The Plate Tectonic Theory and the Three Types of Plate Boundaries


By Robert Hazen, Ph.D.George Mason University

The acceptance of plate tectonics theory was swift and overwhelming. The plate tectonics revolution occurred virtually overnight, in terms of how many years people have been teaching other things. This happened because the evidence piled up in an overwhelming sense, and anyone could see that evidence and understand its implications.

A digital image of the tectonic activity map
The acceptance of the plate tectonic therory was easy because of the series of evidence. (Image: Michael Metzger/Public domain)

Mid-Atlantic Ridge: Divergent Boundaries

Let’s think about some of the implications of plate tectonics. If new crust is formed continuously at the Mid-Atlantic Ridge, where does the old crust go?

The Earth’s surface is broken into about a dozen tectonic plates. These are relatively thin and brittle slabs of material; they’re rock no more than a few tens of kilometers thick, but often many thousands of kilometers across. 

The movement of tectonic plates is accomplished by three different kinds of plate boundaries, where these dozen or so plates contact each other. First of all, you have places like the Mid-Atlantic Ridge; these are divergent boundaries. These are places where new crust is being formed by volcanoes. The crust moves away from both sides, laterally away from ridges of this sort. 

It’s like material on a conveyor belt, but it’s a conveyor belt that goes in two directions at once, and that’s why you get magnetic stripes that are parallel, and symmetrically disposed about the ridges.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Convergent Boundaries and Transform Boundaries

Then you have to have places where the crust is swallowed up. The old crust is taken back into the mantle of the Earth in places called convergent boundaries, where two plates approach each other. This process is occurring, for example, around the Pacific rim, where you have the “ring of fire”.

You have coastal earthquakes and inland volcanoes because as a plate approaches another, it dives underneath; you have a series of earthquakes getting progressively deeper as the plate goes underneath, and melts fairly far inland, maybe 100 or 200 kilometers inland. That causes magma to rise, and causes a series of volcanoes inland from the plate.

Then you have a third kind of plate contact. This is called a transform boundary, and transform boundaries occur where two plates slide against each other.

An image of the global plates motion
A series of earthquakes can happen when a plate approaches another plate. (Image: Macaddct1984/Public domain)

The classic example, one that everybody knows, is the San Andreas fault in California, in which the Pacific plate is going north relative to the North American plate, which is going relatively south.

Learn more about the Erth’s rock cycle.

The Plate Tectonic Theory and Its Source of Energy

A key question that had to be asked for plate tectonics theory was about the source of energy. Here, we can look at the Earth’s inner heat.

In the 1960s, it was realized that when you take a rock and you pressurize and heat it, it is no longer brittle, like it is at the surface; it can be soft, almost like a taffy; almost a plastic-like material. Over long periods of geological time, it can deform; it can even convect, in large convection cells.

In the Earth’s mantle, that’s exactly what happens. You have convection cells, where the Earth’s mantle actually—over tens, and hundreds, and millions of years—forms these great cycles. 

Those epic movements deep within the Earth have the effect of shunting around the plates at the Earth’s surface, almost as an afterthought. You have plate motions, then motions at the surface. 

Everything that we think are big motions, like earthquakes, volcanoes, and so forth, are just the tiniest afterthoughts of these epic, huge motions that are occurring down in the mantle. The development of this convincing, underlying mechanism for plate tectonics assured the almost-universal acceptance of the field.

Learn more about the lithosphere.

All Sorts of Disciplines in the Plate Tectonic Theory

The plate tectonics theory has tremendous power and appeal. It successfully synthesized, for the first time, new data from all sorts of different disciplines.

You had oceanographers, who for the first time were showing the magnet and the topography of the ocean floor; that became a key point of evidence. Paleontologists, whose fossils were matching up all around the Earth in all different sorts of places, suddenly made sense. 

You had petrologists learning the distribution of rocks, and seeing the patterns there. Geomagnetists, people who were studying this arcane field of the Earth’s magnetic field, were suddenly at the very center of the revolution. Economic geologists, because so many economic-geology deposits relate to volcanoes, and since the distribution of volcanoes was now recognized as being directly related to plate tectonics, could look for new ore deposits in places where there were ancient boundaries between plates. 

The theory has integrated earth sciences as never before. The theory provided new ways of analyzing old, puzzling geological data about the evolution of continents, and oceans; even the evolution of life itself, because the distribution of continents affected how life distributed itself around the globe. Most importantly, plate tectonics theory made specific, testable predictions about the Earth.

Common Questions about the Plate Tectonic Theory and the Three Types of Plate Boundaries

Q: What are tectonic plates? 

Tectonic plates are relatively thin and brittle slabs of material; they’re rock no more than a few tens of kilometers thick, but often many thousands of kilometers across. 

Q: What are convergent boundaries?

Convergent boundaries are places where two tectonic plates meet. At these boundaries, according to the plate tectonics theory, the old crust is swallowed up and returned back to the Earth’s mantle.

Q: What are transform boundaries?

According to the plate tectonics theory, transform boundaries occur where two plates slide on top of each other. The San Andreas Fault in California is a prime example.

Keep Reading
The Movement of Plates: Plate Tectonics Mechanism
The Earth’s Inner Structure and Seismic Waves
Plate Tectonics Play a Crucial Role in Sustaining Life on Earth