What does eustasy refer to?

• A. Local/regional sea-level changes due to tectonics.
• B. Changes in sea level due to weather patterns.
• C. Sea-level changes due to gravitational interactions with the Moon.
• D. Global sea-level changes due to ocean volume changes.

Answer: (D)

Eustasy describes global sea-level change that comes from a change in the total volume of the world’s oceans. When water is added to the oceans—such as from melting ice sheets and glaciers—or when ocean water expands as it warms, the overall ocean volume increases and the global mean sea level rises. This is different from regional shifts caused by land moving up or down beneath the sea (tectonics), which change sea level only in certain areas, or from weather-driven effects like winds and atmospheric pressure that can push water around locally and temporarily. Tides, driven by the Moon’s gravity, cause regular fluctuations but don’t produce a sustained global change in ocean volume. So the best description is global sea-level changes due to ocean volume changes.