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

Question

BlackTom AI · Accepted Answer

Depth alone does not guarantee that a magma will undergo more modification; magmatic composition is controlled by processes like partial melting, fractional crystallization, assimilation, and mixing, which often occur in the shallow crust rather than increasing simply with depth. Therefore the statement claiming deeper magmas always undergo more changes is false.

1) Relevant knowledge points and concepts
- Partial melting: only a portion of a rock melts, so the melt’s composition differs from the source (melts are enriched in incompatible elements and silica relative to residues).
- Fractional crystallization: as magma cools, early-formed mafic minerals (olivine, pyroxene) are removed, leaving the residual melt more silicic and viscous.
- Crystal settling/cumulate formation: crystals that form and settle change the composition of the remaining melt.
- Assimilation and magma mixing: ascending magma can melt and incorporate wall rock or mix with other magmas, altering composition.
- Melting mechanisms vary with tectonic setting (decompression melting at ridges, flux melting in subduction zones) and depth affects melting conditions but not in a simple monotonic way.

2) Step-by-step reasoning
- Partial melting produces melts different from source rock (concept 1). This is independent of whether the source is deep or shallow.
- Fractional crystallization and crystal settling require cooling and residence time; these effects are typically stronger in shallow magma chambers where the magma can stall and cool, so shallower magmas often experience more compositional evolution.
- Assimilation of crustal rock also happens when magma rises through and stalls in the crust; the more interaction with crust, often at shallower levels, the more compositional change.
- Depth influences pressure, temperature, and volatile behavior, but greater depth does not universally imply greater change; some deep magmas may ascend rapidly with little modification, while shallow magmas may undergo extensive differentiation.

3) Why the correct answer is correct
The listed statement asserts a direct depth–change relationship that is not generally true. Depth influences melting conditions, but the degree of compositional change depends on residence time, cooling, crystallization, assimilation, and mixing—processes often enhanced at shallower levels.

4) Why other options are true (briefly)
- Option 1: Fractional crystallization and assimilation tend to increase silica and viscosity—true.
- Option 2: Partial melting commonly melts only a small fraction, producing melts compositionally different from the source—true.
- Option 3: Cooling allows crystal formation and settling, changing the residual melt—true.
- Option 4: Magma can melt and incorporate surrounding rock, altering composition—true.

Conclusion: The false statement is "The depth that a magma originates largely determines its compositions as deeper magmas always undergo more changes than shallower ones," which matches the provided answer.

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

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