Posted by: Rob Viens | March 25, 2012

Sailing on the Edge of the Continent

March 25th – Another day at sea – the Beagle was still steadily heading southward towards the Abrolhos Islands on route to Rio. Sometime on this day or the next, the ship crossed onto the South American continental shelf.

“These three days, like the weather, have passed away with quietness & enjoyment. — We are nearly 4 degrees from the coast of Brazil & about 2 from the Albrolhos, from which islands a long shoal extends itself. — The Lead has been regularly cast at every two hours. — to day after finding no bottom at 230 fathoms we suddenly came on the bank with between 30 and 40. We are now steering for the islands.” (Mar 24/25/26)

The map below shows the coast of Brazil, with Salvador near the top and Rio near the bottom.  The blue color represents depth.  Notice that along the cost there is a band of light blue (shallow water) that is found roughly parallel to the coast (zoom out and you will see it to some degree along the edge of all the continents). This band of shallow water is called the continental shelf – it is  literally the part of the continents that is currently covered by the sea.

You’ll also notice that the continental shelf is of varying width, and there is a couple places along the coast of Brazil where it sticks out into the Atlantic several hundred kilometers.  It is one of these shallow “benches” that the Beagle detects with its sounding line/lead  (the timing seems right for it to be the the northern of the two large extensions of the shelf).  Thirty to Forty Fathoms (around 200 feet) is a reasonable depth for the continental shelf.

To understand continental shelves, first you need to know something about the elevation of the Earth’s surface.  The diagram below shows the percent of the Earth’s surface located at different elevations. A couple of things stand out – (1) There is more land below sea level (blues – about 71%) than above, and (2) most of the earths surface is found either (a) within 1 km above sea level or (b) between about 4-5 km below sea level.  These peaks represent the typical elevation of the continental crust and the ocean crust of the Earth.

Histogram of the elevation of the Earth's surface

Why is there a difference in elevation at all between the two types of crust (you may be asking yourself)? To understand that difference we have to look back at our old friends mafic and felsic rocks (for a refresher, see Fernando de Noronha II). The bottom line is that the continents are primarily made of felsic rock, and the oceans are primarily made of mafic rock. Since mafic rock is denser than felsic rock it “sinks” lower in the soft (though very solid) mantle below, reaching a comfortable equilibrium about 4-5 km below sea level. The continents are made of less dense rock that “floats” higher on the mantle, reaching a stable elevation about 0-1 lm above sea level.  Think of it like a ship in the water (though I have to emphasize again the mantle is not liquid like water – its a plastic material that can flow very slowly like silly puddy).  An empty (lighter) ship will float higher on the ocean than one that is loaded with cargo (heavier). Voila – it all comes back to the rocks!

The water that covers the Earth’s surface naturally covers the lower areas – in the case of our planet, that is the ocean crust. And it just so happens that the amount of water on the Earth fills up the ocean basins to just about the level of the continental crust. Hence sea level roughly marks the boundary between the ocean crust and the continental crust. Pretty cool –eh?

Of course, I said “roughly marks the boundary”.  In fact, in modern times sea level is high enough that it actually covers the edge of the continental crust, too, It is this submerged continental crust (i.e., the continental shelf) that the Beagle sailed over 180 years ago today.

You might also note that I said “in modern times”.  That is because, as Darwin reminded us at St. Jago, the level of the ocean changes over time.  About 20,000 years ago, at the height of the last glacial period, there was a lot of water locked up in giant ice sheets covering the northern continents.  This led to a drop in global sea level of over 100 meters, and meant that the areas we call continental shelves today, where actually dry land at the time.  Alternatively, if you go back to the Cretaceous time period (say about 100 million years ago) you’d find that sea level was higher (by over 150 meters), and that you could have sailed a ship though a seaway that connected the Gulf of Mexico with the Arctic Ocean (via Colorado and Edmonton). The things Darwin would have seen if he could have taken that voyage! (Why that happened will have to be the subject of another post…)

Map of the continental shelf and the ocean floor (from etap.org site – resources for home schooling):

Cross section of the continental shelf

The approximate edge of the continental crust is called the continental slope – where the depth of the ocean rapidly drops (relatively speaking – its not literally a cliff) to the deep abyssal plans of the ocean floor. These depths are far below the length of the sounding lines used by the Beagle crew, and in fact were not accurately maps until much later in the 1940’s and 50’s. (RJV)

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Responses

  1. […] shelves, on average, are only about 100 m (~300 ft) deep, and as noted in an earlier post (see Sailing on the Edge of a Continent) are actually a part of the continental crust. The shelf connecting the Falklands to the mainland, […]


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