Volcanoes - Meaning, Types, Causes and Effects

Meaning and overview of volcanoes

• Volcanoes are vents in the earth's crust from which fiery hot magma erupts from the earth's interior.

• Endogenic forces are internal forces of the earth; two types are diastrophic and sudden movements. This chapter focuses on sudden movements, which cause instant changes on the surface.

• The chapter covers: the meaning of volcanoes, types (active, dormant, extinct), causes of eruptions, products of eruptions, structure of a volcano, constructive and destructive effects, important volcanic zones of the world, and key terms.

Causes of volcanic eruptions

• The volcanic eruptions are sudden movements associated with internal forces of the earth. A volcano is a vent in the earth's crust from which magma erupts.

• The main causes of volcanic eruptions are:

  • (i) Heat and Pressure inside the Earth: Temperature and pressure both increase from the surface toward the centre. Rocks are poor conductors of heat, so heat does not easily escape and melts rocks, building up great pressure. This pressure forces the heat to find an escape route through fissures and cracks.

  • (ii) Plate Tectonics: Most eruptions occur near plate margins. The earth's lithosphere consists of movable plates. As these plates slide past or collide with each other due to internal heat, their edges form faults. Along plate boundaries, the crust weakens, allowing hot magma to gush out and cause eruptions. The same forces that cause earthquakes also cause volcanic eruptions.

  • (iii) Magma Chamber: Molten material inside the earth melts weak rocks and creates a large magma chamber. Fresh magma pours into the chamber. Magma contains silicate material, gases, and water vapours; the pressure acts vertically upwards. Once a route is found, magma rises to the surface. Many volcanoes lie along plate boundaries for this reason. Major volcanoes have beneath them a vast magma chamber or reservoir joined to the surface by a conduit called a volcanic vent.

• A key concept: products and pathways of eruption are driven by internal heat, pressure, and magma migration toward the surface.

Products of a volcano

• A volcano erupts various products:

  • (i) Magma and Lava: Magma is molten material beneath the crust; when it reaches the surface, it becomes lava. It contains many materials and gases. Steam is the most abundant gas; when released in large quantities it forms clouds that can influence rainfall. Other gases include hydrogen sulphide, sulphur dioxide, carbon dioxide, and hydrogen.

  • (ii) Solid Matter: Solid fragments are pyroclasts; dust is the fine fraction; lapilli are small stones; bombs are larger ejecta. The rock fragments are picked up by the magma during ascent rather than coming directly from the magma chamber.

Structure of a volcano (and eruption process)

• The hot material erupting from the volcano creates surface and subsurface structures:

  • Vent: An opening in the earth's crust through which lava flows; typically circular, with possible subsidiary vents forming during repeated eruptions.

  • Central Conduit Channel: The main passage through which magma travels to the surface.

  • Volcanic Cone: Erupted fluids around the vent cool and solidify, building a cone-shaped landform around the vent. These are landforms of a volcano that will be discussed.

• Eruption dynamics lead to different shapes and intensities of eruptions depending on magma viscosity, gas content, and pressure.

Eruption of lava: associated features

• The eruption involves magma rising through the conduit and erupting at the summit vent.

• The diagram (Fig. 7.2) shows the structure: Magma Chamber → Central Conduit Channel → Vent.

Types of volcanoes

• Based on eruption frequency, volcanoes are classified into three main types:

  • Active volcanoes: Currently in an active state and have erupted in the recent past. There are about $500$ known active volcanoes, and about $20$ may erupt at any time. Some are in a state of permanent eruption. Examples include Mt. Stromboli and Mt. Etna in Italy; Pinatubo in the Philippines; Mauna Loa in Hawaii.

  • Dormant volcanoes: Sleeping volcanoes that have not erupted in recent history but may become active again. Examples include Mt. Kilimanjaro (Africa) and Mt. Vesuvius (though considered active, it is often treated as dormant because of a long period without eruption).

  • Extinct volcanoes: Have not erupted in the present geological period and are not expected to erupt again. Examples include Mt. Aconcagua (South America) and Mt. Kenya (Africa).

• Specific notes:

  • Mauna Loa is a prominent example of a (highly active) volcano in Hawaii.

  • Some volcanoes show a spectrum of activity, with long periods of quiescence punctuated by eruptions.

Structure of a volcano (revisited)

• The very hot material from eruptions shapes both underground and surface structures; eruptions can be quiet or violent depending on magma characteristics and conduit conditions.

• Vent and central conduit channel work together to control the eruption dynamics and the formation of surface landforms.

Volcanic cones and landforms

• Repeated eruptions result in accumulation of erupted material around the vent, forming the volcanic cone.

Effects of volcanoes

• Volcanic activity has both constructive and destructive effects.

Constructive effects

• Eruptions create various landforms via cooling and solidification of magma and eruption of materials, creating Extrusive Landforms, including:

  • (a) Volcanic Plateaus: Formed by lava flows spreading over wide areas. Examples include the Peninsular Indian Plateau (western Deccan), the South African plateau, the Colombian Plateau, and the Ethiopian plateau.

  • (b) Volcanic Mountains: Built from materials ejected from fissures in the crust. These mountains vary greatly in erosion and eruption style. A notable example is Mauna Loa in Hawaii, which rises $9\,\text{km}$ from the ocean floor. Other famous volcanic peaks include Mt. Fuji (Japan), Mt. Mayon (Philippines), Mt. Merapi (Sumatra), Mt. Agung (Bali), and Mt. Cotopaxi (Ecuador).

  • (c) Volcanic Plains: Vast, smooth plains formed by extensive volcanic flooding from centres. The Western Victorian Plains in Victoria, Australia are exemplary, formed over the last $6{,}000{,}000$ years with the most recent eruption at Mount Napier about $7200\,\text{years ago}$. There are over $400$ volcanic sites in this region.

  • (d) Caldera Lake: Repeated eruptions can blow the summit to form a caldera; the resulting depression may fill with water to become a lake. Example: Crater Lake in the USA, about $10\,\text{km}$ wide, and one of the deepest lakes in the USA.

  • (e) Hot Springs: Hot magma heats groundwater, forming thermal springs rich in minerals; sulfur springs are considered beneficial for health and tourism (example: Manikaran in Kullu valley, Himachal Pradesh).

  • (f) Geysers: A geyser is a fountain of hot water and steam that erupts when underground water contacts hot rocks. Example: Old Faithful Geyser in Yellowstone National Park (USA). The vent temperature has reached $95.6^{\circ}\mathrm{C}$ in eruptions.

Other constructive effects

• Lava and ash deposits enrich soil, creating fertile agricultural land.

• Heat from volcanic activity provides a potential for geothermal energy generation.

• Dramatic volcanic scenery attracts tourists, boosting local economies.

Destructive effects

• Eruptions destroy life and property, including humans, animals, and vegetation, due to lava and ash/dust.

• Agricultural fields become covered with ash and dust, reducing productivity.

• Explosive eruptions can generate huge ash clouds that cause heavy rains, floods, and landslides.

• Volcanic gases pollute the environment and can cause health problems.

• Explosive island-arc eruptions can generate tsunamis that devastate coastal areas and farmland.

Important volcanic zones of the world

• Circum-Pacific Belt (Pacific Ring of Fire): The largest concentration of active volcanoes, with over 80% of the total active volcanoes located here. Notable figures: about $40$ active volcanoes in the Andes, over $100$ in the Philippines, $70$ in Indonesia, and $40$ in Japan."Circum-Pacific Belt" is synonymous with the Pacific Ring of Fire.

• Mid-world Mountain Belt: Home to Mt. Vesuvius, Mt. Etna, and Mt. Stromboli; the only active volcano in India (Andaman and Nicobar Islands) is in this belt. The Hawaiian Islands, the Canary Islands, and Mt. Kenya are also in this belt.

Terms to remember

• Plate Tectonics: The theory that explains the movements of the lithospheric plates and the resulting landforms.

• Magma: Molten material moving beneath the earth's crust.

• Lava: Magma that reaches and erupts onto the surface.

• Extrusive forces: Forces acting on the surface that create extrusive landforms.

• Extra notes on terminology:

  • Endogenic vs Exogenic forces (contextual in broader geography; topic here focuses on endogenic sudden movements).

Exercises

• Choose the correct option: 1) The forces arising from the interior of the earth:

  • (a) Endogenic forces

  • (b) Exogenic forces

  • (c) Platonic forces
    2) A vent in the earth's crust from which fiery hot magma erupts:

  • (a) Plate

  • (b) Tectonics

  • (c) Volcano
    3) Which of the following is NOT a cause of volcanic eruption:

  • (a) Heat from the sun

  • (b) Plate Tectonics

  • (c) Magma chamber

  • (d) Heat and pressure inside the Earth
    4) Which amongst the following is not clearly captured in the transcript:

  • [Incomplete in provided transcript]

Notes and references:

• Examples and figures referenced in the chapter include Fig. 7.1 (Tectonic plates and distribution of volcanoes) and Fig. 7.2 (Structure of a volcano) and Fig. 7.3 (Saint Helena eruption) and Fig. 7.4 (Geysers and Hot Springs).

• Real-world exemplars mentioned: Mt. Stromboli, Mt. Etna, Pinatubo, Mauna Loa, Mt. Kilimanjaro, Mt. Vesuvius, Mt. Fuji, Mt. Mayon, Mt. Merapi, Mt. Agung, Mt. Cotopaxi, Crater Lake, Old Faithful, Manikaran.

• Numerical notes:

  • Active volcanoes: $500$ known active volcanoes; about $20$ may erupt at any time.

  • Mauna Loa height: rises $9\,\mathrm{km}$ from the ocean floor.

  • Caldera lake example: Crater Lake is about $10\,\mathrm{km}$ wide.

  • Geyser temperature: $95.6^{\circ}\mathrm{C}$ at the vent.

  • Western Victorian Plains formed over the last $6{,}000{,}000\,\mathrm{years}$; most recent eruption Mount Napier about $7200\,\mathrm{years ago}$.

  • Circum-Pacific Belt hosts >$80\%$ of active volcanoes; regionally distributed as noted above.