The idea that our planet’s continents drift around the globe, periodically glomming together
and breaking apart, is at least 200 years old.
But most geologists didn’t believe it until the 1960’s, when mounting evidence made
it clear that the Earth’s crust is broken up into fragments, and that those fragments,
called tectonic plates, are moving.
And these days we directly track that motion – with millimeter precision – from space.
The common, simplified explanation for why tectonic plates are moving is that they’re
carried along on currents in the upper mantle, the slowly flowing layer of rock just below
Earth’s crust.
Converging currents drive plates into each other; diverging currents pull them apart.
This is mostly true; hot mantle rock rises from the core and moves along under the crust
until it grows cool and heavy and sinks back down again.
But the plates aren't just passively riding these conveyer-belt-like currents around like
a bunch of suitcases at the baggage claim.
They can’t be, because some of the plates are moving faster than the currents underneath
them.
For example, the Nazca plate – a chunk of ocean crust off the west coast of South America
– is cruising eastward at about 10cm per year, while the mantle underneath it oozes
along at just five.
Neither tectonic plates nor luggage can move faster than the belt they’re riding on unless
something else is helping to push or pull them along.
And some of Earth’s plates, it turns out, are pulling themselves.
When an ocean plate collides with another ocean plate or a plate bearing the thick crust
of continental landmasses, the thinner of the two plates bends and slides under the
other.
As the edge of the seafloor sinks into the mantle, it pulls on the plate behind it, the
same way a chain dangling further and further off a table will eventually start to slide.
The bigger the sunken portion of the plate becomes, the harder it pulls and the faster
the remaining plate behind it moves.
You can find where this is happening by looking at google earth – the incredibly deep, narrow
ocean trenches visible off the coasts of some continents and island chains mark the creases
formed as ocean crust plunges downward, bending the edge of its neighbor in the process.
What’s more, those chunks of seafloor are actually helping to drive convection in the
mantle beneath them.
Sunken slabs of ocean crust block flowing rock from moving further sideways, forcing
it to turn downward and sink.
Eventually those slabs get too heavy and break off, plunging slowly toward the core and creating
a suction force that pulls mantle material along behind it.
So, in some ways, seafloor crust really is more like part of the conveyor belt than something
riding on top of it.
The continents, on the other hand, are baggage.