Composite cones have steep slopes and experience ________ eruptions because they are composed of ________ magma.

Question

BlackTom AI · Accepted Answer

Composite (stratovolcano) cones have steep slopes and experience violent eruptions because they are composed of felsic (or intermediate-to-felsic) magma.

Relevant concepts: magma composition controls viscosity and gas retention; silica-rich magmas (felsic: high SiO2) are viscous and trap volatiles, promoting explosive eruptions. Volcano morphology reflects eruption style: viscous lava solidifies near the vent producing steep slopes and alternating layers of lava and pyroclastic debris form composite cones.

Step-by-step reasoning:
1. Identify the volcano type: composite (stratovolcano) cones are tall, steep-sided mountains built from alternating layers of lava flows, ash, and tephra.
2. Link form to magma behavior: steep slopes occur because lava that is viscous does not travel far from the vent before solidifying, piling up and building steep slopes.
3. Connect viscosity to composition: felsic magmas (rhyolite, dacite, and andesite toward intermediate) have higher silica content than mafic magmas, making them much more viscous.
4. Relate viscosity to explosivity: viscous felsic magmas trap dissolved gases; as pressure decreases, gas expansion leads to fragmentation of magma and violent explosive eruptions (pyroclastic flows, ash columns).
5. Synthesize: composite cones therefore typically erupt violently and are fed by felsic/intermediate magmas.

Why the correct answer is correct: “violent; felsic” directly matches the physical cause—felsic composition yields high viscosity and gas trapping, producing violent eruptions and the steep, layered structure of composite cones (examples: Mount St. Helens, Mount Fuji).

Why the other options are incorrect:
- “violent; mafic”: mafic magma is low in silica, low viscosity, and typically produces effusive (mild) eruptions, not violent ones.
- “mild; felsic”: felsic magmas cause explosive, not mild, eruptions.
- “mild; mafic”: describes shield volcanoes, not composite cones.

Conclusion: The correct choice is violent; felsic, which matches the provided answer.

Composite cones have steep slopes and experience eruptions because they are composed of magma.单项选择题

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Composite cones have steep slopes and experience ________ eruptions because they are composed of ________ magma.单项选择题