What rock characteristics would you NOT expect to find at the surface of a mid-ocean ridge?

Question

BlackTom AI · Accepted Answer

Mid-ocean ridges erupt mafic, basaltic lava that cools very rapidly when it contacts seawater, producing fine-grained extrusive rocks rather than coarse-grained plutonic rocks.

1) Relevant knowledge points and concepts
- Extrusive vs. intrusive igneous textures: extrusive (volcanic) rocks cool quickly at or near the surface and develop aphanitic (fine-grained) or glassy textures; intrusive (plutonic) rocks cool slowly at depth and develop phaneritic (coarse-grained) textures.  
- Mid-ocean ridges are divergent plate boundaries where new oceanic crust forms by upwelling mafic magma; the surface expression is primarily basalt (extrusive, mafic).  
- Typical surface textures at ridges include aphanitic basalt, pillow basalts, vesicular textures, and glassy margins from quenching in seawater.

2) Step-by-step reasoning
- Step 1: Identify the tectonic setting: mid-ocean ridge = seafloor spreading, lava erupts at/near the seafloor.  
- Step 2: Determine expected rock composition: mantle-derived melts are mafic, so basaltic composition is expected.  
- Step 3: Determine cooling environment: erupting into cold ocean water causes rapid cooling → small crystal sizes.  
- Step 4: Match texture with environment: rapid cooling → aphanitic or glassy textures; slow cooling (required for phaneritic) is not present at the seafloor.

3) Calculations/intermediate steps
- No numerical calculations are required; the logical sequence above demonstrates cause (rapid cooling) → effect (fine-grained texture).

4) Why the correct answer is correct
- "Phanteritic texture" denotes a coarse-grained (phaneritic) intrusive texture formed by slow cooling at depth. That environment does not exist at the exposed surface of a mid-ocean ridge, so phaneritic textures would not be expected at the ridge surface.

5) Why the other options are incorrect
- Basalt: expected — it is the common extrusive mafic rock produced at ridges.  
- Mafic composition: expected — mantle-derived melts are mafic at ridges.  
- Ignneous rock (igneous rock): expected — ridge-sourced basalt is igneous. All three are consistent with ridge-surface geology.

6) Conclusion
Therefore, the rock characteristic you would NOT expect at the surface of a mid-ocean ridge is phanteritic texture, which matches the provided answer.

What rock characteristics would you NOT expect to find at the surface of a mid-ocean ridge? 单项选择题

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