The Living World
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Tectonic Hazards

The Structure of the Earth

The Earth is made up of four main layers:

  • Inner core –  A solid and dense layer of iron and nickel, with temperatures reaching about 5500°C
  • Outer core – A semi-molten metal layer with temperatures ranging between 5000 and 5500°C
  • Mantle – A semi-molten layer that is less dense than the outer core
  • Crust – There are two types of crust that vary in thickness: oceanic crust and continental crust.


The Earth’s crust consists of two types

  • Oceanic crust
  • Continental crust

The oceanic crust is thinner (around 5 – 10km) but heavier and denser than the continental crust. Whereas, the continental crust is thicker (ranging from 25 – 90km) but older and lighter.

The oceanic crust is continuously created and destroyed due to plate movement. It sinks (subducts) under the continental crust, which forms trenches. This process explains why the continental crust is much older, as it is not destroyed like the oceanic crust.

Plate tectonics

The Earth’s crust is divided into several tectonic plates. These plates float on the semi-molten mantle below.

Previously, tectonic theory suggested that the movement of the plates was caused by convection currents in the mantle. However, the current theory, known as slab pull theory, suggests that the movement occurs due to the weight of denser oceanic plates subducting and dragging the rest of the plate with it.

  • Plate boundaries, otherwise known as margins, are the areas where two plates meet.

Global Distribution of Earthquakes and Volcanoes

Earthquake distribution

Earthquakes can occur at all types of plate boundaries. However, the majority of earthquakes (approximately 90%) take place along the Pacific “Ring of Fire.” 

This area encircles the Pacific Ocean and is known for its intense seismic activity. Other regions, such as the Mediterranean, the Himalayas, and the Mid-Atlantic Ridge, also experience significant earthquake activity.

Volcano distribution

Volcanoes are primarily found at constructive and destructive plate boundaries. Around 75% of active volcanoes are located along the “Ring of Fire.”

Also, there are hotspots that occur away from plate boundaries. Hotspots are plumes or columns of magma that escape through the Earth’s crust, resulting in volcanic eruptions.

Physical Processes at Plate Boundaries

The three main types of plate boundaries are:

  • Constructive
  • Destructive
  • Conservative (transform)

Destructive plate boundaries

Destructive plate boundaries involve the convergence of an oceanic plate and a continental plate. When these plates collide, the oceanic plate subducts beneath the continental plate, forming a trench.

Volcanoes and mountains can emerge near the subduction zone. The subducting oceanic plate melts in the mantle, creating magma that rises to the surface.

This magma escapes through weaknesses in the rock, leading to the formation of composite volcanoes. The process can also generate earthquakes.

Constructive plate boundaries

At constructive plate boundaries, the plates move apart from each other. Magma from the mantle rises and forms new land. This land often takes the form of shield volcanoes due to the type of lava involved and the way it flows and cools.

The movement of the plates over the mantle can cause earthquakes.

Conservative plate boundaries

Conservative plate boundaries occur when two plates move past each other or are side by side, moving at different speeds.

Friction between the plates causes them to become stuck, building up pressure. When the pressure is released, it generates a significant amount of energy, leading to earthquakes.

Unlike destructive and constructive plate boundaries, conservative plate margins do not have volcanoes.

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