Geology Reference
In-Depth Information
Divergent plate boundary: The boundary between two crustal plates that are mov-
ing apart from each other
Convergent plate boundary: The edge along which two crustal plates meet when
they are moving toward one another
Transform plate boundary: The boundary between two crustal plates that are
neither moving toward each other nor apart — they are sliding back and forth
next to each other
Be careful not to confuse the edge of a continent with the edge of a crustal
plate. In some cases, such as along the west coast of South America, the two
edges are the same. When the edge of the continent is also the edge of the plate,
it's called an active continental margin. It is active because the plate boundary is
engaged in geologic processes (such as earthquakes and volcanoes) as a result of
its interaction with another crustal plate.
But in some cases, like along the eastern coast of North America, the crustal plate ex-
tends into the ocean and includes continental lithosphere as well as oceanic litho-
sphere. In this case, the edge of the continental portion of the plate is called a passive
continental margin because it is not actively participating in plate boundary processes.
In the following sections, I explain what happens to the crustal plates and what features
are created at the earth's surface at each of these boundary types.
Driving apart: Divergent plate boundaries
When two crustal plates move apart from each other, the edge along which they are sep-
arating is a divergent boundary. The particular characteristics of a divergent boundary
vary depending on whether the two plates are continental crust or oceanic crust.
Ripping open the ocean floor
In many places around the earth, two plates are moving apart from each other. This is
most common on the sea floor, where this separation allows molten rock (magma) to
erupt along the boundary, harden, and create new ocean floor. (I describe this process
in detail in Chapter 10.) The buildup of this new rock material creates a mid-ocean ridge
along the undersea divergent boundary. Mid-ocean ridges form the longest continuous
mountain chains on earth, winding through all the oceans.
Search WWH ::




Custom Search