Earthquakes and Volcanoes » Causes of Earthquakes and Volcanoes

What you'll learn this session

Study time: 30 minutes

The structure of the Earth and plate tectonic theory
Different types of plate boundaries and their associated features
The causes of earthquakes and how they are measured
The formation of volcanoes and different volcanic eruption types
Case studies of significant earthquake and volcanic events
The global distribution of earthquakes and volcanoes

Structure of the Earth and Plate Tectonics

The Earth's crust is not one solid piece but is broken into sections called tectonic plates that float on the semi-molten mantle below. The movement of these plates is responsible for earthquakes and volcanic activity.

Key Definitions:

Tectonic plates: Large sections of the Earth's crust that move relative to one another.
Plate boundaries: Areas where tectonic plates meet and interact.
Seismic activity: The occurrence of earthquakes in a specific area.

🌎 Earth's Structure

The Earth consists of four main layers:

Inner core: Solid iron and nickel (5,000-6,000°C)
Outer core: Liquid iron and nickel
Mantle: Semi-solid rock (makes up 84% of Earth's volume)
Crust: Thin, solid outer layer (5-70km thick)

📈 Plate Tectonic Theory

Developed in the 1960s, plate tectonic theory explains how the Earth's crust moves. Key points:

The Earth's crust is divided into about 15 major plates
Plates move at rates of 1-10cm per year
Movement is driven by convection currents in the mantle
Most seismic and volcanic activity occurs at plate boundaries

Types of Plate Boundaries

There are three main types of plate boundaries, each creating different geological features and hazards.

↔ Divergent Boundaries

Plates move apart

Creates rift valleys and mid-ocean ridges
Magma rises to fill the gap
Forms new crust
Example: Mid-Atlantic Ridge

→← Convergent Boundaries

Plates move towards each other

Oceanic-Continental: Subduction, volcanic arcs
Oceanic-Oceanic: Island arcs, deep trenches
Continental-Continental: Mountain ranges
Example: Andes Mountains

⇄ Transform Boundaries

Plates slide past each other

No creation or destruction of crust
Causes friction and stress build-up
Results in frequent earthquakes
Example: San Andreas Fault

Causes of Earthquakes

Earthquakes occur when energy stored in rocks is suddenly released due to movement along faults. This energy travels as seismic waves, causing the ground to shake.

The Earthquake Process

Most earthquakes are caused by the following process:

Stress build-up: As plates move, friction prevents them from sliding smoothly, causing stress to accumulate.
Elastic deformation: Rocks bend and store energy like a stretched elastic band.
Rupture: When the stress exceeds the rock's strength, it breaks suddenly along a fault.
Energy release: Stored energy is released as seismic waves that radiate outward.

💥 Earthquake Terminology

Focus (Hypocentre): The point beneath the Earth's surface where the earthquake originates.
Epicentre: The point on the Earth's surface directly above the focus.
Seismic waves: Energy waves that travel through the Earth (P-waves, S-waves and surface waves).
Aftershocks: Smaller earthquakes that follow the main earthquake.

📊 Measuring Earthquakes

Earthquakes are measured in two main ways:

Richter Scale: Measures the magnitude (energy released) on a logarithmic scale. Each whole number increase represents 10 times more ground motion and about 32 times more energy.
Mercalli Scale: Measures the intensity (observed effects) from I (not felt) to XII (total destruction).
Moment Magnitude Scale: Modern replacement for the Richter scale for larger earthquakes.

Causes of Volcanoes

Volcanoes form when magma from the Earth's mantle rises to the surface. This typically happens at plate boundaries where pressure and heat conditions allow rock to melt.

Volcanic Formation

The main processes that lead to volcanic activity include:

Subduction zones: When an oceanic plate sinks beneath another plate, water is carried down, lowering the melting point of the mantle and creating magma.
Divergent boundaries: As plates move apart, pressure is reduced, allowing the mantle to melt and rise.
Hot spots: Areas where mantle plumes bring heat close to the surface, causing melting (e.g., Hawaii).

🌋 Types of Volcanoes

Volcanoes come in different shapes depending on the type of eruption:

Shield volcanoes: Broad, gently sloping sides formed from fluid lava flows (e.g., Mauna Loa).
Composite volcanoes: Steep, symmetrical cones with alternating layers of lava and ash (e.g., Mount Fuji).
Cinder cones: Small, steep-sided cones made of loose fragments (e.g., Paricutin).
Calderas: Large depressions formed when a volcano collapses after eruption (e.g., Yellowstone).

💥 Eruption Types

Volcanic eruptions vary in explosiveness:

Effusive: Gentle outpouring of fluid lava with little explosive activity.
Explosive: Violent eruptions with ash, gas and pyroclastic flows.
Factors affecting explosiveness: Magma viscosity (thickness), gas content and water interaction.
VEI (Volcanic Explosivity Index): Measures eruption size from 0 (non-explosive) to 8 (mega-colossal).

Case Study: The 2004 Indian Ocean Earthquake and Tsunami

Date: 26 December 2004

Location: Off the west coast of northern Sumatra, Indonesia

Magnitude: 9.1-9.3 (third-largest ever recorded)

Cause: Megathrust earthquake at a subduction zone where the Indian Plate is being forced under the Burma Plate

Effects:

Triggered a series of devastating tsunamis up to 30 metres high
Affected 14 countries around the Indian Ocean
Killed approximately 230,000 people
Displaced 1.7 million people
Caused $10 billion in damage

This earthquake demonstrated the catastrophic potential of subduction zone earthquakes and led to the development of the Indian Ocean Tsunami Warning System.

Case Study: Mount Pinatubo Eruption (1991)

Location: Luzon island, Philippines

Type: Explosive eruption at a convergent plate boundary

VEI: 6 (colossal) - second largest eruption of the 20th century

Key events:

After 500 years of dormancy, showed signs of activity in April 1991
Successful evacuation of 60,000+ people before the main eruption
Climactic eruption on June 15 ejected 10 cubic kilometres of material
Ash cloud reached 35 km into the atmosphere
Caused global cooling of 0.5°C for two years due to sulfur dioxide emissions

This eruption is considered a success story for volcanic monitoring and prediction, as early warning signs were detected and acted upon, saving thousands of lives.

Global Distribution of Earthquakes and Volcanoes

Earthquakes and volcanoes are not randomly distributed across the Earth but follow specific patterns related to plate boundaries.

The Ring of Fire: A horseshoe-shaped belt around the Pacific Ocean where approximately 75% of the world's volcanoes and 90% of earthquakes occur.
Mid-ocean ridges: Underwater mountain chains where new oceanic crust is forming, characterized by frequent but usually small earthquakes and volcanic activity.
Continental rift zones: Areas where continents are being pulled apart, such as the East African Rift Valley, which features volcanic activity and earthquakes.
Hot spots: Isolated areas of volcanic activity away from plate boundaries, such as Hawaii and Yellowstone.

Understanding the causes and distribution of earthquakes and volcanoes is crucial for predicting these events and mitigating their impacts on human populations and the environment.

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