Plate tectonics significantly influences sea level, both globally (eustatic sea level) and locally, primarily through changes in ocean basin volume and land elevation.
Mechanisms of Influence
Plate tectonics affects sea level in several key ways:
- Changes in Ocean Basin Volume:
- Mid-ocean ridge activity: Faster seafloor spreading at mid-ocean ridges creates larger, more buoyant ridges. These ridges displace more water, leading to a higher sea level. Conversely, slower spreading results in smaller ridges and lower sea levels. This is a long-term process, operating over millions of years.
- Formation and destruction of oceanic plateaus: The creation of large oceanic plateaus reduces ocean basin volume, raising sea level. Subduction of these plateaus increases ocean basin volume, lowering sea level.
- Vertical Land Movement:
- Tectonic Uplift and Subsidence: Plate collisions and subduction zones can cause uplift in coastal areas, making it appear that sea level is falling locally, even if global sea levels are rising. Similarly, subsidence (sinking) due to tectonic activity can cause relative sea level rise.
- Isostatic Adjustment: While not directly plate tectonics per se, isostatic adjustment is a related process. After the removal of large ice sheets (post-glacial rebound) or the deposition of significant sediment loads, the Earth's crust slowly rebounds or subsides, respectively, affecting relative sea level.
- Changes in Continental Configuration:
- Supercontinent Cycles: The formation and breakup of supercontinents influence global climate and ocean circulation, indirectly affecting sea level through changes in ice volume.
Eustatic vs. Relative Sea Level
It's important to distinguish between eustatic (global) sea level change and relative sea level change.
- Eustatic Sea Level: Refers to the average height of the ocean across the globe. It's affected by changes in the volume of water in the ocean (e.g., melting glaciers) and the volume of the ocean basins (as influenced by plate tectonics).
- Relative Sea Level: Refers to the sea level at a particular location relative to the land. This is affected by both eustatic changes and local vertical land movement (tectonic uplift or subsidence).
Examples
- Japan: Located in a tectonically active region, Japan experiences both uplift and subsidence due to plate interactions, leading to varying rates of relative sea level change along its coast.
- Scandinavia: Experiences ongoing isostatic rebound following the last glacial period. The land is rising, resulting in a perceived decrease in sea level relative to the land.
Table: Plate Tectonic Processes and Sea Level Effects
| Plate Tectonic Process | Effect on Ocean Basin Volume | Effect on Global Sea Level | Effect on Local Sea Level |
|---|---|---|---|
| Increased Mid-Ocean Ridge Activity | Decrease | Rise | May be masked by regional uplift/subsidence |
| Decreased Mid-Ocean Ridge Activity | Increase | Fall | May be masked by regional uplift/subsidence |
| Continental Collision/Uplift | No direct effect | Indirect (climate change) | Relative sea level fall |
| Subduction/Subsidence | No direct effect | Indirect (climate change) | Relative sea level rise |
In summary, plate tectonics plays a crucial role in shaping sea level, both globally and locally, through its influence on ocean basin volume and vertical land movement.
