Explain the difference between magma viscosity and gas content and how these factors influence eruption style.

• A. Higher viscosity traps gases and leads to explosive eruptions; low viscosity allows gas to escape and yields effusive lava flows; gas pressure drives fragmentation
• B. Low viscosity allows gas to escape and yields effusive lava flows
• C. Viscosity has no effect; eruption style is random
• D. Gas content is irrelevant to eruption style

Answer: (A)

Magma viscosity and the amount of dissolved gases control how magma releases its energy during an eruption. When viscosity is high, magma resists flow, so gases stay trapped and pressure builds up inside. That pressure can cause the magma to fragment violently as it erupts, leading to explosive events that eject ash, pumice, and pyroclastic flows. In contrast, low-viscosity magma flows more easily, allowing gases to escape gradually rather than build up to fragmentation; this produces effusive lava flows that spread out as lava rather than exploding violently. The amount of gas dissolved in the melt amplifies these effects: more dissolved volatiles mean more potential pressure to release, increasing the likelihood of fragmentation when the magma does erupt. So the combination of high viscosity and high gas content tends toward explosive eruptions, while low viscosity with gas escape favors effusive lava flows.