By Robert Hazen, Ph.D., George Mason University
Divergent boundaries mark elongated zones along which new crust is formed as volcanic material. Two plates are going to diverge above a region where mantle convection brings heat to the surface. If one can imagine huge convection going on in the mantle, then as the material is brought upward and moved side-to-side, that’s where divergent boundaries occur.
Mid-Atlantic Ridge
The Earth’s surface displays many prominent divergent boundaries. Of course, the most famous, because of its discovery in the 1950s, was the Mid-Atlantic Ridge. It extends for 60,000 kilometers, and it’s now spreading at a rate of roughly two to four centimeters per year. That’s about an inch per year.
You can actually make satellite measurements that can detect the increasing distance. This is how it works. You have two points that are set in the rock; one in North America and one in Europe. You have laser satellites that can measure distances to better than a fraction of an inch; and therefore, you can measure the distance between North America and Europe, and year by year that distance is getting longer.
By the way, the Mid-Atlantic Ridge passes right through the volcanic island of Iceland, and you can actually study the volcanism associated with the spreading right at the surface of the Earth, at that point.
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Juan de Fuca Ridge
One of the most intensively studied divergent boundaries is the Juan de Fuca Ridge. That’s a ridge that’s just a few hundred kilometers off the coast of Oregon and Washington state. One of the reasons this ridge is so intensively studied is that there are great oceanography departments at both the University of Washington and the University of Oregon.
They’re well-equipped with ships that can easily get out to these ridges, and rather than sailing halfway across the Atlantic—1,500 miles to the middle of the Mid-Atlantic Ridge—they need to sail just a few hundred kilometers off the Washington and Oregon coast. This is one of the most accessible ridges of this kind available.
Why Is Juan de Fuca Ridge Important?
Juan de Fuca Ridge is the first place where magnetic anomalies were discovered; so when people describe the spreading of the mid-ocean ridge, and the magnetic anomalies on either side of the ridge, it was the Juan de Fuca Ridge where the very first data was collected.
Also, because it’s so close to a coastline, there’s constant monitoring of the chemistry of the ridge, of the volcanic activity; there are seismometers down there. If there happens to be a new volcanic eruption, where a particularly strong volcanic event occurs, people can very quickly be at the Juan de Fuca Ridge.
There also is a very great deal of interest in the biology of the Juan de Fuca Ridge because it’s one of the places where you find fascinating, deep-sea hot-hydrothermal-vent communities.
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East Pacific Rise: The Major Divergent Boundary
Another important spreading center is the East Pacific Rise; that’s a major divergent boundary in the South Pacific Ocean. There we have the highest spreading rates known, as high as 17 centimeters—that’s about seven inches per year.
Another intriguing aspect of the East Pacific Rise is that the spreading is not exactly symmetrical. One side of that spreading center moves about twice as fast as the other side, and we don’t fully understand that phenomenon, that dynamic yet. It’s one of those little interesting things that the Earth throws us every so often, another curve ball.
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The Indian Ocean Ridge: The Mysterious Divergent Boundary
Another major divergent boundary is located in the Indian Ocean. However, this one is very poorly studied. We know almost nothing about the ridge in the Indian Ocean because it’s a very remote locality, far from any big oceanographic facility; and also, apparently, the weather conditions in the Indian Ocean can be quite severe, so it’s a more dangerous and difficult place to do oceanography.
All of these ocean-ridge systems display complex hydrothermal systems. This is where seawater filters in through cracks and fissures in the ocean floor, and then gets heated up by the volcanic activity; it comes bubbling to the surface, sometimes in mineral-laden hot waters called “black smokers.”
There are also rich biological communities at these black smokers; these are communities of organisms that thrive in the deep, hot water. Apparently, astonishingly, these are communities that don’t require sunlight to live. The old biological paradigm that all life gets its energy from the Sun is now proven to be wrong. The black smoker communities get their energy from the Earth’s inner heat.
Common Questions about Divergent Boundaries and the Formation of New Crust
The Mid-Atlantic Ridge is the most famous divergent boundary because of its discovery in the 1950s. This divergent boundary extends for about 60,000 kilometers and is now expanding at a speed of two to four centimeters per year.
Juan de Fuca Ridge is only a few hundred kilometers off the coast of Oregon and Washington state, and is thus easily accessible. It is where the magnetic anomalies were discovered for the first time. Also, since it’s close to a coastline, there’s constant monitoring of the chemistry of the ridge. This site is also very important from a biological point of view because it contains hot-hydrothermal-vent communities.
The East Pacific Rise is the major divergent boundary in the South Pacific Ocean. It has the highest spreading rates, as high as 17 centimeters per year. The most interesting point about the East Pacific Rise is that its expansion isn’t symmetrical; one side of the spreading center moves about twice as fast as the other side.
