5.18: Continental Margins

Last updated
Save as PDF

Page ID: 5396

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

alt

Can plate tectonics explain the differences in these beaches?

Plate tectonics explains why some beaches have lots of cliffs and some do not. A beach with lots of cliffs is near a plate boundary. A gentle beach is not. There are exceptions to this rule, but it works in some cases.

Continental Margins

Think of a continent, like North America. Surrounding the continent are continental margins. Continental crust grades into oceanic crust at continental margins. Continental margins are under water. Almost all of North America sits on the North American Plate (Figure below). Both sides of the continent have continental margins, but each is very different. One continental margin of North America is an active margin. The other is a passive margin. Can you guess which is which?

Map of the North American plate and its surrounding plates

The North American plate and the plates that surround it.

Active Margins

If a continental margin is near a plate boundary, it is an active margin. The continental margin of western North America is near a set of plate boundaries. There are convergent boundaries, like where there is subduction off of the Pacific Northwest. There is a transform boundary, the San Andreas Fault. The small amount of the North American continent that is not on the North American Plate is across the San Andreas Fault. It is on the Pacific Plate. Western North America has a lot of volcanoes and earthquakes. Mountains line the region. California, with its volcanoes and earthquakes, is an important part of this active margin (Figure below).

Big Sur is an active margin

Big Sur, in central California, has beautiful cliff-lined beaches.

Passive Margins

There are no volcanoes and very few earthquakes on the eastern edge of North America. The continental margin is a smooth transition from continental to oceanic lithosphere. The continental margin there becomes oceanic lithosphere, but both are on the North American Plate. There is no plate boundary. The far eastern edge of the North American Plate is the mid-Atlantic Ridge. The portion of a plate that does not meet another plate has no geological activity. It is called a passive margin (Figure below).

Daytona Beach is Florida is a passive margin

The eastern U.S. is a passive margin. Daytona Beach in Florida is flat and sandy, typical of a passive margin.

Summary

Continental margins can be active or passive depending on whether they are near a plate boundary.
Volcanoes and earthquakes are common at active margins. Active margins are near plate boundaries.
Passive margins are passive. They have little or no geological activity.

Review

Describe the continental margin of Western North America.
Describe the continental margin of Eastern North America.
Why are there mountain ranges at passive margins?