Which term describes global sea-level changes due to ocean volume changes?

• A. Tectonics
• B. Isostasy
• C. Subduction
• D. Eustasy

Answer: (D)

Global sea-level changes caused by changes in ocean volume are described by eustasy. Eustatic changes reflect shifts in the total amount of seawater in the oceans and affect the global average height of the sea surface across all basins. When ice sheets and glaciers melt or rivers and groundwater release more water into the oceans, ocean volume increases and global sea level rises; conversely, water stored on land as ice or crustal water losses can lower the global sea level. Temperature also influences volume: warmer water expands, raising sea level globally, while cooling contracts it. These effects are global and basin-wide, unlike changes driven by land movement. Tectonics, by contrast, moves the crust and can cause uplift or subsidence in particular regions, leading to changes in sea level that are not global. Isostasy refers to the crust adjusting to loads, causing local vertical movements, and subduction is a tectonic process where one plate sinks beneath another—neither term describes global ocean-volume-driven sea-level change.