Volcanism: Styles and Landforms
Introduction & Conceptual Foundation
Volcanism is the geodynamic process by which molten rock (magma), volcanic gases, and pyroclastic materials rise from the Earth's interior (mainly the asthenosphere) and are extruded onto the surface or intruded within the crust. Magma that reaches the surface is called lava.
The style of a volcanic eruption and the resulting landforms are determined by magma viscosity, which is controlled by silica content and temperature:
Magma Viscosity & Silica Content
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Acidic (Felsic) Magma Basic (Mafic) Magma
- High Silica ($>65\%$) - Low Silica ($<45\%$)
- Highly viscous, flows slowly - Low viscosity, highly fluid
- Traps gases, building pressure - Gases escape easily
- Highly explosive, central eruptions - Gentle, fissure eruptions
Classification based on Eruptive State
- Active Volcanoes: Erupt frequently or show ongoing seismic signs of activity. Defined geologically as having erupted during the Holocene epoch (last 11,700 years). Examples include:
- Barren Island (Andaman & Nicobar): India's only active volcano, located 130 km east of Port Blair, erupting periodically since 1991.
- Mount Stromboli (Italy): Known as the "Lighthouse of the Mediterranean" due to its continuous gentle eruptions.
- Ojos del Salado (Chile/Argentina): The highest active volcano in the world.
- Dormant Volcanoes: Volcanoes that have been inactive for centuries but retain the potential to erupt in the future (e.g., Mount Fuji in Japan, Mount Kilimanjaro in Tanzania).
- Extinct Volcanoes: Volcanoes that show no historical record of eruption and lack a physical magma supply, indicating they will not erupt again (e.g., Mount Kenya in Kenya, Arthur's Seat in Scotland).
Styles of Volcanic Eruptions
Volcanic eruptions are broadly categorized into two styles based on the geological outlet:
1. Central Eruptions (Explosive)
Magma is ejected through a single vertical pipe or vent. Because the lava is highly viscous and acidic, it blocks the vent, building up gas pressure until it erupts explosively. Central eruptions are associated with convergent boundaries and are classified by increasing explosivity:
- Hawaiian Type: Most gentle; highly fluid basic lava with minimal gas explosion.
- Strombolian Type: Moderate explosions with periodic ejecta of glowing cinders.