Long-Term Sea Level Change (100s of 1000s of years to millions of years) is influenced by factors that modify the size and shape of ocean basins. As highlighted previously, 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 two quick paleogeographic animations below.
Plate Tectonics in action (2:31)
Earth 100 Million Years from Now (3:18)
Today, we have the large Pacific Ocean, the 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 and, although significant, would have operated on time scales beyond those commonly experienced by human beings. For those who would like a review of plate tectonics, it is recommended that you check out one of the National Geographic videos available online. One such video includes National Geographic: The Story of Earth. These videos will help provide some context for the discussion below. If you don't understand the concepts presented here, the video might be a good place to clear some things up for you.
The Story of Earth video is long if you watch it in its entirety, and maybe you will want to do that. However, at the minimum, you should watch the Introduction of the film to 1:30 (one minute, 30 seconds) and then move forward to watch the section on plate tectonics that begins about 20:00 and runs through about 32:00. This section explores not only how plate tectonics works, but also how plate tectonics can be linked to ocean formation/destruction, climate change, and sea level change. So, pay attention to how the first super-continent called Rodinia formed and what happened after it formed. Many of the topics and observations below are highlighted in this portion of the video (1:30) entitled, The Story of Earth.
Specific observations & facts to know about Long-Term Sea Level Change
Changes in tectonic activity that modify the size of ocean basins include:
- 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.
The rate at which volcanism occurs on the seafloor and along mid-ocean ridges (seafloor spreading rates) will play a role in where sea level occurs on continents and other land masses.
- 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).
The rate of cooling of volcanic rocks under the seafloor
- 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.