By Robert Hazen, Ph.D., George Mason University
Convergent boundaries mark places where the motion of two plates is toward each other. It’s a situation that arises above zones of downwelling mantle convection. While divergent plates occur when mantle comes up and spreads apart, there are places where the mantle convection goes down, the plates converge, and one of them plunges beneath the other one.
Three Types of Convergent Boundaries
There are three kinds of boundaries that one can imagine in a convergent situation. First, you can have two oceanic plates, two chunks of the ocean that come in contact. The second possibility is that one of the plates carries just oceanic material, while the other one carries continental material.
The third kind is when both plates carry continents. Because continents cover only a small fraction of the Earth’s surface, less than one-third, the continent-continent type of collision is much less common. Indeed, something like 95 percent of all convergent boundaries have at least one oceanic plate involved.
If neither plate carries a continent, then one of the oceanic plates plunges beneath the other one. You have a situation where you have an oceanic plate approaching another one, and one oceanic plate plunges beneath the other. If one of the two plates carries a continent and the other one doesn’t, invariably the oceanic plate plunges beneath the one carrying the continent. The continent is the lighter material, and it’s the one that’s going to stay on top.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
How the Plate Tectonics Is Driven
According to the prevailing view, the cold plates are dense. You may think of plate tectonics as being driven by the spreading centers, where new material is formed and pushed to the side. But a lot of geologists now think that plate tectonics is driven by the fact that cold plates, as they subduct into the mantle, are dense, and they actually are pulled down by gravity.
Instead of pushing the plates apart, at divergent boundaries, it’s possible that the dynamic is pulling the plates apart, with gravity playing a role, and of course, the convection cell driving the process all along. It’s interesting—is this a push effect? Is it a pull effect? That’s one of those details that geologists constantly debate, and it’s an active area of forefront research.
Rates of subduction are a few centimeters a year; maybe one inch, maybe two or three inches, in some places.
Learn more about the plate tectonics.
Process of Subduction
The axis along which one plate subducts includes most of the deepest trenches on the ocean floor, and you can imagine why this is true. Places like the Marianas Trench, the long trench which is so deep on the ocean floor it could hold Mount Everest with thousands of feet to spare, is a place where one plate plunges beneath another one.
As it does so, it buckles the leading edge of the plate it’s plunging underneath, brings it down; and you get a deep ocean trench, in that sort of plate.
This process of subduction is accompanied by numerous, deep earthquakes, and the earthquakes get deeper and deeper as the descending plate plunges deeper and deeper within the Earth.
Volcanoes and Island Arc
Subduction zones can be associated with intense volcanism. As one plate plunges deeper and deeper, it gets hotter and hotter. It also carries with it lots of water, in the rocks, in the materials that are being taken down.
When the materials get hot, they tend to melt. When they melt, they form magmas, which then try to rise to the surface, and so you get a whole series of volcanoes 100 or 200 kilometers inland; and that chain of volcanoes can be seen in many places around the world.
When subduction leads to a chain of volcanoes, the result is called an island arc, and there are many examples of these arc-like arrangements of volcanic islands. For example, the Aleutian Islands off Alaska; Japan; the Philippines; and many of the islands of the South Pacific are this island-arc type.
Subduction also leads to continental volcanoes. The examples are found along the East Pacific Rim; the Andean volcanoes of South America; and the Cascades of the Northwest Pacific area. This kind of subduction also leads to many of the world’s most famous volcanoes; for example, Mount Vesuvius in Italy.
Learn more about lithospheric plates.
Converging Continental Plates
Converging continental plates doesn’t happen often, but when it does, it’s spectacular. Very occasionally, when two plates are converging and they carry continents, the continental material can’t be subducted. The continental material is just too light to be carried down into the mantle, and so it has no place to go.
As the two plates come together, the two continents collide with each other and start hitting each other; colliding, crumpling up, becoming ever more contorted, and ever more piled on top of each other, until you form a huge mountain range. The tallest mountain ranges in Earth’s history have been the result of colliding plates: the collision and the convergence of two plates, each of which carried a continent.
Today, that’s occurring where the Indian plate is colliding with China; and the Himalayan Mountains, the tallest mountains in the world, are the result of piling layer upon layer of continental material into each other. That’s why you find fossils at the top of Mount Everest, because these are layers of rocks that have been folded, and contorted, and pushed together, and raised up.
Common Questions about Convergent Boundaries
There are three types of convergent boundaries. One type of convergent boundary consists of two oceanic plates that are in contact with each other. The other type consists of an oceanic plate and a continental plate, and the third type, which is very rare, consists of two converging continental plates.
When subduction leads to a chain of volcanoes, the result is called an island arc. There are many examples of these arc-like arrangements of volcanic islands, like Aleutian Islands off Alaska; Japan; the Philippines; and many of the islands of the South Pacific.
When two continental plates meet, they form a convergent boundary. Each plate carries continental material. These continental materials are very light and therefore can’t be subducted into the mantle. What happens is that the two plates begin to hit each other, collide and pile up to form a large mountain range.
