Continental margin

A continental margin is the outer edge of the continental crust. It sits where the land meets the oceanic crust under coastal waters. This zone is one of the three major parts of the ocean floor. The other two parts are the deep-ocean basins and the mid-ocean ridges. Continental margins are quite large. They make up about 28% of the total oceanic area.

The margin consists of three distinct geological features. The first is the continental shelf. This is a shallow, relatively flat area near the coast. It transitions from the shore out toward the open ocean. The width of these shelves varies greatly. They can range from 0.03 km to 1500 km wide. The shelf ends at a point called the shelf break. At this break, the angle of the ground increases drastically. The mean angle of shelves worldwide is only 0° 07'.

After the shelf break, the continental slope begins. This is a steep descent toward the deep ocean. The slope can sit 1 to 5 km above the deep-ocean floor. This area often features submarine canyons. These are deep, V-shaped cuts in the seafloor. Some canyons cut deeply into the continental shelf. They can even reach the abyssal plain. At the base of this slope, the angle decreases again. The seafloor begins to level out into the continental rise. The rise is the outermost part of the margin.

Scientists categorize margins into two types: active and passive. Active margins occur at lithospheric plate boundaries. These areas experience high tectonic activity. This includes frequent earthquakes and volcanic eruptions. Active margins can be convergent or transform. Convergent margins occur when oceanic plates meet continental plates. The denser oceanic crust subducts, or slides, below the continental crust. Transform margins are rarer. They happen when plates move parallel to each other in opposite directions. These create a continental borderland with many faults, islands, and basins. The West Coast of North America is an active margin.

Passive margins are located in the interior of lithospheric plates. They sit away from plate boundaries. Because of this, they lack major tectonic activity. These margins often face mid-ocean ridges. They are much wider and less steep than active margins. They feature low-relief land that extends miles from the beach. Long river systems often flow into these areas. The East Coast of the United States is a passive margin. These margins are excellent places for sediment to gather.

Sediment plays a massive role in shaping these margins. As continental crust weathers and erodes, it breaks into sand and clay. Rivers carry these particles into the ocean. About 80% of the sediment in a stream load is trapped on continental margins. On passive margins, these deposits can be several kilometers thick. They consist of terrigenous and carbonate deposits. These layers are vital for studying paleoceanography. This is the study of the history of the oceans. On active margins, tectonic activity often prevents these sediments from being well preserved.

The continental shelf is the most economically valuable part of the ocean. It is shallow and accessible for humans. This makes it highly productive for fisheries. It is also the most studied part of the margin. The United Nations Convention on the Law of the Sea, or UNCLOS, manages these areas. UNCLOS helps define the boundary of a country's legal continental shelf. This allows countries to claim underwater resources. These resources include oil, gas, sand, and gravel. Some areas also contain heavy minerals or metallic resources.