The Difference Between an Active and Passive Continental Margin

The main difference between an active and passive continental margin is the amount of tectonic activity occurring at the margin. Active continental margins are located at convergent plate boundaries, where one plate is being subducted under another. This process causes earthquakes, volcanoes, and mountain building. Passive continental margins are located at divergent plate boundaries, where two plates are moving away from each other. This process creates a rift valley, which eventually fills with sediment to form a continental shelf.

Earth's Crust

The earth's crust is broken into sections, called plates. There are two (2) basic types:
⇒  Oceanic plates which are composed of basalt, and continental plates, which are mostly granite. 
⇒ The Continental plates are in motion, and literally bounce around on the surface like giant air hockey pucks. The overall rate may be extremely slow (one to ten centimeters per year), but the long term effect is a continual re-shaping of the surface of our planet.

Active Continental Margin

An active continental margin is found on the leading edge of the continent where it is crashing into an oceanic plate. An excellent example is the west coast of South America. Active margins are commonly the sites of tectonic activity: earthquakes, volcanoes, mountain building, and the formation of new igneous rock. Because of the mountainous terrain, most of the rivers are fairly short, and the continental shelf is narrow to non-existent, dropping off quickly into the depths of the subduction trench.

Active continental margins are characterized by a number of features, including:

• Mountain ranges. Active continental margins are often home to mountain ranges, which are formed when the two colliding plates are forced upwards.
• Volcanoes. Active continental margins are also often home to volcanoes, which are formed when magma rises from the mantle and erupts through the crust.
• Tectonic activity. Active continental margins are characterized by high levels of tectonic activity, including earthquakes and landslides.

Passive continental margins are found along the remaining coastlines. Because there is no collision or subduction taking place, tectonic activity is minimal and the earth's weathering and erosional processes are winning. This leads to lots of low-relief (flat) land extending both directions from the beach, long river systems, and the accumulation of thick piles of sedimentary debris on the relatively wide continental shelves. Again South America provides a great example. The Amazon River, whose source is in the Andes Mountains (the active margin) drains east across the interior of South America to the coast, where it enters the Atlantic Ocean and deposits the tremendous volume of sedimentary materials it eroded from the continent. 

Passive continental margins are characterized by a number of features, including:

• Wide continental shelves. Passive continental margins have wide continental shelves, which are areas of shallow water that extend from the coast to the continental slope.
• Sedimentary rocks. Passive continental margins are often composed of sedimentary rocks, which are formed when sediments are deposited on the continental shelf and slope.
• Low levels of tectonic activity. Passive continental margins are characterized by low levels of tectonic activity. This is because there is no subduction taking place, which is a major source of earthquakes and other tectonic activity.

Some examples of active and passive continental margins:

Active continental margins:

• West coast of North America
•  Andes Mountains
•  Japan

Passive continental margins:

• East coast of North America
•  Atlantic coast of Africa
•  Gulf of Mexico

The North American plate also serves to illustrate this difference. The west coast is the active margin, and is the location of earthquakes, volcanoes, and mountains. The Eastern Seaboard is a passive margin, as is the Gulf Coast. These are the low-relief areas of our continent, and are the locations of such exciting places as New Jersey, the Barrier Islands, the swamps of Florida, and the Mississippi delta.