Which of the following is a factor that can lead to the formation of magma?

Question

BlackTom AI · Accepted Answer

Decompression melting (release of pressure) lowers the melting point of mantle rocks without requiring additional heat, so rising rock crosses its solidus and partially melts to form magma.

1) Relevant concepts
- Melting behavior: rocks have a solidus (temperature where melting begins) and a liquidus (complete melting). Melting depends on temperature, pressure, and composition.
- Decompression melting: when mantle rock ascends (e.g., at mid-ocean ridges or above a mantle plume), pressure decreases faster than temperature falls, so the rock's temperature can exceed its new, lower solidus and produce melt.
- Other mechanisms: flux melting (addition of volatiles) and conductive heating (heat transfer) can also produce magma, but the question’s choices focus on specific factors.

2) Step-by-step reasoning
- At depth, mantle rock may be hot but held solid by high pressure (solidus is at a higher temperature under high pressure).
- If that rock rises toward the surface, pressure drops. The solidus temperature drops with pressure.
- If the rock’s actual temperature is now above the lowered solidus, partial melting occurs and magma forms.
- This process is commonly observed at mid-ocean ridges (plate divergence) and in mantle plumes.

3) Calculations
- No numeric calculation is required; the mechanism is qualitative (temperature vs. pressure relative to the solidus).

4) Why the correct answer is correct
- “Release of pressure” directly describes decompression melting, a primary and well-established mechanism that generates magma in Earth’s mantle. Reduced pressure lowers the melting point, enabling hot rock to melt.

5) Why other choices are incorrect
- Heating of rock: while added heat can cause melting (e.g., from intrusions or mantle plumes), it is not the specific factor named by the correct choice here; the best single answer is decompression by pressure release.
- Change in rock composition: composition affects melting temperature but is not itself a common trigger for magma formation in the same direct way.
- Exposure to sunlight: irrelevant to subsurface rock melting.

Conclusion: Release of pressure is the correct factor that can lead to magma formation, matching the provided answer.

Which of the following is a factor that can lead to the formation of magma? 单项选择题

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