Which of the following statements concerning the ways that magma can be formed or altered within the Earth is FALSE?

Question

BlackTom AI · Accepted Answer

Fractional crystallization removes early-forming mafic minerals (olivine, pyroxene, Ca-rich plagioclase) from a melt, leaving the residual magma enriched in silica and generally more viscous. This core concept explains why the statement claiming most post‑formation changes decrease silica and make magma more fluid is false.

1) Relevant concepts
- Fractional crystallization: early minerals crystallize and are removed, enriching the remaining melt in silica (SiO2).
- Partial melting: partial melts are often more silicic than their source because felsic minerals melt preferentially.
- Volatile solubility: magma holds more H2O and CO2 at high pressure; as magma rises and pressure falls, volatiles degas.
- Assimilation and magma mixing: can modify composition, often increasing silica if crustal material is incorporated.
- Residence time: deeper magmas have more opportunity to cool and fractionate than shallow, rapidly erupted magmas.

2) Step-by-step reasoning
- Fractional crystallization tends to remove mafic components, therefore increasing silica content of the residual melt and generally increasing viscosity (making magma less fluid), not decreasing silica.
- Partial melting of mantle or crust commonly yields melts that are more silica-rich than the bulk source because felsic minerals melt first.
- High-pressure magmas retain more dissolved water; during ascent, they lose volatiles (degassing), consistent with the given option about water solubility.
- Deep-formed magmas have longer cooling and crystallization times, so they can fractionate more than shallow magmas.
- Rising hot magma can transfer heat to surrounding rock and cause partial melting (assimilation).

3) Intermediate logical steps
- Remove mafic minerals → residual melt silica increases → viscosity increases.
- High pressure → higher volatile solubility; drop in pressure → volatile loss.

4) Why the correct answer is correct
The statement "Most changes after initial formation tend to decrease silica and make magma more fluid" contradicts established processes (fractional crystallization, partial melting, assimilation) that commonly increase silica and viscosity. Therefore it is false.

5) Why the other options are correct
- Deep magmas do undergo more fractional crystallization (true).
- Magmas hold more water at high pressure and degas on ascent (true).
- Hot rising magma can partially melt surrounding rock (true).
- Partial melting produces melts different from the source composition (true).

6) Conclusion
The false statement is "Most of the changes that occur in magma after it initially forms tend to decrease its silica content and make the magma more fluid." This matches the provided answer.

Which of the following statements concerning the ways that magma can be formed or altered within the Earth is FALSE?单项选择题

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