This is the seventh in a series of reposts from gregladen.com on global warming.
This installment is about sea level rise and fall, in the past. Sea level change that results from the formation and melting of glaciers not only has an enormous impact on the physical nature of the landscape, but it also would not have gone unnoticed by people living ever pretty far from the sea!
With large amounts of the world’s water trapped in glaciers (mainly continental glaciers), the sea level drops. When that ice melts, the sea level rises.
As you know, the earth is covered by two kinds of surface: Continents, which are relatively tall and buoyant and which have a tendency to move around, and sea floor, which is structurally different from the continents. But if you look at the oceans, you will see that they cover both sea floor and parts of the continents. The parts of the continents that are covered by sea floor are typically referred to as “continental shelf.” All this … this continental shelf … really is the edge of the continents themselves that happen at the moment to be covered with the sea. There are places, like the coast of California, where there is no shelf, and other places, like the coast of New England, much of the Caribbean and large parts of the Gulf of Mexico, that have extensive shelf. If you removed all the water from this shelf, you could fit a couple of more New England states between Boston (now on the coast) and the new coast line.
There have been times in the past when the ocean may have covered even more of the continents then they do now, and thus, there were large inland seas. This may have been because the continents were shorter, or because more of the water was out there in the ocean instead of trapped in glacial ice, or it may have been because the sea floor was somewhat raised (I can’t vouch for that theory, but I’ve seen it referred to recently).
Notice that continents get built up because of mountain building, and on the surfaces, and there can be a build up of sediments when a basin forms … an area ringed by highlands in which an inland sea would form given enough water. An example of a basin is the “basin and range” area of the American West, with the Great Salt Lake being one of the major low spots …. streams or rivers in this area do not flow to the ocean, they flow the low point in the basins. Another example is the Okavongo basin in southern Africa, and still another is the Lake Chad basin in north-Central Africa.
Continents get shorter via erosion. If there was no building up of continents, all of the continents would eventually erode down to sea level.
Continents also build up from beneath. The hot molten stuff underneath the continents can intrude into the continental mass and harden there. Really obvious examples of this are certain “pushup” ranges. These are not true mountains (which are caused by folding and mushing of the continents) but rather, they are relatively buoyant masses of deep molten stuff pushing up and hardening as they go. The Rwenzori mountain on the Congo-Uganda border is an example of one of these, most likely (the geology there is still in dispute). The Adirondack Mountains is another example. My favorite example is Mount Desert Island, in Acadia National Park, Maine. This is a small pushup, and you can drive around it and see the edges, along with evidence of what happens when a huge mass of molted hot deep-earth stuff slowly works its way to the surface, eating (melting) the rock into which it intrudes.
So, the continents are always building up and they are always being worn down. In the mean time, as you know from previous posts on this topic, glacial periods are coming and going (over the last couple of million years). This means that the sea level is constantly moving up and down along the edge of the continent.
Since the continent is always eroding, and it is bouyant, a given place on the edge of the continent tends to become higher and higher in elevation over time. As you wear down the surface, the continent rises. For this reason, in many areas, the earlier oceanic shoreline (the “strandline”) is found raised above the place where later high sea level shorlines rest against the continent. A spectacular example of this is the Niger Delta. If you look at a geological map of that region, you see delta deposits dating from tens of millions of years ago way inland.
One practical application of this phenomenon, for humans, is the manufacture of caves. Erosion along the shorline will sometimes form caves in the intertidal zone. Humans can’t live in these caves, because they become filled with water a couple of times a day. But eventually a glacial period comes along, and the sea level drops, the continent continues to erode (and in fact erodes more quickly because the shelf is exposed) and raises up. In this manner, the cave rises up. So even with the next high sea level stand, during an interglacial, that cave may be high and dry.
The southern and northern coasts of Africa are both dotted with these caves. The famous Klasies River Mouth cave in South Africa, an important archaeological site, is one example. Haua Fteah, a very important archaeological site in Libya, is another.
It appears that continental glaciers form slowly, but melt quickly. This means that the regression of the sea level (lowering) is slow, but the transgression (rising sea level) can be quite quick. One of the favorite feeding grounds of whales off the coast of New England is Georges Bank. This is a sea mount on the continental shelf. When sea levels were low, this was a high spot … a hilly area … well away from the ocean. When the sea level rose, it became an island. About this time we believe it was occupied by people … their artifacts are pulled up in nets now and then. Then it became a smaller island. Then it was swallowed by the sea.
Did the people of George’s Island have boats? If not, did they invent boats? If not, did they all just drown? If they got off the island, did they bring the story with them? If so, it is lost to history now, demonstrating that not only is it not true that all origin stories involve a flood, but that not all floods make it into origin stories!
Another spectacular example of sea level rise may have been the Nullarbor Plain of Australia. This is a big flat dry area in southern Australia. About half (or a bit less?) of the plain is under the sea, the other portion is dry land. When the sea level was lower, the entire thing was dry. The Nullarbor plain is very very flat, and the portion that is flooded by the sea now would likely have been flooded during a time when post-glacial sea levels were rising very quickly. It is very likely that there would have been times when people were camped inland, thinking “Hey, let’s walk down to the sea tomorrow. It’s about a day and a half walk, we’ll be there by Saturday.” Then they go to bed. Then they wake up on the coast. If they’re lucky.
The graphic (above) is from the Global Warming Art Project and was prepared from various data sources by Robert A. Rhode. Since I got this form Wikipedia, I have no idea if Robert A. Rhode actually exists. But I am very familiar with the sea level data, and this is a good chart. Notice that over the last 20 thousand years, the sea level has risen about 120 meters. There were many times when the sea level rise rate was as much as 10 cm. a year or more, and brief periods when it may have been close to 1 meter a year.