Features of the Seafloor

Parts of the Seafloor

How parts of the seafloor are classified is fairly simple:  the continental margin and the deep seafloor.

Do you remember the densities of continental and oceanic crust?  Which one is heavier?  If you don't, review Plate Tectonics: The Layers of the Earth page.

There is a height difference between the top of the continental crust and the top of the oceanic crust due to the difference in densities.  All of the oceanic crust is located under water, but not all of the continental crust is. 

The portion of the continental crust that is under water is called the continental margin.  The amount of continental crust that is under water is dependent upon a few factors:

• Sea level - during periods of global warming, the ocean level is higher, therefore more of the continental margin is under water.  During periods of global cooling, sea level drops, and the continental margin gets narrower.
  
  
  • Lower sea level
     
    
    Image source:  Sonjia Leyva, own work, © 2020.
    
    
     
  • Higher sea level
     
    

    Image source:  Sonjia Leyva, own work, © 2020.

    
     
    
• Topography - some coastal areas are flat, others are full of cliffs, hills, or mountains.  The sea can extend inward more on gently sloping areas, creating a wide continental margin.  Exceptionally wide continental margins are often called "Land Bridges".

Take a look at these two Google Earth images.  The first shows the Doggerland region in Europe.  Archaeology shows that this area was inhabited by Mesolithic people as far back as 12,000 years ago.  Melting of the ice sheets began to inundate this area, causing people to migrate elsewhere around 6,000 years ago.  Today, the region is covered in approximately 100 meters of seawater.  The Doggerland area is referred to as a "Land Bridge".  On the image, the light blue areas represent shallow water - the continental margin and, in this case, a land bridge.

Eduard Suess believed that South America, Africa, and India were once connected and were separated as sea levels rose and a land bridge was connected between them.  You learned in the Plate tectonics lecture why this isn't true - those continents were separated by mid-ocean ridges.  The Doggerland area is a true land bridge, creating a nice, wide continental margin for western Europe.

Image source:  "Satelite Imagery" Google Data: SIO, NOAA, U.S. Navy, NGA, GEBCO, IBACO, Landsat / Copernicus, ORION-ME, U.S. Geological Survey by is in the Public Domain

Next is the Bering Land Bridge.  Like the Doggerland area, this land bridge formed as sea levels rose as the ice sheets melted.  The area between Russia (left) and Alaska (right) has formed a wide continental margin.  Now, look south (bottom of image) where the Aleutian Islands form an arc below the land bridge.  Recall that the light blue areas represent shallow water - the continental margin.  Note that the amount of light blue south of the island chain is very narrow;  this area has a narrow continental margin.

Image source:  "Satelite Imagery" by Google Data: SIO, NOAA, U.S. Navy, NGA, GEBCO, IBACO, Landsat / Copernicus is in the Public Domain