Suction and pull drive movement of Earths plates, U-M researchers
As anyone with a smattering of geological knowledge knows, Earths
crust is made up of plates that creep over the planets surface
at a rate of several inches per year. But why do they move the way
they do? Even experts have had trouble teasing out the exact mechanisms.
A model developed by U-M researchers and published in the Oct.
4 issue of Science provides a relatively simple explanation.
Its been known that slabs (portions of plates that
extend down into the Earth) drive convection in Earths mantle,
and ultimately the motion of the surface plates, but it hasnt
been well established exactly how that happensthe ideas have
been fairly vague, says Clinton Conrad, a postdoctoral fellow
in the department of geological sciences. In this paper, weve
been able to describe more precisely how slabs interact with the
When two plates collide, one is forced beneath the other into
the mantlethe plastic-like layer between Earths crust
and core that flows under pressurecreating what geologists
call a subduction zone. Because subducting slabs are colder and
more dense than surrounding mantle material, they tend to sink like
a lead ball in a vat of molasses.
There are two main ways these sinking slabs might influence plate
motion. If a slab is attached to a plate, the slab can directly
pull the plate toward the subduction zone. A slab that is not well
attached to a plate, on the other hand, cant pull directly
on the plate. Instead, as it sinks, it sets up circulation patterns
in the mantle that exert a sort of suction force, drawing nearby
plates toward the subduction zone much as floating toys are drawn
toward the outlet of a draining bathtub.
|How mantle slabs drive plate motion
To understand the relative importance of slab pull and slab suction
forces, Conrad and Carolina Lithgow-Bertelloni, an assistant professor
of geological sciences with whom he worked on the project, developed
models in which: 1) only slab suction was operating; 2) only slab
pull was operating; and 3) both slab suction and slab pull were
at work. Then they compared the plate motions that would result
from each of these scenarios with actual plate motions. The best
fit was the model that combined slab pull and slab suction forces.
The model also explained an observation that has baffled geodynamicists
for some time. The way the observation was originally framed
was that plates that have continents on them are slow, compared
to plates that are only oceanic, says Lithgow-Bertelloni.
But the real issue is whether or not the plates have slabs attached,
she says. Overriding plates, which have no slabs, are slower than
subducting plates, which have slabs. The explanation? Subducting
plates move faster because the pull effect acts directly on them,
making them move rapidly toward the subduction zone. Overriding
plates are also drawn toward the subduction zoneby the suction
effectbut at the same time, the pull effect creates forces
in the mantle that counteract that motion. The net effect is that
overriding plates move more slowly toward the subduction zone than
subducting plates do.
Weve been able to explain that the difference in speed
occurs because slab pull generates mantle flow that counteracts
the motion of the overriding plate, says Lithgow-Bertelloni.
We also found that this effect is only important for slabs
in the upper 600 to 700 kilometers of the mantle. Any slabs deeper
than 700 kilometers do not contribute to this effect. Theyre
important for driving flow in the mantle, but theyre not important
for the pull.
For more information, visit http://www.geo.lsa.umich.edu/dept/faculty/lithgowbertelloni/index.html,
http://pubs.usgs.gov/publications/text/understanding.html or http://www.platetectonics.com/.