It seems a bit odd, but the tectonic plates that cover huge areas of Earth's surface have
something in common with your fingernails.
Our nails grow at an average rate of about 3 centimeters a year. The tectonic plates
move at about the same rate, essentially traveling a few centimeters across Earth's surface every
year.
In this lesson we will describe the relative motions of plates along different types of
plate boundaries and we will explain why we have fast and slow plates.
Here's a world map with the major plate boundaries indicated. Could you draw arrows on this map
to illustrate the relative motion directions associated with these plate boundaries? We
will start you off with a few examples. Can you add some more arrows?
We can use our knowledge of plate boundaries to constrain the plate motions. Can you identify
where different types of plate boundaries are present? For example, convergent, divergent,
or transform plate boundaries.
You might have come up with a map that looks something like this. The divergent boundaries
are shown in red, blue lines indicate convergent boundaries, and green indicates a transform boundary.
Rule #1 is that tectonic plates move away from the oceanic ridges that mark the locations
of divergent plate boundaries. And rule #2 is that plates generally move toward oceanic
trenches that characterize many convergent boundaries, such as those around the rim of
the Pacific Ocean.
Now let's look at a map with a little more detail about both the direction of plate motions
and also the rates of motion. While the direction of motion relative to the oceanic ridges is
consistent in each location, the rates of motion are not. For example, the North Atlantic
ocean is opening at a rate of a couple of centimeters or about one inch per year. In
contrast, the oceanic ridge that makes up the East Pacific Rise spreads 15 centimeters
or 6 inches each year. That is, the Pacific and Nazca plates each move about 7.5 centimeters
away from each other each year.
Notice too that the Nazca plate is moving toward the trench that marks the western limit
of the South American plate. The Nazca plate will actually get a little smaller each year
as South America will slowly over-ride its eastern margin, forcing the trench to migrate
to the west, steadily reducing the size of the Nazca plate.
A similar process is taking place in other locations. We see the same thing in the Indian
Ocean where the Indian-Australian plate migrates toward the Java trench. And in the western
Pacific Ocean, where the Pacific plate is consumed beneath the Kurile and Mariana trenches.
Elsewhere, relative plate motions are essentially parallel to portions of plate boundaries defined
by transform faults. We see examples of this along the margins of the smaller Caribbean
plate, along transform segments in ocean ridges, and along the San Andreas fault separating
the North American and Pacific plates.
The Nazca and Pacific plates move faster than other plates. While the plates around the
Atlantic Ocean move relatively slowly. This helps explain why the Pacific Ocean basin
is wider than the Atlantic. But what's the explanation for this contrast in rates?
Before we figure out why some plates are fast and some are slow, think about why plates
move at all. Recall that plates are in motion as a result of convection currents in the
mantle. Convection releases internal heat left over from the planet's formation and
heat generated by the decay of radioactive elements in the core and mantle.
OK, so we have convection, but why do some plates move faster than others? The answer
to that question brings us back to our consideration of the differences between the South American
and Nazca plates.
In particular, we are interested in the structure of the plates and the processes operating
along their margins. Every major plate is bounded on at least one side by an oceanic
ridge. The opposite side of several plates is often defined by an oceanic trench. Some
plates descend into the mantle below the trench at a subduction zone, while others over-ride
the trench. For example, the Nazca plate is consumed in a subduction zone that is over-ridden
by the western margin of the South American plate. The Nazca plate descends into the subduction
zone while the South American plate remains at the surface.
All major plates form at oceanic ridges and are pushed outward and downward from the high
elevations by a process we call ridge push. After traveling along the seafloor for millions
of years, the slab of cold dense lithosphere is then pulled down into the subduction zone.
Think of the slab of lithosphere acting almost like a giant weight, pulling the plate behind
it down into the mantle by a process we label slab pull. Plates that are driven by ridge
push alone tend to move slowly, while plates with a combination of ridge push and slab
pull have faster rates of motion.
So a final examination of the plate map allows us to identify fast moving plates like Nazca
and the Pacific plate. These relatively rapid motions are due to the combination of the
push forces from the ridge and pull forces along the opposing convergent boundaries.
In contrast, slow moving plates, such as the North, and South American plates, only experience
the push force from the oceanic ridge in the Atlantic Ocean. Both plates have active margins
associated with trenches but they over-ride these margins and are not attached to a down-going
slab, so they don't have the benefit of slab pull.
So we had two learning objectives for today. How confident are you that you could complete
both of these tasks?