When two plates slide on top of each other

Through Robert Hazen, Ph.D., George Mason University

Tectonic plates are large chunks of the lithosphere – about 50 or 100 kilometers thick, but thousands of kilometers in diameter – and lithospheric plates are displaced when they straddle the asthenosphere, which is mobile and moving. Transformation boundaries are a kind of plate boundary. They are basically faults where two plates slide over each other.

An image of the San Andreas Fault in Hanford, USA
The San Andreas Fault is the longest highly active transformational frontier on the Earth’s surface. (Image: oliverdelahayeShutterstock)

The Lithosphere and the Asthenosphere

The lithosphere and asthenosphere are the two major layers associated with plate tectonics.

The lithosphere consists of the crust and the upper part of the mantle; it is a relatively thin layer, about 50 to 100 kilometers thick. It is also relatively cold, less than 1000 degrees centigrade, and at these temperatures the rock is brittle. That is, if you take a hammer and hit it, it will break; it will shatter, like a brittle piece of ceramic. This solid rock layer is the lithosphere.

It covers a softer layer, the asthenosphere soft and warm. This layer of asthenosphere extends deep into the mantle. It’s hotter – temperatures are usually over 1000 degrees – and it’s hot enough to make the rock relatively soft and plastic; more like a taffy.

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Transform the boundaries

Transformation limits are faults where two plates slide over each other. Such boundaries are inevitable whenever you have a sphere divided into diverging and converging boundaries. The most common is the San Andreas fault; it is the longest highly active transformation frontier on the Earth’s surface.

Major earthquakes occur along the San Andreas fault every few decades. Geologists predict that another severe earthquake is to occur in Southern California in the next quarter century or so; stress builds up too much.

There are also transformation limits along ocean ridge systems.

Transformation limit locations

If you look at a seabed map, you will see that the ridges are shifted from time to time; and these offsets are transformation limits of a shorter kind. They involve areas where the plaques meet; they move relative to each other, but they neither diverge nor converge.

Geologists have also identified several silent transformation boundaries, those that do not move. The plates meet, they are in contact; but at least for now, they don’t move much. There is such a passive border between the Eurasian plate and the African plate. At some point in the future these boundaries may move, there may be giant earthquakes along them; but at least for now they seem to be calm.

Learn more about the rock cycle.

Volcanic areas called hot spots

An image of the Hawaii hotspot.
There are around 100 hot spots around the world. (Image: Bourrichon / Public domain)

Large-scale mantle convection drives plate tectonics and controls most of Earth’s volcanoes. You have volcanoes at divergent borders, where a new crust is formed, and therefore you have a ridge of volcanoes. You have volcanoes along converging borders that are about 100 to 200 kilometers inland from the actual border.

But there are other well-known volcanic areas, including Hawaii and Yellowstone, which lie in the middle of the plates. What is happening with these volcanoes? These areas are called hot spots. They are found all over the world; there are about 100 of them in the world, and they represent another type of mantle convection, which is still quite poorly understood.

Appearance of a hot spot

Hot spots appear to appear when a narrow plume of magma rises from the depths of the mantle. Perhaps the core-mantle boundary is the source of these hot spots; it would be 3,000 kilometers lower. The location of the hot spots seems to be absolutely independent of the tectonic movements of the plates.

For example, if you look at the Hawaii mountain range, you see that the mountain range appears to have moved northwest. The reason is that the plate itself moves and the hot spot remains fixed. You have a fixed hot spot; As the plate gradually moves, the new volcanic islands that are forming appear more and more to the southeast, in this 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 occurs, is the largest; this is where all the action takes place. As you move further away from Hawaii, you see more and more eroded islands. If you study sonar even further, there is a huge string of volcanic islands underwater; that is, things that were once islands, but are now eroded below ocean level.

It stretches hundreds of kilometers to the northwest. This indicates that the hot spot has been fixed for a long time and the Pacific plate has moved on it.

There is still a lot to understand about hot spots, and their origin remains a hot topic of research.

Common questions about processing limits and hot spots

Q: What are processing limits?

Transform the boundaries are faults where two plates slide against each other. Such borders are inevitable due to divergent and converging borders. The most common transformation boundary is the San Andreas fault; it is the longest highly active transformation frontier on the Earth’s surface.

Q: What are the limits of silent transformation?

Silent transformation limits are borders that do not move. The plates are in contact at these limits but do not move much at this time. Geologists have found passive transformational boundaries between the Eurasian Plate and the African Plate.

Q: What are hot spots? How do they arise?

Hot spots are volcanic areas that lie right in the middle of the plates. They seem to arise when a narrow plume of magma rises from the depths of the mantle. The location of the hot spots seems to be absolutely independent of the tectonic movements of the plates.

Keep reading
Volcanic eruptions: dramatic and violent events on the Earth’s surface
The internal structure of the Earth and seismic waves
Understanding plate tectonics and its importance on Earth


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