Continental shelf

A continental shelf is a submerged portion of a continent located under relatively shallow water. These areas are often called shelf seas. They are not part of the deep ocean basin itself. Instead, they represent the flooded margins of the continents.

These shelves cover about 27 million square kilometers. This area represents roughly 7% of the total ocean surface area. They are vital to the planet's health and economy.

The transition from land to the deep ocean follows a specific sequence. The shelf usually ends at a point called the shelf break. At this break, the seafloor drops away much more steeply. This steep section is known as the continental slope. The average angle of the slope is about 3 degrees. However, it can range from 1 degree to as high as 10 degrees.

Below the slope lies the continental rise. This area is a flatter region where sediment accumulates. This sediment cascades down the slope in what are called turbidity currents. These currents move material from the shelf and slope to form thick piles. The rise can extend as far as 500 km from the slope. Together, the shelf and the slope form the continental margin.

Geologists often divide the shelf into three distinct zones. These are the inner, middle, and outer continental shelves. Each zone has its own specific marine biology and geomorphology. The character of the seafloor changes dramatically at the shelf break. Interestingly, most shelf breaks occur at a very uniform depth. This is likely a result of past ice ages. During those times, sea levels were much lower than they are now.

Scientists have learned much about these structures over time. For example, the physical mechanisms that form submarine canyons were not understood until the 1960s. These canyons are deep cuts found on the continental slope. The way these shelves are shaped also depends on their location. Passive continental margins, like the Atlantic coasts, have wide and shallow shelves. These are built from thick sedimentary wedges created by long periods of erosion. In contrast, active continental margins have narrow and steep shelves. These are often caused by frequent earthquakes that move sediment into the deep sea.

The width of these shelves varies greatly across the globe. The Siberian Shelf in the Arctic Ocean is the largest in the world. The Sunda Shelf is another massive area that connects Borneo, Sumatra, and Java to Asia.

Some places have almost no shelf at all. This happens near subduction zones, such as off the coast of Chile. In these zones, an oceanic plate dives beneath the continental crust. The average width of all continental shelves is about 75 km. However, the depth is generally limited to waters shallower than 200 meters.

Continental shelves are incredibly important for life on Earth. They are much more productive than the deep ocean. This is because sunlight can reach the shallow waters. This allows for photosynthesis.

While they cover less than 10% of the ocean, they support 15% to 20% of global primary productivity. The water column is called the neritic zone. The seafloor is called the sublittoral zone. Much of the sediment on these shelves is relict sediment. This means 60% to 70% of it was deposited during the last ice age. At that time, sea levels were 100 to 120 meters lower than today.

Humans also rely heavily on these underwater regions. Most commercial activities take place on the continental shelf. This includes offshore drilling for hydrocarbons like oil and gas. It also includes the extraction of metallic and non-metallic ores. Because they are relatively accessible, they are the best-understood part of the ocean floor. There are also legal rules for who owns these areas. The United Nations Convention on the Law of the Sea defines the shelf legally. It belongs to the specific country adjacent to the shore. This helps nations manage their exclusive economic zones and resources.