Tectonics

I've spent the last few weeks looking at how sea level could change from changing the amount of water in the oceans by melting ice. Now I want to look into something completely different - tectonics - how can the movement of tectonic plates affect sea level? In terms of sea level change in our lifetimes this is one of the less important factors, but it's still important to take it into account.


Long term geological patterns in sea level change are thought to be mainly the result of changing spreading rates at mid-oceanic ridges. When mean spreading rates are faster more new crust is created, newer crust is warmer and more buoyant so the topography of the ocean basins in higher. This makes sea level appear higher at the coast (Moucha et al, 2008). Or indeed vice versa.



So here you can see the relative ages of the sea floor moving away from the mid-oceanic ridges (in the middle of the red), unfortunately it doesn't come with a key but red is younger and the oldest ocean crust is dark blue (NOC). So you can see that there is alot more younger crust in the Pacific Ocean than the Atlantic. According to one of my old lecturers mid-oceanic ridges are where all the sexy stuff happens, I'm yet to be convinced.

However, it has been argued by various groups that this theory is too simplistic. It doesn't take into account subduction zones which can also change the height of the oceanic lithosphere and therefore sea level. In fact, faster subduction rates at the same time as fast spreading rates, and the subduction of older, colder, oceanic crust may have a counteractive effect on sea level. Subduction of old oceanic crust can cause subsidence of the overlying lithosphere, and therefore a relative sea level rise. (Husson and Conrad, 2006).


Husson and Conrad (2006) used a dynamic model to determine the effects of changing velocity of tectonic plates on sea level. They concluded that short term changes in plate velocity had little effect on the volume of ocean basins - only up to 22 metres change in relative sea level (still sounds like quite a lot to me). Whereas for longer term changes in plate velocity, the amount of subducted crust decreases. The model suggested larger variations in ocean basin volume, and sea level changes of up to 78 metres.


But this isn't all, the age of the sea floor (and therefore it's buoyancy) is also related to the layout of the continents. When the continents are joined together in a supercontinent formation, such as Pangaea, sea levels are generally low because the sea floor is older (Miller et al., 2005). Supercontinent formations  also promote ice on land, and also mean more mountain building as the continents collide also resulting in lower sea levels (Miller et al., 2005). Ice sheets are also more likely to form when there is land at the poles - it doesn't matter whether this is in a supercontinent or dispersed formation - Antarctica is a really good example of this - it's the coldest place on the planet. Ice sheet formation locks up water on land, and so lowers the sea level.


Pretty serious stuff, however before you all go out and invest in a boat it is worth remembering that this all on geological time-scales. The IPCC put the current effect of tectonics on contemporary sea level rise at only 0.1 mm a year. Earthquakes are the only way that tectonics can cause rapid sea level change. They do this by causing rapid vertical land movements and tsunamis which can cause sudden sea level rise, but even then they can't effect the global mean sea level (Melini et al, 2004).