Continental Margins

The continental margin is the underwater edge of a continent where the land meets the ocean. It marks the transition from the shallow continental crust to the deep ocean floor. Even though it’s underwater, this zone is still part of the continent.

Passive Margins

Passive Margins are found along coastlines that are not near plate boundaries, such as the east coast of North and South America. Passive margins have little to no volcanic or earthquake activity. They form where the ocean and continent are part of the same tectonic plate and move together quietly. A passive continental margin has three main parts: the continental shelf, the continental slope, and the continental rise.

Continental Shelf

This is the gently sloping, shallow area that extends outward from the shoreline. It’s often less than 200 meters (650 feet) deep and rich in marine life because sunlight can reach the sea floor. Many important ecosystems and oil and gas deposits are found here.

2. Continental Slope

Beyond the shelf, the sea floor drops sharply down the continental slope. This steep region marks the true edge of the continent, where the thick continental crust begins to thin and transition into the denser oceanic crust.

3. Continental Rise

At the base of the slope lies the continental rise, a broad, gentle area made up of sediments that have slid or drifted down from the shelf and slope. Over time, these layers of mud, sand, and silt create a smooth transition to the deep ocean floor, called the abyssal plains.

Features of a Passive Margin

Deep beneath the ocean’s surface, powerful underwater processes shape the seafloor. Three important features connected to these processes are turbidity currents, submarine canyons, and abyssal fans.

Turbidity Currents

A turbidity current is a fast-moving underwater avalanche of sediment—made up of mud, sand, and water—that rushes down the continental slope. These currents are triggered by things like earthquakes, storms, or the collapse of sediment on the slope. As the dense mixture flows downhill, it erodes the seafloor and carries material far out into the deep ocean.

2. Submarine Canyons

The paths carved by turbidity currents are called submarine canyons. They are steep, V-shaped valleys that cut into the continental shelf and slope, resembling river canyons on land. Some submarine canyons, like the Monterey Canyon off California, are as deep and dramatic as the Grand Canyon. They act as natural channels, guiding sediments from the continent down to the ocean basin.

3. Abyssal Fans (Deep-sea Fans)

When the turbidity current finally slows at the base of the continental slope, it drops its load of sediment, forming a large, fan-shaped deposit called an abyssal fan (or deep-sea fan). These fans can stretch for hundreds of miles and are similar to river deltas, but they form underwater on the deep ocean floor.

Together, turbidity currents, submarine canyons, and abyssal fans show how gravity and moving sediment continue to reshape Earth’s seafloor long after the continents themselves stop moving.

Active Margins

Active Margins occur along coastlines that lie on or near tectonic plate boundaries, like the west coasts of North and South America. Active margins form where an oceanic plate is being forced under a continental plate in a process called subduction.

Features of an Active Margin

Because active margins occur along tectonic plate boundaries, they have steep, dramatic, and geologically active features, unlike the calm passive margins. Key features include:

Narrow Continental Shelf

The shelf is thin or almost nonexistent because sediments are quickly carried into the deep trench.

2. Steep Continental Slope

The seafloor drops sharply into the ocean trench.

Click on the image above for a more in-depth look.

3. Volcanoes

Subduction causes melting in the mantle, producing magma that can form chains of volcanic mountains on the continent (continental volcanic arcs) or on nearby islands (volcanic island arcs).