Gas escapes more easily from which type of eruption?

• A. Typical composite volcano such as Mount St. Helens
• B. Typical Hawaiian shield volcano such as Mauna Loa
• C. Cinder cone eruption
• D. Lava dome eruption

Answer: (B)

Gas escape happens most readily when the magma is low in silica and has low viscosity. Basaltic magma, which feeds Hawaiian-style eruptions, can flow easily and lets dissolved gases come out of solution as bubbles that escape with the lava. This means pressure doesn’t build up enough to drive a violent explosion, so gases vent continuously during effusive lava flows.

In contrast, higher-silica magmas that feed typical composite volcanoes are more viscous. Their thick magma traps gases and prevents easy escape, allowing pressure to build until an explosive release occurs. Lava domes and some other eruption styles involve viscous magma as well, which tends to trap gases more than it lets them escape.

So, the eruption type with the easiest gas escape is the Hawaiian-style shield eruption driven by low-viscosity basaltic magma.