Why do some volcanoes erupt gently with flowing lava while others explode violently?
AExplosive volcanoes are bigger than gentle ones
BThe difference depends on the magma's composition — thin, low-silica magma flows easily and releases gas gently, while thick, high-silica magma traps gas until pressure builds to an explosion
CGentle volcanoes are dormant while explosive ones are active
DThe weather determines whether an eruption is gentle or explosive
Magma composition controls eruption style. Basaltic magma (low silica) is thin and runny, so gas bubbles escape easily and lava flows gently. Rhyolitic or andesitic magma (high silica) is thick and sticky, trapping gas bubbles. When enough pressure builds, the gas explodes violently, shattering the magma into ash and rock fragments.
Question 2 True / False
The Hawaiian Islands formed over a hot spot rather than at a plate boundary.
TTrue
FFalse
Answer: True
Hawaii sits in the middle of the Pacific Plate, far from any plate boundary. A stationary plume of hot mantle material (a hot spot) melts through the plate above it. As the Pacific Plate slowly moves northwest over the hot spot, new islands form while older ones are carried away, creating a chain of islands that gets progressively older to the northwest.
Question 3 Short Answer
Name two hazards from volcanoes other than lava flows.
Think about your answer, then reveal below.
Model answer: Pyroclastic flows (fast-moving, superheated clouds of gas, ash, and rock fragments that can travel over 100 km/h and reach temperatures of 700 degrees Celsius) and lahars (volcanic mudflows caused by eruptions melting snow/ice or mixing with rainwater, which can bury entire valleys). Other hazards include ash fall, toxic gas emissions, and tsunamis triggered by eruptions.
Lava flows are actually among the least deadly volcanic hazards because they usually move slowly enough for people to evacuate. Pyroclastic flows and lahars are far more dangerous because of their speed and reach.