Why is it in News?
- Mount Semeru, one of Indonesia’s most active volcanoes, erupted again on Wednesday, releasing ash clouds, pyroclastic flows, and volcanic debris.
- Located in Java, Semeru is part of the Pacific “Ring of Fire,” making it prone to frequent eruptions.
- The eruption renewed concerns over Indonesia’s high volcanic risk, evacuation readiness, and the science behind explosive eruptions.
Relevance
- GS1 (Geography): Physical geography, volcanism, tectonic processes.
- GS3 (Disaster Management): Hazard assessment, early warning, mitigation.
What causes volcanic eruptions?
- Heat inside Earth melts rocks into magma in the mantle.
- Magma is lighter than surrounding solid rock → rises through cracks.
- It accumulates in magma chambers beneath volcanoes.
- As more magma enters the chamber, pressure builds.
- When pressure > strength of the overlying rock → magma forces its way out through vents.
- Once it reaches the surface, it is called lava.
Why are some eruptions explosive and others gentle?
a) Low-viscosity magma (runny/thin) → Gentle eruptions
- Basalt-rich, low silica.
- Gases escape easily → low pressure buildup.
- Produces lava flows (e.g., Hawai’i volcanoes).
b) High-viscosity magma (thick/sticky) → Explosive eruptions
- Andesitic/rhyolitic, high silica.
- Traps gases → enormous pressure builds.
- Sudden release = explosive eruption, throwing ash, pumice, tephra.
- Produces pyroclastic flows (like Semeru).
Why Semeru is so explosive?
- High-silica magma → very viscous.
- Closed conduit system traps gases effectively.
- Located on a subduction zone (Indo-Australian plate under Eurasian plate), which naturally produces gas-rich, sticky magma.
- Generates deadly pyroclastic flows, ash columns, lahars.
Indonesia’s Volcanic Vulnerability
- Sits on the Ring of Fire with 120+ active volcanoes.
- Subduction of tectonic plates produces high-pressure volcanic systems.
- Dense population on volcanic slopes increases risks.
Overview
A. Causes of volcanic eruptions
- Mantle convection & heat → melting of rocks.
- Buoyancy of magma → upward movement.
- Gas pressure in magma chambers.
- Weak zones / fractures created by tectonic movements.
B. Types of volcanic eruptions
- Effusive (Hawaii-like) – lava flows, low danger.
- Explosive (Semeru, Krakatoa) – ash columns, pyroclastic flows.
- Phreatomagmatic – interaction with water increases explosivity.
C. Hazard profile of explosive eruptions
- Pyroclastic density currents: fastest and deadliest.
- Ash clouds: aviation risk, respiratory hazards.
- Lahars: volcanic mudflows; long-term destruction.
- Climate impacts: large eruptions can inject aerosols → global cooling.
D. Why some volcanoes erupt repeatedly
- Constant magma supply due to subduction tectonics.
- Structural weakness of volcanic conduits.
- Recharge of magma chambers over time.
