Seafloor Bottom

Unit 1: Sea Floor Features

Oceans 11

​Continental Margins

Continental margins refer to the region of transition from the land to the deep seafloor

• i.e. between continental and oceanic crust.  ​

​Continental Shelf

The shallow, flooded edge of the continent.

• Geologically the shelf is still part of the continental crust, but it is often overlaid with marine sediments.

• On average, the shelf extends about 80 km from the coast; some margins have very little shelf, while some extend roughly 1500 km.

• The depth of the shelf generally remains below about 150 m, and the floor of the shelf is fairly flat.

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The flat topography is the result of changes in sea level; throughout history the shelves have been both submerged and exposed, and as sea level rose and fell, wave action, ice sheets, and other erosional processes smoothed out the shelf surface. Wave action and the movement of sediments over the shelf have continued this smoothing process.

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Continental shelves only make up about 6% of the ocean’s surface area, but they are biologically one of the richest parts of the ocean; their shallow depth prevents nutrients from sinking out, and their proximity to the coast provides significant nutrient input. The continental shelf ends at the shelf break, which is the point where the angle of the seafloor begins to get steeper. The shelf break averages about 135 m deep.

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Multiple Choice

What is the name of the region of transition from the land to the deep seafloor?

• Continental Margin

• Continental Shelf

Multiple Choice

What is the average length of a continental shelf?

• 150 m 

• 1500 km

• 80 km

Multiple Choice

True or False? 

Continental shelves are biologically one of the richest parts of the ocean; their shallow depth prevents nutrients from sinking out, and their proximity to the coast provides significant nutrient input.

• True

• False

​Continental Slope

​After the shelf break, the seafloor takes on a steeper angle (about 4^o) as it descends to the deep ocean. This steeper portion of the margin is the continental slope, and it extends from the shelf break down to 3000-5000m. In some parts of the ocean, large submarine canyons have been carved into the continental slope; for example, Monterey Canyon in Monterey Bay, California, is a submarine canyon similar in size to the Grand Canyon! These canyons may be carved out by turbidity currents, which are essentially landslides of sediment, rocks, and other debris down the face of the slope.

​Continental Rise

​At the bottom of the slope is the continental rise. This area represents where the continental crust meets the oceanic crust, as the slope begins to level off to become the deep ocean floor. The rise consists of a thick layer of accumulated sediment coming from the continent, so it is difficult to tell where the slope ends and the rise begins.

Multiple Choice

This steeper portion of the margin is the continental slope, how far does it extend down from the shelf break?

• 200 - 400 km

• 300 - 500 km

• 2000 - 4000 m

• 3000 - 5000 m

Multiple Choice

Which area represents where the continental crust meets the oceanic crust?

• Continental Shelf

• Continental Slope

• Continental Rise

​Abyssal Plain

​After the rise comes the abyssal plain, or the deep ocean floor, lying between 4500 – 6000 m. The abyssal plain includes most of the ocean floor, and is the flattest region on Earth. It is flat due to millions of years of sediment accumulation on the bottom, which buries many bottom features (Figure 1.2.2).

​Abyssal Plain

​Continuing your journey across the ocean basin, you would descend the steep continental slope to the abyssal plain. At depths of over 10,000 feet and covering 70% of the ocean floor, abyssal plains are the largest habitat on earth. Sunlight does not penetrate to the sea floor, making these deep, dark ecosystems less productive than those along the continental shelf. But despite their name, these “plains” are not uniformly flat. They are interrupted by features like hills, valleys, and seamounts (underwater mountains that are also hotspots for biodiversity).

Multiple Choice

Select the false (untrue) fact regarding Abyssal Plains

• The deep ocean floor

• Flattest region on earth

• A dark ecosystem

• 3rd largest habitat on Earth

​Mid-Ocean Ridge

​Rising up from the abyssal plain, you would encounter the mid-ocean ridge, an underwater mountain range, over 40,000 miles long, rising to an average depth of 8,000 feet. Tracing their way around the global ocean, this system of underwater volcanoes forms the longest mountain range on Earth.

​Ocean Trenches

​After scaling the mid-ocean ridge and traversing hundreds to thousands of miles of abyssal plains, you might encounter an ocean trench. The Mariana Trench, for example, is the deepest place in the ocean at 36,201 feet.

Multiple Choice

Underwater mountain ranges are called what?

• Continental Rise

• Mid-ocean ridge

• Ocean Trench

Multiple Choice

True or False?

The Andes are the longest mountain range on Earth

• True

• False