Which of the following would cause sea level to fall (lower)?

Question

BlackTom AI · Accepted Answer

Increasing the volume of ocean basins lowers global sea level because the same amount of water spreads into a larger container. Relevant concepts: sea level depends on (1) total water mass in the oceans, (2) the volume of the ocean basins (capacity), and (3) water density/temperature (thermal expansion). A change that increases basin volume causes sea level to fall; changes that add water or reduce basin capacity cause sea level to rise.

Step-by-step reasoning:
1. Represent sea level height h as h = Vwater / A_ocean, where Vwater is ocean water volume and A_ocean is surface area of the ocean basin at a reference level. If basin volume (capacity) increases without adding water, the equilibrium h must decrease.
2. Tectonic forces that expand ocean basins (seafloor spreading, rifting) increase Vbasin. With Vwater unchanged, Δh ≈ −ΔVbasin / A_ocean. Example calculation: if basin volume increased by ΔV = 1.0 × 10^12 m^3 and ocean area A_ocean ≈ 3.61 × 10^14 m^2, then Δh ≈ −(1.0×10^12)/(3.61×10^14) ≈ −0.0028 m (≈ −2.8 mm). This demonstrates how increased basin volume produces a fall in sea level.
3. Evaluate other options:
- Land ice melts: adds water previously stored on land into the oceans, increasing Vwater and raising sea level—so incorrect.
- Ocean water warms: thermal expansion increases water volume (even without added mass), raising sea level—incorrect.
- Sediment eroded from land and deposited in oceans: deposits fill basin space, reducing basin capacity (Vbasin decreases), so sea level rises, not falls—incorrect.

Why the correct answer is correct: Only expanding ocean basins directly increase basin capacity and thus lower equilibrium sea level for a fixed water mass. Therefore the correct choice is "Tectonic forces cause ocean basins to expand," which matches the provided answer.

Which of the following would cause sea level to fall (lower)? Single choice

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