What is the difference between a magma intrusion (dike) and an eruption in volcanic hazard assessment?

• A. A magma intrusion is surface movement of magma that forms lava domes.
• B. Eruption is subsurface movement that deforms ground.
• C. A magma intrusion is subsurface movement that may deform ground and trigger instability; eruption is the surface release of magma; both pose hazards but different mechanisms and timescales.
• D. There is no difference between intrusion and eruption.

Answer: (C)

In volcanic hazard assessment, it’s essential to separate underground magma movement from the surface eruption. A magma intrusion, like a dike, moves through the crust below the surface. This movement can push and crack surrounding rock, causing ground deformation, tilting of the landscape, and increased seismic activity, which can destabilize slopes or systems like reservoirs and roads. An eruption, on the other hand, is the magma reaching and erupting at the surface, releasing lava, ash clouds, pumice, and volcanic gases. This creates immediate surface hazards and atmospheric effects.

Both pose risks, but through different mechanisms and timescales. Intrusions may occur quietly and be detected by ground deformation and seismic signals hours to years before an eruption, signaling potential future volcanic activity. Eruptions are rapid, surface events with direct, often dramatic hazards. That distinction—that an intrusion is subsurface movement that can deform ground and trigger instability, while an eruption is the surface release of magma—captures the different processes and timing involved.