Changing Coasts and Sea Levels

Changing Coasts and Sea Levels

Experts on events happening near coasts often say that “Change is the only constant”. The Sea Grant Program at the Woods Hole Oceanographic Institution, on Cape Cod, reported that about 75% of the U.S. coastline is eroding, with only about 25% stable or advancing. For Massachusetts, 68% of the coastline was listed as eroding, 30% advancing, and a mere 2% stable. As we will discuss soon, much of the retreat is being driven by sea-level rise, which is being driven by human-caused global warming. But, the rising sea level is interacting with a very complex coastal system, and we’ll look at a little of that complexity, too, with land still moving vertically because of the ice age, coasts responding to natural and human-caused changes in sediment supply, and more.  

Up in Maine, the rocky coasts of Mt. Desert Island are among the few places that would be classified as “stable,” although very slow erosion is occurring as the sea pounds the granite headlands. But if you look further back in time, the size of the changes becomes evident. Glacial ice overran the highest peaks in the park during the ice age. Sea level was lowered 300-400 feet (100 m or a bit more) at that time to supply the water that grew the ice sheets, but the land of Mt. Desert Island was pushed down 600-700 feet (roughly 200 m) or even more by the weight of the ice. 

Ice-age ice extended south of Maine, beyond Cape Cod, and that southern ice began melting before the ice left Maine, so the sea began rising, and then loss of ice on Maine caused the rocks of Mt. Desert Island to begin rising faster than the sea; these rocks are still rising slowly today. Thus, as the ice retreated, the already much-raised sea first flooded in across broad regions of Maine and adjacent parts of the east coast. Beaches and sea caves formed along the edge of the sea, and deltas were deposited. Then, these coastal features were raised out of the ocean as the land rebounded. Such coastal features can be found today in Acadia to almost 300 feet (almost 100 m) above the modern sea level, and similar features occur all along coastal Maine, often extending well inland. We include a video of similar features from Greenland; the features in Maine are covered with blueberry fields, or trees, houses, roads and such, and although the features are quite easily identified by experienced geologists, the features are not as clear as those in Greenland to the beginning geologist. 

Video: Raised Deltas and Beaches (2:48 minutes)

You can see and hear the story of raised deltas and beaches in Maine, Greenland and elsewhere in this short video.

Click here for a transcript of the Raised Deltas and Beaches video.

Dr. Richard B. Alley: The Ice Age led to the development of beaches and deltas that are now found well above sea level, as in Acadia or in South Greenland, as shown here. Let’s see how. We will start with some land next to the ocean, under the air. You might find a moose on your land—or not. And at some time in the past, an Ice Age happened. Water that evaporated from the ocean fell as snow on the land. This lowered the ocean as the water was transferred into the ice sheet. The ice sheet immediately began sinking under its own weight, but it took thousands of years (and even longer) for the full sinking to occur—to get way down to the bottom, even though the top was way up. Then, the Ice Age ended, the ice melted, the water went back into the ocean and the ocean extended farther inland than it had originally because the land had not finished bobbing up from being pushed down under the ice. When the ocean pounded on the shore, it made a beach—or maybe a delta was built there—far inland. And then as the land continued to rise, that beach or delta was raised way above sea level, where you can find it today—maybe with a moose in Acadia, or maybe not.

So we go back to this picture of beaches in Greenland. In some of these beaches you can find shells. You can date those shells using radiocarbon or other techniques. These are about 8,000 years old from Greenland. This is a delta that formed in Greenland, where a stream was flowing into the ocean. And here's a very similar delta viewed from the air—a very nice picture from the Maine Geological Survey. This Maine delta looks red because the picture was taken in the fall and the delta is covered with blueberries that grow in the sandy soil and the blueberry leaves turn red in the fall. The little yellow box on the far right there is around some features in the delta in Maine that look almost identical to those features in the delta in Greenland. So, if you go to Acadia, you go to Maine, or you go to Greenland, you can see features that were formed on the coast; they’re now well inland and well up because the Ice Age ended.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0(link is external)

Regions that were slightly beyond the reach of the ice-age glaciers were pushed up during the ice age to form a forebulge, where the hot, soft, deep rocks pushed out from beneath the sinking ice sheets bulged up the land just before the ice. In those forebulge regions, the land now is sinking, as the deep, hot rocks flow back to their starting point; where forebulge sinking has combined with rising sea level as the ice melted, the total sea level rise has been especially large. Far from the ice sheets, sea-level rise has been about what you would expect based on the amount of water returned to the ocean by the melting ice sheets. (If you took a more-advanced course, you would learn that the entire surface of the Earth was warped by shifting water from the oceans to the ice sheets and back during the ice-age cycle, so the changes are all a bit more complicated than you might expect, just as a wine glass balanced anywhere on a cheap air mattress or water bed may tip over if you sit anywhere on the bed). 

Video: Forebulge (2:09 minutes)

To see the description of the Earth’s “water bed” responding to the ice age, watch this very short video.

Click here for a transcript of the Forbulge video.

Dr. Richard B. Alley: These are data from NASA showing vertical motions of the land along the coasts. Because the Ice Age ended, the areas shown in red are rising and the areas shown in blue are sinking as much as about 5 millimeters a year or an inch and five years. Here's why. Let's start with some air over some land and then let's drop an ice sheet on it. And when you drop the ice sheet on the weight of the ice pushes down so the land begins to sink, the deep, hot mantle flows out to the sides and that bulges up the land before the ice sheet in what we call the forebulge. It takes thousands of years and even longer for the total motion to occur until it comes into balance. When you melt the ice, you may still just have land, the ocean may come in or a lake may form and things start to go back where they were before. If you were on an island in the middle there, you would be rising, perhaps very rapidly, and this motion can continue for thousands of years and longer. Today, globally, sea level is rising by more than 3 millimeters a year or an inch in about 7 years, mainly because of global warming. A house on land that had been been pushed down may be rising almost as fast as the sea. It may be rising even faster than the sea and coming out of the sea. But, a house on the former forebulge is flooding really fast as it sinks while the sea rises. Offshore of Pennsylvania we have a little bit of sinking going on, so sea level rise is faster than it otherwise would be. Whereas in the places of the former ice sheet we have the land rising almost as fast as the ocean or even faster than the ocean.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0(link is external)

By now, you may be getting the idea that what happens to a particular coast is fairly complex. If mountain-building is pushing the coast up, it rises; if mountain-building is pushing the coast down, it sinks. Where plates meet, when the edges lock together and build toward an earthquake, the motion may drag one side down and push the other side up; the earthquake that follows will suddenly reverse the offset—in the great Tohoku earthquake of Japan in 2011, parts of the Japanese coast moved as much as 8 feet toward North America, and offshore the largest motions of the sea floor were more than 150 feet horizontally and more than 20 feet vertically. Where cities are built on deltas, as in New Orleans, the compaction of the mud causes sinking. Much additional sinking is caused by pumping water or oil or gas out of the ground; as the fluids are removed, the ground compacts. This is happening a little on Cape Cod, and is quite dramatic in some places. Such pumping may have contributed to problems in and near New Orleans, in Venice, and elsewhere. (Pumping fluids back into the ground can partially offset this problem, and is being used in some places, but generally does not completely fix the problem.) 

So, coasts may be going underwater, or rising out of the water, because of sea-level changes as described below, and because of the land going up or down. But, coasts also may advance or retreat because of issues related to the waves moving sand and other sediment.

Beaches inevitably lose a little sediment to deep water, somewhat like losing socks behind a clothes dryer, because it is easy to drop something that falls way down there, and hard to get it back. Waves can pick up sand from below the ocean’s surface and carry that sand to the beach, but waves cannot reach sand in very deep water (no deeper than roughly half the distance between a wave’s crest and the next one). If sediment happens to slide or bounce deeper than that, then that sediment cannot be brought back easily. (The sediment can go into a subduction zone, make new mountains, and be eroded to make new sand that reaches a beach by longshore drift, but that takes millions of years or longer.)

Thus, a “happy” beach requires a supply of sediment to balance the loss to deep water. Normally, that supply comes from the material delivered to deltas by rivers, and carried to the beach by longshore drift. But if there is not enough sediment coming this way, the beach will narrow as it loses sediment to the deep ocean, and the waves will crash across the sand to erode the material behind it, gaining sediment in this way.

In some cases such as Acadia, longshore drift does not supply enough sand to sustain a beach, and the rocks are too hard for the waves to break rapidly to supply a beach. Then, the little bit of sand produced by the waves ends up in deep water, and many of the cliffs have no beaches. (There are a few small “pocket” beaches at Acadia in protected places, but most of the coast doesn’t have beaches, with the waves pounding directly on granite.) In other cases such as Cape Cod, the waves crossing the beach hit sand and gravel left by the glaciers, easily eroding the loose material to supply beaches.

In some places, dams on rivers have greatly reduced the delivery of sediment to the longshore drift, so the nearby coasts are eroding. You may recall that the dams on the Elwha River, draining Olympic National Park, caused the loss of beaches along the nearby coast. At Cape Cod, there really aren’t any rivers that humans could dam. The glaciers made a big pile of sediment in a place where rivers are not supplying much sediment to deltas, and so the Cape eventually will be lost to deep water.