Kilauea (Hawaii) is one of the world's most active volcanoes yet rarely produces explosive eruptions, while Mount St. Helens erupts far less frequently but explosively. What best explains this difference?
AKilauea is a smaller volcano, so insufficient pressure builds up for explosive eruption
BKilauea erupts so frequently that pressure never accumulates to dangerous levels
CKilauea's basaltic magma has low viscosity, allowing dissolved gases to escape gradually rather than building explosive pressure
DMount St. Helens sits above a subduction zone where water injection generates more gas than Hawaiian hot spot magma
The key variable is silica content, which controls viscosity. Basaltic magma (low silica) is fluid — dissolved gases escape continuously as magma rises, releasing pressure. High-silica andesitic/rhyolitic magma (like Mount St. Helens) is viscous and traps gases until pressure becomes catastrophic, driving explosive eruption. Eruption frequency and volcano size are consequences of composition, not independent controls. Option D has some truth (subduction zones produce water-rich, more viscous magma) but the mechanism is still the silica-viscosity-explosivity link.
Question 2 Multiple Choice
The 1815 Tambora eruption temporarily cooled global climate by ~0.5°C. What is the primary mechanism?
AVast ash clouds blocked sunlight over the eruption site and surrounding regions for years
BThe CO₂ released by the eruption triggered a short-term greenhouse cooling feedback
CSO₂ injected into the stratosphere formed sulfate aerosol particles that reflected incoming solar radiation globally
DPyroclastic debris reduced surface albedo across the Northern Hemisphere
It is specifically SO₂ reaching the stratosphere that causes global cooling — not ash, not CO₂. SO₂ reacts with water vapor to form sulfate aerosol particles that persist for 1–3 years (unlike tropospheric ash, which rains out within weeks) and scatter incoming solar radiation back to space. CO₂ from volcanoes is actually a greenhouse gas, but volcanic CO₂ output is too small to matter on short timescales. Only large eruptions (VEI 6+) inject SO₂ high enough to reach the stratosphere and produce global effects.
Question 3 True / False
Pyroclastic flows are generally slower and less lethal than lava flows, which is why most volcanic fatalities historically come from lava.
TTrue
FFalse
Answer: False
False — this is directly addressed as one of the most dangerous misconceptions in volcanology. Pyroclastic flows travel at 100–700 km/h and consist of superheated gas mixed with fragmented rock, offering essentially no chance of escape. They are among the deadliest volcanic hazards, responsible for events like the destruction of Pompeii and the 1902 eruption of Mount Pelée that killed ~30,000 people. Lava flows, while dramatic, are typically slow enough for evacuation and rarely cause fatalities. Lahars (volcanic mudflows) and pyroclastic flows together account for the vast majority of volcanic deaths.
Question 4 True / False
A volcano located far from any plate boundary, like those in the Hawaiian Islands, is expected to be explained by processes unrelated to plate tectonics.
TTrue
FFalse
Answer: False
False. Hot spot volcanism is part of the broader plate tectonic framework — a stationary thermal anomaly in the mantle melts through the tectonic plate as it moves overhead, creating a chain of volcanic islands. The Hawaiian chain pattern — progressively older and more eroded islands to the northwest, active volcano at the southeast end — is itself direct evidence of the Pacific Plate's motion over a relatively stationary hot spot. Far from being unrelated to plate tectonics, intraplate hot spot volcanism is one of the strongest independent confirmations of plate motion.
Question 5 Short Answer
Why does silica content so strongly determine the style of a volcanic eruption?
Think about your answer, then reveal below.
Model answer: Silica polymerizes into chains in magma, dramatically increasing its viscosity. High-viscosity magma traps dissolved gases (H₂O, CO₂, SO₂) that cannot escape as the magma rises and pressure decreases. Gases accumulate until the pressure becomes catastrophic and the magma erupts explosively. Low-viscosity basaltic magma allows gases to escape continuously as the magma ascends, releasing pressure gradually and producing effusive lava flows rather than explosions.
Viscosity is the master variable in eruption style because it controls whether volatiles can degas or must accumulate. The analogy to a carbonated drink is useful: open it slowly and shake gently (low viscosity) and gas escapes quietly; shake it vigorously and open suddenly (high viscosity equivalent) and it erupts. Silica content is the primary control on viscosity — which is why knowing whether you're dealing with basalt or rhyolite predicts eruption behavior better than any other single property, and why magma composition determines both the shape of the volcano and the nature of its hazards.