MEV 002: Block-1 (Unit 02) – Earthquake adds and volcanoes

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 UNIT 2: EARTHQUAKES AND VOLCANOES

2.0 Introduction

The Earth is a dynamic planet, constantly reshaped by internal and external forces. Among the most powerful and dramatic manifestations of this dynamism are earthquakes and volcanoes. Both are geological processes rooted in the movement of tectonic plates and internal heat of the Earth. While earthquakes shake the ground without warning, volcanoes erupt with the release of molten rock, gases, and ash. Understanding these phenomena is essential not only for geologists but also for policymakers, urban planners, and disaster management authorities to mitigate the risks they pose.

2.1 Objectives

By the end of this unit, learners should be able to:

  • Understand the causes and mechanisms of earthquakes.
  • Describe the structure and features of volcanoes.
  • Identify the types of plate boundaries associated with seismic and volcanic activity.
  • Explain how earthquakes and volcanoes are measured, classified, and geographically distributed.
  • Evaluate the environmental and human impacts of these natural events.

2.2 Causes of Earthquakes

Earthquakes are caused by the sudden release of energy in the Earth's crust, producing seismic waves. This energy accumulates due to stress along geological faults, usually associated with tectonic activity.

2.2.1 Types of Plate Boundaries

  • Convergent Boundaries: Plates collide, causing intense pressure and subduction.
  • Divergent Boundaries: Plates move apart, forming new crust (e.g., mid-ocean ridges).
  • Transform Boundaries: Plates slide past each other, generating shear stress.

2.2.2 Concept of Stress, Strain, and Rock Failure

  • Stress is the force per unit area exerted on a rock.
  • Strain is the deformation that results from stress.
  • When stress exceeds the rock's strength, it fails, resulting in a fault and an earthquake.

2.2.3 Focus and Epicentre

  • Focus (or Hypocenter): The point within the Earth where the earthquake originates.
  • Epicentre: The point on the Earth’s surface directly above the focus.

2.2.4 Seismic Waves

  • P-Waves (Primary): Compressional waves; fastest.
  • S-Waves (Secondary): Shear waves; slower, cannot travel through liquids.
  • Surface Waves: Cause the most damage; travel along the Earth's surface.

2.2.5 Shallow and Deep Focus Earthquakes

  • Shallow Focus (<70 km): More destructive, common in tectonic plate boundaries.
  • Deep Focus (>300 km): Less common and usually occur in subduction zones.

2.3 Location of an Earthquake Epicentre and Focus

Seismologists use data from multiple seismograph stations to triangulate the location of the earthquake’s epicentre. The difference in arrival times of P- and S-waves helps estimate the distance from each station to the epicentre.

2.4 Measuring the Size of an Earthquake

Earthquakes are measured in two main ways:

  • Magnitude: Quantifies the energy released using the Richter or Moment Magnitude Scale (Mw).
  • Intensity: Measures the effects on people and structures using the Modified Mercalli Intensity Scale.

2.5 Nature of Destruction

The destructive effects of earthquakes depend on several factors:

  • Magnitude and depth
  • Distance from the epicentre
  • Soil type and topography
  • Building materials and construction standards

Destruction may include collapsed buildings, landslides, fires, tsunamis, and ruptured infrastructure.

2.6 Effects of Earthquakes

  • Human Casualties: Injuries and loss of life.
  • Economic Impact: Damage to infrastructure, loss of productivity.
  • Environmental Consequences: Landslides, altered water courses, soil liquefaction.
  • Social Displacement: Temporary or permanent relocation of populations.

2.7 Volcanoes

A volcano is a vent through which magma, ash, and gases escape to the Earth's surface. Volcanism is closely linked to tectonic activity, especially at convergent and divergent plate boundaries.

2.8 Nature, Extent, and Causes of Volcanism

Volcanism is primarily caused by:

  • Subduction of oceanic plates
  • Rifting at divergent boundaries
  • Mantle plumes and hotspots

2.8.1 Volcanic Features: Background and Associated Terminology

  • Magma (underground), Lava (surface flow)
  • Crater, Caldera, Vent, Fissure

2.8.2 Volcanic Features Formed in the Crust

  • Dykes, Sills, Batholiths, Laccoliths

2.8.3 Volcanic Features Formed on the Surface

  • Lava Plateaus, Shield Volcanoes, Cinder Cones, Stratovolcanoes

2.8.4 Nature and Extent of Volcanoes/Volcanism

Determined by the tectonic setting, magma composition, and eruption frequency.

2.8.5 On the Basis of Frequency of Eruption

  • Active: Currently erupting or likely to erupt.
  • Dormant: Inactive but may erupt again.
  • Extinct: No historical record of eruption.

2.8.6 On the Basis of Morphology/Appearance

  • Shield Volcanoes: Broad, gently sloping.
  • Composite Volcanoes: Tall, layered with lava and ash.
  • Cinder Cones: Small, steep, built from pyroclasts.

2.8.7 On the Basis of Tectonic Settings

  • Found along subduction zones, rift valleys, and hotspots.

2.8.8 On the Basis of Volcanic Eruption Style/Strength

  • Effusive Eruptions: Gentle lava flow.
  • Explosive Eruptions: Violent, pyroclastic flows and ash clouds.

2.8.9 On the Basis of Mechanisms of Eruptions

  • Magma pressure buildup, water-vapour explosions, tectonic-induced cracking.

2.8.10 Causes of Volcanism

  • Plate movement, hotspot activity, crustal melting, decompression melting.

2.9 Geographic Distribution of Volcanoes

Most volcanoes are located along plate boundaries, especially:

  • Pacific Ring of Fire
  • Mid-Atlantic Ridge
  • East African Rift Valley

2.9.1 Volcanoes of India

  • Barren Island (Andaman & Nicobar) – Active volcano.
  • Deccan Traps – Ancient flood basalt province.

2.10 Volcanism and Climate

Volcanic eruptions can impact climate by:

  • Injecting aerosols like sulfur dioxide into the atmosphere, reducing solar radiation.
  • Short-term cooling effects, as observed after the 1991 Mount Pinatubo eruption.

2.11 Effects of Volcanic Eruptions

  • Lava Flows: Destroy everything in their path.
  • Ash Falls: Disrupt air travel, affect human health, block sunlight.
  • Pyroclastic Flows: Fast-moving, deadly clouds of hot gas and rock.
  • Mudflows (Lahars): Devastating when ash mixes with water.
  • Environmental Impact: Alter ecosystems, climate, and topography.

2.12 Let Us Sum Up

Earthquakes and volcanoes are significant natural hazards resulting from the Earth's internal geological activity. While they pose substantial threats, a deep understanding of their causes, mechanisms, and effects enables better preparedness and risk mitigation. Studying their geographic patterns and classifications also helps in planning human settlements and disaster-resilient infrastructure.

 

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