We once thought that mantle convection below
the tectonic plates could drive plate motions.
Early textbooks showed mantle convection cells,
like in a beaker of hot liquid on a Bunson
burner, pushing plates along from below. Current
dynamic models have plates moving as part
of a gravity-driven convection system that
pushes young hot plates away from spreading
ridges and pulls old cold plates down into
subduction zones.
Remember that lithospheric plates, also called
tectonic plates, have a layer of crust on
top of lithospheric mantle, the outermost
rigid part of the mantle. These move as a
single unit. The hotter asthenosphere beneath
the plates is solid but less-rigid mantle
rock that can slowly flow.
Now let’s look at the broader picture.
This map shows how the seafloor increases
in age with distance away from spreading ridges,
such as the East Pacific Rise or the Mid-Atlantic
Ridge, where new ocean plate is forming Spreading
ridges stand 2500 meters higher than deep
ocean basins. At a spreading ridge, the ocean
depth is only about 3000 meters . Ocean depth
increases with age of the underlying plate,
so that where the plate is more than 80 million
years old, the overlying ocean increases to
5500 meters deep.
Let’s examine the formation of new ocean
plate and how that plate cools with age.
As hot mantle rock rises to lower pressure,
a small portion of this upwelling asthenosphere
melts to form magma that builds the 7-km-thick
oceanic crust at the edges of two diverging
plates along the ridge axis. Beneath the crust
at the spreading ridge, there is only a thin
layer of lithospheric mantle because it is
unusually hot in the upwelling zone. This
hot, and therefore lower-density, mantle rock,
supports the 2500-meter elevation of the spreading
ridge As the plate slowly moves away from
the ridge, it cools by conducting heat through
the crust to the cold ocean water above. At
the same time, the underlying asthenosphere
cools and adds to the bottom of the lithospheric
plate. Thus, although the crust maintains
its thickness during migration away from the
ridge, the lithospheric plate thickens and
cools to create ocean basins that exceed five
kilometers in depth. A simple way to think
about elevation is that the plate is “young,
hot, and high” at the spreading ridge and
becomes “old, cold, and low” during the
aging and cooling process. Mathematical modeling
of this cooling process illustrates the ocean
plate becoming cooler and thicker with age.
The temperature at the bottom of the plate
is about 1300° centigrade. Notice that most
of the cooling process occurs between age
zero at the ridge and about 80 million years
when the ocean plate has grown to about 100
km thick. The upper plate is less than 600°,
thus is the only part of the plate cold and
brittle enough to fracture and produce earthquakes.
Though still rigid, the lower plate is warmer
and can deform in a ductile or plastic fashion.
So what force pushes ocean plates away from
spreading ridges? Any mass on a slope is affected
by gravity, seen most dramatically with avalanches
and landslides. Spreading ridges are broad
undersea mountain ranges and, although the
flanks of a spreading ridge are a relatively
gentle slope, the mass on that slope is humongous.
The force of ridge push is zero at the ridge
but increases quickly with distance and age,
pushing the cooling and thickening ocean plate
away from the ridge.
Now, what about “slab pull”?. We’ll
consider a 30-million-year-old ocean plate
subducting at 5 centimeters per year into
hotter asthenospheric mantle beneath a continental
plate. As the ocean plate, subducts, the warming
process takes many millions of years as the
slab descends. The deeper part it is continuously
replaced, in a conveyor-belt fashion, by cooler
plate from above. Mathematical modeling again
illustrates the temperatures within
the subducting ocean plate.
In this example, lithospheric mantle rock
in the subducting plate at 150 km depth is
1000° cooler than the asthenospheric mantle
at the same depth. The cooler temperatures
mean that the density of rocks in the subducting
slab is greater than the density of the hotter
asthenosphere. While gravity pulls down on
all rocks, it pulls down harder on more-dense
rocks. This enhanced gravitational force
on the cooler and denser rocks in the subducting
plate is the slab pull gravity force.
Earthquakes, including great megathrust earthquakes,
occur on the shallow part of the boundary
between the converging plates and within the
shallow parts of both plates near that boundary.
In addition, there is a zone of rock cooler
than 600°C within the subducting plate that
remains brittle and within which earthquakes
can occur to depths of hundreds of kilometers.
Observations of intraplate earthquakes and
other indications of stress with tectonic
plates suggest that the slab pull force is
usually larger than the ridge push force.
It is noteworthy that fast-moving plates,
like the Pacific Plate, generally have a fast
spreading ridge pushing on one side while
a subduction zone pulls on the other side.
In this big picture view, we see that lithospheric
plates are part of a planetary scale thermal
convection system. The energy source for plate
tectonics is Earth’s internal heat while
the forces moving the plates are the “ridge
push” and “slab pull” gravity forces.
