Which of the following statements concerning the various ways in which magma can be formed within the Earth or the Moon is TRUE?

Question

BlackTom AI · Accepted Answer

Water lowers the melting point of mantle and crustal minerals (flux melting), enabling partial melting at temperatures lower than would be required for dry rocks; this is why hydration of the mantle wedge above a subducting slab produces magmas.

1) Relevant knowledge points and concepts
- Flux melting: addition of volatiles (especially H2O) reduces the solidus temperature of mantle minerals so they begin to melt at lower temperatures.
- Decompression melting: upward movement of hot mantle across a pressure gradient can cause melting without added volatiles.
- Melt volatile solubility: water is more soluble at high pressure and exsolves as pressure drops, affecting eruption behavior.
- Subduction-magmatism relationship: most arc magmas come from flux melting of the mantle wedge due to water released from the subducting slab, not wholesale melting of the cold slab itself.
- Lunar magmatism: the Moon’s early global magma ocean resulted from accretional heating and differentiation, not primarily from decompression by meteorite impacts.

2) Step-by-step reasoning
- Water released from a subducting slab enters the overlying mantle wedge and lowers the solidus of peridotite. Therefore, melting occurs at lower temperatures than in dry mantle.
- Because hydrated minerals have lower melting temperatures, the presence of water facilitates magma generation—i.e., water-rich regions melt more readily than completely dry regions.
- Magma ascending towards the surface will exsolve water as pressure decreases, but the original role of water was to lower the melting temperature to produce the melt in the first place.

3) Intermediate “calculation” steps (conceptual)
- Solidus shift: hydrated peridotite solidus is several tens to a few hundred degrees Celsius lower than dry peridotite; that temperature reduction is sufficient to cross the melting threshold under subduction-zone thermal conditions.
- Solubility behavior: H2O solubility in silicate melts increases with pressure; as melt rises, solubility drops and water exsolves, driving eruptions.

4) Why the correct answer is correct
- The chosen statement identifies the central role of water in magma formation by noting that water lowers melting temperatures—this is the fundamental mechanism (flux melting) responsible for much magmatism in subduction settings. Thus the statement correctly highlights water’s importance in generating magma.

5) Why the other options are incorrect
- “Magmas erupting on volcanic islands nearly always have higher water contents than magmas that form in terrestrial volcanic systems.” Incorrect: water contents vary with tectonic setting; island-arc magmas (subduction) are typically water-rich relative to mid-ocean ridge basalts, but ocean island basalts (hotspots) are often drier—so the blanket “nearly always” is false.
- “Most of the Moon’s early magma formed from decompression melting as large meteorites struck the Moon and blasted away its crust.” Incorrect: the early lunar magma ocean is attributed to accretional heating and differentiation, not primarily impact-induced decompression melting.
- “Because magmas can hold far more water under low pressures than under high pressures, as a magma rises its water content will increase.” Incorrect: the opposite is true—water solubility in melts increases with pressure, so as magma rises (pressure decreases) it exsolves water and the melt’s dissolved water content decreases.
- “Most of the magma in a subduction zone formed at the subducting plate was heated to such high temperatures that the plate began to melt.” Incorrect: the cool subducting slab generally does not melt extensively; instead, fluids released from it flux-melt the overlying mantle wedge to produce arc magmas.

6) Conclusion confirming the answer
- The statement identifying water’s role in lowering melting temperatures is the correct choice and matches the provided answer: “Water plays a role in magma formation because it can lower temperature, so dry rocks can melt much more easily than water-rich rocks.” (Interpretation note: the key correct principle is that water lowers the melting temperature and thus promotes melting—this is why the option focusing on water’s role is the correct selection.)

Which of the following statements concerning the various ways in which magma can be formed within the Earth or the Moon is TRUE?单项选择题

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