NOTES ON SEA LEVEL AND CORAL ISLANDS

The following is a comment by Jonathan Scott on Not A Lot of People Know That. I thought it was so interesting that it deserved a wider airing.

Sea level is relative. Relative Sea Level is referenced by a position on land. Now the sea level indeed can and does rise AND fall but so does the land. Sea level rise and fall is eustasy and land rise and fall is isostasy. The Upper Crust of the Earth floats on a more plastic Lower Crust and Mantle. Push it down in one place and for equilibrium it will rise in another. 

All very simple and logical. The UK for example is rebounding from the ice sheet which covered a large proportion of the land mass during the first part of the current Ice Age and pushed the land in the North down. The net rebound effect today is that Scotland is rising and the South of England sinking, nothing at all to do with NET sea level rise due to increased volume of water. 

A second land movement is caused by tectonics, usually when there is an interaction at the margins of two continental tectonic plates.. 

Next comes sediment behaviour on coastal planes. A river is really an emulsion of sediment suspended in water and when the sediment is deposited finally it contains a lot of water and that water wants to get out. Where rivers meet the sea the velocity suddenly is reduced because the river collides with the sea dropping its sediment load usually to form Deltas. 

Deltas are another thing which subside all on their own. With a little help from humans they can and do subside substantially faster! . Deltas prograde out into the ocean, get slope failure and also dewater so they subside. Put some humans running around on top and sinking water wells and guess what happens, the sediments dewater all the more quickly. (The political scientists of the BBC does try it on every 6 months or so to blame the high rate at which the Mekong delta is sinking incredibly on sea level rise alone!) 

If you do not have a grasp of the relative influence of eustasy, isostasy tectonics and sediment type and consolidation processes at the coast you cannot possibly predict forward how relative sea level will behave.

So now onto the climate poster boys, those sinking Pacific atols. These are the emotional symbols of Western man’s wickedness falsely touted by wicked people among the plethora of hand wringing activist groups. 

However, there is a lot more going on than you will ever hear from the wailing political scientists, profiteers and their chums spreading the media message, Atolls are extinct volcanoes. They form on basaltic oceanic crust over a hot spot in the Earth’s mantle. Heat causes the crust to rise and like a teenage pimple the volcano erupts and as a consequence builds up and up, in some cases forming an island. 

Now! The oceanic plates are not stationary. Some move faster than others. So the volcano is not stationary but moves relative to the hotspot. Once the volcano has moved off the hotspot three things happen, Firstly, the volcano stops erupting, secondly it begins to sink because the heat source under it is no longer there and third it begins to erode or should I say new eruptions no longer keep pace with erosion.

 In time a combination of sinking and erosion takes it down to sea level. Corals can form only in relatively shallow water. While there is an volcano with steep sides, any reefs will be small and constantly destroyed. Only when the top of the volcano is eroded away does sufficient area within the correct depth(and energy ) range permit a decent rim of coral to form.

Next comes the problem. The coral grows up towards the sunlight at a maximum rate per species controlled by several factors ( water temperature salinity, nutrients etc). Sea level is not a constant but varies up and down. The live coral wants to be as close to sea level as possible because it needs sunlight so very often it gets exposed sub-aerially and wonder of wonders bleaching occurs. 

The fact we have coral islands shows that in the recent past sea level was much higher. So back to what the atoll is doing. The atoll may sink at a similar rate to coral growth or the coral grows faster than the atoll sinks but inevitably the coral will lose the race and inevitably the island disappears below the waves. 

Not all rates of sinking atolls are the same and in some instances TODAY quite a number of islands made from coral are actually growing. As for the sinking or drowned atoll, after some time it can the be called a guyot. These can be found often hundreds of metres below sea level often with fossilised coral reefs attached to them. 

The hotspots do not emit lava constantly but like everything else in the universe they are pulsed. Because of this and because of the plate movements a volcanic island chain is formed which allows the casual observer to track the movement of the tectonic plate. 

The Hawaiian island chain is one example of a long lived hot spot at work. There are also isolated volcanic islands which are not part of a chain but represent individual short lived Mantle plumes.

So given that atolls are what they are, a complex combination of extinct sinking and eroding volcano and different coral species growing at different rates in different geographical locations on top of them, how on earth can they be used as a poster boy for NET sea level rise especially when real empirical data shows some of those poster children actually growing, except by people who will happily and wilfully deceive for profit or for power, or for both.