1.1.2 Tectonic Hazards

Many natural hazards derive from Earth’s tectonic plates and their movement around Earth’s crust and upper mantle. Understanding these hazards let us know why some natural hazards are more impactful than others, and why many natural hazards only exist in particular places.

Earth’s Structure

The Earth’s structure is composed of distinct layers, each with its own unique chemical and physical properties and characteristics.

These layers, from the innermost to the outermost, are the inner core, outer core, mantle, and crust:

• Inner Core: The innermost layer of the Earth is the inner core. This layer is primarily composed of solid iron and nickel, and is approximately 1200 km thick.
• Outer Core: Surrounding the inner core is the outer core. Unlike the inner core, the outer core is primarily in a liquid state. It consists of molten iron and nickel.
• Mantle: The mantle is the layer that lies above the outer core. Over 2000 km thick, it is composed of silicate minerals and rocks and is responsible for convection currents that drive the movement of tectonic plates on the Earth’s surface. 
• Crust: The outermost layer of the Earth is the crust, which is the layer we live on. It varies in thickness, with oceanic crust being thinner and denser and continental crust being thicker and lighter. The crust is composed of a variety of rocks and minerals and is divided into several tectonic plates that float on the semi-fluid mantle beneath them. The crust and the top of the mantle are known together as the lithosphere. These plates interact at plate boundaries, leading to geological features such as mountains, earthquakes, and ocean basins.

Plate Tectonics Theory

Plate tectonics is a scientific theory that explains the movement and interaction of Earth’s lithospheric plates. The theory states that the Earth’s surface is divided into several rigid plates that float on the semi-fluid asthenosphere beneath them. These plates are in constant motion. The motion, however, is extremely slow. The average plate moves 1.5 cm a year, which is believed to be caused by the convective currents within the Earth’s mantle or ridge push – slab pull, where denser plate slides down into the mantle.

There are three main types of plate boundaries: constructive, destructive, and conservative. These are where most of Earth’s earthquakes and volcanic eruptions happen.

Constructive Plate Margins

At constructive plate margins, two plates move away from each other, creating a gap. This movement occurs along mid-ocean ridges or rift valleys. As the plates separate, magma from the Earth’s mantle rises to fill the gap. When this magma cools and solidifies, it forms new rock, creating new oceanic crust. This process can result in volcanic activity along the mid-ocean ridges and the formation of volcanic islands and shield volcanoes. Earthquakes at constructive plate margins are generally less powerful and occur due to the fracturing of the crust as it pulls apart.

Destructive or Convergent Plate Margins

Destructive (also known as convergent) plate margins occur when two tectonic plates move toward each other. This collision often leads to one of the plates being forced beneath the other in a process called subduction. As the subducting plate sinks into the Earth’s mantle, it melts and generates intense heat and pressure. This, in turn, triggers volcanic eruptions and the formation of mountain ranges. Destructive plate boundaries are known for their geological activity, such as earthquakes, tsunamis, and volcanic eruptions forming composite volcanoes.

Conservative Plate Margins

Conservative plate margins occur where two plates slide past each other horizontally, either in different directions or in the same direction at different speeds. The movement is not smooth and results in the accumulation of stress along the plate boundary. When the stress is released suddenly, it causes earthquakes. Unlike constructive or destructive plate margins, conservative plate margins do not typically exhibit volcanic activity.

Global Distribution of Earthquakes and Volcanic Eruptions

Earthquakes occur worldwide, but their distribution is not uniform, they tend to be concentrated at plate margins, particularly destructive and conservative. The most intense earthquakes tend to originate from subduction zones, from destructive plate margins, where one plate is forced beneath another plate. Earthquakes occur when tectonic plates pushing or pulling against each other build up energy which is released suddenly when it can no longer be contained by the rocks.

Despite this, around 90% of the total earthquakes Earth experiences occur in what’s referred to as the ‘Pacific Ring of Fire’. In this case the Pacific plate is forced (subducted) beneath the surrounding continental plates.

A similar pattern can be seen from volcanoes, with 75% of Earth’s total volcanoes located around the Pacific Ring of Fire. At destructive plate boundaries, where plates collide, often lead to volcanic formations of arcs and ridges whereas constructive boundaries, where plates move apart, create conditions for volcanic activity along mid-ocean ridges and rift zones.

What Causes Movement of Tectonic Plates?

The movement of tectonic plates is caused by heat from inside the Earth, which generates convection currents in the mantle. These convection currents occur when hot molten rock near the core rises toward the surface, cools down, and then sinks again. This circular motion drags the plates above it, causing them to move in different directions.

Another important cause of plate movement is called ridge push. This occurs at divergent (constructive) plate boundaries where two plates move apart and magma rises to form new crust, such as at mid-ocean ridges. As the new crust cools, it becomes denser and begins to slope away from the ridge. Gravity causes the new rock to slide down this slope, pushing the rest of the plate away from the ridge. This process helps to move the plates across the Earth’s surface.

A third process, slab pull, happens at convergent (destructive) plate boundaries where an oceanic plate sinks beneath another plate into the mantle in a process called subduction. The sinking plate is cooler and denser than the surrounding mantle, so it sinks easily and pulls the rest of the plate down with it. This pulling force helps to drive the overall movement of the plate.