The overwhelming majority of our Earth’s surface – 71% – is water. Of that water, 97% of it is located in our oceans. Somewhat counterintuitively, this means that the vast majority of Earth’s topographic features – valleys, plains, mountains – are located under water as well. Today, let’s take a “deep dive” and examine some ocean floor features you should know about.Oceanic Topography
Today we’ll take a look at many prominent submerged features. Before taking the plunge, stop for a moment to examine the above diagram from the National Oceanic and Atmospheric Administration. It will serve as a convenient visual guide as we descend into the deep.
Continental shelves are large landmasses that surround each of the continents. This area is usually very shallow (comparatively), typically less than a few hundred feet. Continental shelves account for roughly 8% of all submerged features. Interestingly, continental shelves can drastically vary in size. For example, the continental shelf off the coast of Siberia extends 1,500 km (930 miles) into the Arctic Ocean. Conversely, off the coast of Africa, the continental shelf extends only 10 km (6 miles) into the Atlantic Ocean.
Continental shelves also serve as bastions of biological diversity. According to some estimates, about 90% of the world’s fish are found along the continental shelf. More interestingly, practically all of the world’s oceanic plants and most types of algae also live along the continental shelf.
Perhaps the most famous of continental shelves is the Bering Strait. A mountain of evidence supports the hypothetical land bridge that anthropologists, geologists, and climate scientists believe allowed humans to cross over to North America from Asia some 17,000 years ago. Although the strait is now underwater, it is just barely submerged. The deepest section is less than 55m (180 feet) underwater.
The end of the continental shelf is referred to as the continental slope. Geologists may refer to this location as the “seaward border of the continental shelf.” This ocean feature accounts for roughly 9% of the entirety of the ocean floor. On average, the ocean’s continental slope descends into the sea at an angle of 4°. This may not seem like much, but over the course of 100km (60 miles) of the continental slope, the ocean depth will increase by 70 km (43 miles)!
While the average slope of descent for continental slopes is 4°, some are much steeper. About 1,600 km (1000 miles) off the Cape of Good Hope, Africa, the continental slope drops some 20,000 feet over the course of only 16 km (10 miles). This corresponds to an average slope of 70°!
Abyssal plains are the most common land feature on planet earth. They make up half of all of the ocean floor. The International Hydrographic Organization (IHO) defines abyssal plains as “extensive, flat, gently sloping or nearly level region at abyssal depths.” Abyssal depths are approximately 3,000-6,000 meters (10,000-20,000 feet). When the IHO says that these plains are flat – they’re not kidding. Compared to continental slopes, which fall roughly 2,800 meters (9,000 feet) for every 1,000 meters (3,000 feet), abyssal plains fall, on average, less than 1 meter per 1000 meters!
Abyssal plains are also the largest habitat on earth. Even so, extremely little is known about the organisms that inhabit this zone. This is mainly due to the stark lack of sunlight that penetrates to these depths. Sunlight only reaches roughly 1,000 meters (3,000 feet) down into the ocean. Considering that the abyssal plains exist, at the shallowest depth, at 3,000 meters, absolutely no sun reaches the floor here.
Abyssal hills are exactly what they sound like: relatively small hills that rise out of the abyssal plain. These features comprise roughly 30% of the ocean floor. Typically, they rise no more than a few hundred meters above the abyssal plain and are less than 100 meters (300 feet) in width.
Breaking down the term seamount we see two parts: sea and mount. The “sea” part references the fact that these features are under water. The “mount” part stems from “mountain.” Putting these two together, you have the definition of a seamount: an underwater mountain! Importantly, seamounts cannot break the water’s surface.
Deep, deep below the water’s surface lie vast, unexplored regions of the ocean floor: trenches. These underwater canyons are the deepest spots in the ocean. In fact, from sea level, the surface of the earth extends further down than it does up! Mt. Everest is the tallest point on earth (elevation wise) at 8,848 meters (29,029 feet). In comparison, the Mariana Trench stretches down 11,034 meters (36,201 feet) below the surface. If we were to place Mt. Everest in the bottom of the trench, its summit would still be about 1.6 km (1 mile) below the surface!
Volcanic islands are usually referred to by their common name: islands. By definition, seamounts that break the water’s surface are called islands. In places like Hawaii, one can actually see the process of new land being formed as lava cools as it enters the ocean. Hawai’i Volcanoes National Park is a prime example of this. Islands make up a relatively tiny fraction of the total area, but they are very numerous. The amount of islands in the world is practically impossible to estimate. Presently, the estimation ranges from about 5,000 oceanic islands to over 100,000. This figure also changes over geologic time. Islands tend to be somewhat unstable, and as global sea levels fluctuate over millions of years, new islands are exposed while others are submerged.
[Featured image via Pixabay from Mariamichelle]
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