By Robert Hazen, Ph.D., George Mason University
Tectonic plates are wide pieces of lithosphere—about 50 or 100 kilometers thick, but thousands of kilometers across—and the lithospheric plates are shunted about as they ride upon the asthenosphere, which is mobile and moving. Transform boundaries are a kind of plate boundary. These are basically faults where two plates slide by each other.
The Lithosphere and the Asthenosphere
The lithosphere and the asthenosphere are the two great layers associated with plate tectonics.
The lithosphere includes the crust and the very top part of the mantle; it’s a relatively thin layer, about 50 to 100 kilometers thick. It’s also relatively cold, less than 1,000 degrees centigrade, and at those temperatures, the rock is brittle. That is, if you take a hammer and hit it, it will shatter; it will break, like a brittle piece of ceramic. This strong rock layer is the lithosphere.
It overlies on top of a softer layer, the soft, hot asthenosphere. This asthenosphere layer extends deep into the mantle. It’s hotter—temperatures are typically over 1,000 degrees—and that’s hot enough to make the rock relatively soft and plastic; more like a taffy.
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Transform Boundaries
Transform boundaries are faults where two plates slide by each other. Such boundaries are inevitable whenever you have a sphere that’s broken into divergent and convergent boundaries. The most common one is the San Andreas Fault; it’s the longest highly active transform boundary on the surface of the Earth.
Major earthquakes occur along the San Andreas Fault every few decades. Geologists predict that another severe quake must occur in Southern California sometime in the next quarter-century or so; the stress is just building up too much.
There are also transform boundaries along the ocean-ridge systems.
Locations of Transform Boundaries
If you look at a map of the seafloor, you’ll see that the ridges are offset from time to time; and those offsets are transform boundaries of a shorter sort. They involve areas where plates meet; they’re moving by each other, but they’re not diverging or converging.
Geologists have also identified several quiet transform boundaries, ones that aren’t moving. Plates meet each other, they’re in contact; but at least for the time being, they’re not moving very much. There’s such a passive boundary between the Eurasian plate and the African plate. At some time in the future, these boundaries may move, there may be giant earthquakes along them; but at least for now they seem to be quiet.
Learn more about the rock cycle.
Volcanic Areas Called Hot Spots
Large-scale mantle convection drives plate tectonics, and it controls most of the Earth’s volcanoes. You have volcanoes at divergent boundaries, where a new crust is being formed, and so you have a ridge of volcanoes. You have volcanoes along convergent boundaries that are roughly 100 to 200 kilometers inland from the actual boundary.
But there are some other very well-known volcanic areas, including Hawaii and Yellowstone, that are smack in the middle of plates. What’s going on with these volcanoes? These areas are called hot spots. They’re found all over the globe; there are about 100 of them around the globe, and they represent a different kind of mantle convection, which is as yet rather poorly understood.
Arising of a Hot Spot
Hot spots seem to arise when a narrow plume of magma rises from deep within the mantle. Perhaps the core-mantle boundary is the origin of these hot spots; that would be 3,000 kilometers down. The location of hot spots seems to be absolutely independent of plate tectonic motions.
For example, if you look at the Hawaii chain of mountains, you see that the chain of mountains seems to have moved off to the northwest. The reason is because the plate itself is moving, and the hot spot is remaining fixed. You have a fixed hot spot; as the plate moves progressively, the new volcanic islands that are formed appear to be more and more to the southeast, in that chain of islands.
Learn more about earthquakes and volcanoes.
The Fixed Hot Spot
If you look at the volcanic islands, the present-day Big Island of Hawaii, where active volcanism is occurring, is the largest; it’s the one where all the action is taking place. As you go farther and farther away from Hawaii, you see more and more eroded islands. If you do sonar studies even farther out, there’s a huge string of undersea volcanic islands; that is, things that were once islands, but are now eroded to below ocean level.
That extends for hundreds of kilometers off to the northwest. This indicates that the hot spot has been fixed for a long time, and the Pacific plate has been moving over it.
There’s an awful lot yet to be understood about hot spots, and their origin remains a very hot topic of research.
Common Questions about Transform Boundaries and Hot Spots
Transform boundaries are faults where two plates slide by each other. Such boundaries are inevitable because of divergent and convergent boundaries. The most common transform boundary is the San Andreas Fault; it’s the longest highly active transform boundary on the surface of the Earth.
Quiet transform boundaries are boundaries that do not move. The plates are in contact at these boundaries but don’t move much at this time. Geologists have found passive transform boundaries between the Eurasian plate and the African plate.
Hot spots are volcanic areas that are right in the middle of plates. They seem to arise when a narrow plume of magma rises from deep within the mantle. The location of hot spots seems to be absolutely independent of plate tectonic motions.
