3. UNIT-6 VOLCANOES

UNIT-6 VOLCANOES

Volcano is an opening or vent in the earth’s crust through which molten rock material, gases, and ash are expelled to the surface. The process by which magma rises and erupts is known as volcanism. Volcanic activity is closely related to internal heat and tectonic movements within the earth.

Objectives

• After studying this unit, you should be able to:
• Define volcanoes and volcanic activity
• Describe types of volcanoes
• Understand volcanic materials and eruptions
• Explain the impact of volcanoes on Earth

1. Introduction

A volcano is a natural opening or vent in the Earth's Crust through which molten rock, gases, and other volcanic materials escape from the interior of the Earth. The molten rock beneath the surface is called magma, and when it reaches the surface it is known as lava. Volcanic eruptions may be quiet and gradual or violent and explosive depending on the composition of magma and the pressure of gases within the magma chamber.

Volcanic activity plays an important role in shaping the Earth’s surface. It contributes to the formation of new landforms, islands, and mountain ranges, and also influences soil fertility and mineral resources.

2. Causes of Volcanism

Volcanism is mainly caused by the high internal temperature of the Earth and the movement of tectonic plates. Heat generated inside the Earth melts rocks in the Earth's Mantle, producing molten material known as magma.

Several factors contribute to volcanic eruptions:

• Internal Heat of the Earth : Heat generated from radioactive elements melts mantle rocks.
• Movement of Tectonic Plates : According to Plate Tectonics, plates move apart, collide, or slide past each other, creating conditions for magma formation.
• Pressure of Dissolved Gases : Magma contains gases such as water vapour, carbon dioxide, and sulphur dioxide. When pressure increases, these gases expand and force magma upward.

Ø Weak Zones in the Crust : Fractures or cracks in the Earth’s crust allow magma to rise toward the surface.

When the pressure inside the magma chamber becomes greater than the strength of surrounding rocks, a volcanic eruption takes place.

3. Types of Volcanoes Based on Activity

Volcanoes are classified according to their activity into three main types.

3.1 Active Volcanoes

Active volcanoes are those that are currently erupting or have erupted in recent geological time. These volcanoes may show signs such as earthquakes, gas emissions, or lava flows.

3.2 Dormant Volcanoes

Dormant volcanoes are also called sleeping volcanoes. They have not erupted for a long period but still have the potential to erupt again in the future.

3.3 Extinct Volcanoes

Extinct volcanoes are those that have not erupted for thousands of years and are not expected to erupt again because the supply of magma has stopped.

(Source; The image was generated by an AI model (DALL·E by OpenAI))

4. Types of Volcanoes Based on Structure

Volcanoes are also classified according to their shape and structure.

4.1 Shield Volcano

Shield volcanoes are broad volcanoes with gentle slopes formed by repeated eruptions of fluid lava. The lava spreads over large areas before cooling.

4.2 Composite Volcano (Stratovolcano)

Composite volcanoes have steep slopes and are formed by alternating layers of lava, volcanic ash, and other materials. These volcanoes often produce explosive eruptions.

4.3 Cinder Cone Volcano

Cinder cone volcanoes are small and steep-sided volcanoes formed by accumulation of volcanic fragments such as ash and cinders around a central vent.

(Source; The image was generated by an AI model (DALL·E by OpenAI))

5. Volcanic Products

Volcanic eruptions release different types of materials, which are generally classified into gaseous, liquid, and solid products.

5.1 Gaseous Products

Volcanic gases include:

• Water vapour
• Carbon dioxide
• Sulphur dioxide
• Hydrogen sulphide

These gases play a major role in volcanic eruptions and atmospheric changes.

5.2 Liquid Product

The main liquid product of volcanoes is lava. Lava flows across the Earth’s surface and later cools to form igneous rocks.

5.3 Solid Products

Solid materials ejected during eruptions are known as pyroclastic materials. These include:

• Volcanic ash (fine particles)
• Lapilli (small fragments)
• Volcanic bombs (large molten fragments)
• Rock blocks (large solid fragments)

6. Volcanic Landforms

Volcanic activity creates many types of landforms both on the surface and beneath it.

6.1 Extrusive Landforms

Extrusive landforms are formed when lava erupts onto the Earth’s surface. Examples include:

• Lava plateaus
• Volcanic cones
• Craters and calderas

6.2 Intrusive Landforms

Intrusive landforms are formed when magma cools and solidifies beneath the surface. Examples include:

1. Batholiths

2. Laccoliths

3. Dykes

4. Sills

1. Batholiths: Batholiths are very large bodies of intrusive igneous rock formed when huge masses of magma cool slowly deep inside the Earth's Crust. They usually have irregular shapes and may extend over more than 100 km² at the surface.

Key points

• Formed deep underground from slowly cooled magma
• Mostly composed of granite or granodiorite
• Often form the core of mountain ranges
• Exposed on the surface after erosion removes the overlying rocks

2. Laccoliths: A laccolith is a dome-shaped igneous intrusion formed when magma enters between layers of rock and pushes the upper layers upward.

Key points

• Has a flat base and dome-shaped top
• Formed when magma accumulates between rock layers
• Causes the overlying strata to bulge upward
• Usually smaller than batholiths

3. Dykes: A dyke is a vertical or steeply inclined sheet of igneous rock formed when magma fills cracks or fractures in surrounding rocks.

Key points

• Magma cuts across existing rock layers
• Usually narrow but may extend for many kilometres
• Acts as a pathway for magma to reach the surface

4. Sills: A sill is a horizontal sheet of igneous rock formed when magma intrudes between existing layers of rock and solidifies.

Key points

• Lies parallel to surrounding rock layers
• Formed by magma spreading sideways between strata
• Often seen in layered sedimentary rocks

 

7. Effects of Volcanoes

Volcanoes have both constructive and destructive effects.

7.1 Constructive Effects

• Formation of new landforms and islands
• Development of fertile soils suitable for agriculture
• Availability of mineral deposits
• Production of geothermal energy

7.2 Destructive Effects

• Lava flows destroy vegetation, buildings, and infrastructure
• Ash fall damages crops and affects air travel
• Release of toxic gases may harm humans and animals
• Loss of life and property during major eruptions

8. Distribution of Volcanoes

Volcanoes are mainly distributed along tectonic plate boundaries. The most important volcanic region is the Pacific Ring of Fire, which surrounds the Pacific Ocean and contains the majority of the world’s active volcanoes.

Other important volcanic regions include:

• Mid-Atlantic Ridge
• Mediterranean volcanic belt
• Oceanic island chains formed by mantle hotspots

These regions correspond to areas of intense tectonic activity where magma can easily reach the Earth’s surface.