Today, the Earth’s ocean is made up of the large Pacific, Atlantic, Indian, and Arctic Oceans. These bodies of water were not always in their current shape and configuration. As a result, you can imagine the large-scale changes in sea level that would have accompanied their assembly since the last super-continent (Pangea) began to break up some 250 million years ago. These changes would have been very slow but significant, operating on time scales beyond those experienced by human beings.
Long-Term Sea Level Change (hundreds of thousands to millions of years) is influenced by factors that modify the size and shape of ocean basins. Global or eustatic sea level can change as the result of changes in the number, size, and shape of ocean basins. Throughout Earth's history, the global ocean has been modified by plate tectonics. Often, large continents assembled from smaller ones produced more expansive oceans between them. These expansive ocean bodies were subsequently dissected when super-continents rifted and formed smaller oceans out of the formerly vast oceans. For visualization purposes, please watch the quick (3:18) paleogeographic animation below (no audio).
The tectonic processes at work on the Earth influence the size of ocean basins and therefore, sea level in many, complex ways. The following list gives an idea of some of these processes and their interactions and feedback mechanisms:
- rifting of tectonic plates at divergent plate boundaries;
- assembly of micro-continents, volcanic terrains, continents - especially supercontinents like Rodinia, Pangea, etc.;
- subduction of tectonic plates at ocean trenches at convergent plate boundaries;
- eruption and formation of large igneous provinces that originate from massive extrusions of lava, oceanic plateaus, hotspot volcanic island chains, etc.;
- high rates of volcanism on the seafloor volumetrically displace water out of the ocean basin producing higher sea levels (called transgression of sea level);
- low rates of volcanism allow water to return to the ocean basin and sea levels drop (called regression of sea level);
- when rocks cool from a molten state, they contract in volume; this allows subsidence to occur, especially along the mid-ocean ridges, and sea levels fall;
- when rates of volcanism are low, rocks tend to cool faster and sea levels drop as subsidence occurs.
- conversely, when rates of volcanism are high, it takes longer for the rocks to cool, and sea level remains higher for longer periods of time after the rate of volcanism subsides.
For more information
Take a look at a schematic of four factors that influence sea level. Here is more explanation of this concept.