upon reaching the surface, the cooling lava and ash accumulate, leading to the formation of a volcano.
• At destructive plate boundaries, the lava typically has high viscosity, resulting in explosive eruptions.
• These eruptions tend to give rise to composite or stratovolcanoes.

Volcanoes at hot spots 

At a hot spot, a tectonic plate traverses a magma plume.

• The magma ascends to the surface via fissures in the crust.
• As the tectonic plate gradually moves over the magma plume, it can result in the formation of a series of islands, such as Hawaii.

Causes of Earthquake Hazards

• Earthquakes may happen in various locations but are most common at or near plate boundaries.
• Earthquakes occur at all types of plate boundaries: divergent (constructive), convergent (destructive), collision, and transform (conservative).

Plate Boundary Types and Earthquake Characteristics

• At divergent (constructive) plate boundaries, earthquakes are typically milder as the plates are moving away from each other.
• Conversely, at convergent (destructive), collision, and conservative (transform) plate boundaries, earthquakes tend to be more intense.