Jellinek and Michael Manga, associate professor of earth
and planetary science at UC Berkeley, modeled the mantle
in a vat of viscous oil and showed by analogy that a thin
layer of dense, low-viscosity rock at the base of the mantle
could anchor plumes for long periods of time, perhaps for
the entire age of the Earth.
The two scientists reported their experimental findings
on Sunday, Dec. 8, at the annual meeting of the American
Geophysical Union in San Francisco. The meeting ends Dec.
10. The team also detailed their experiment and conclusions
in an article in the Aug. 15 issue of Nature.
Over the past few years, evidence from earthquake vibrations
bouncing around the Earth's interior have revealed a layer
of dense material at the base of the mantle that clumps
around the feet of the known plumes. It stands out because
seismic waves slow down when they pass through the material,
indicating rock that is hotter than the mantle above. Several
scientists have suggested that if this dense material is
10 to 100 times less viscous than the overlying mantle,
the clumps could anchor the plumes and explain their long
lifetimes. The dense layer, which sits on top of the Earth's
core, is about 60 to 200 miles (100-300 kilometers) thick.
Jellinek and Manga sought to test this hypothesis with
a series of more than 40 experiments in a vat of polybutene
- an oil used in two-stroke engines and as a lubricant -
that simulated a scaled-down mantle. The vat, about the
size of a 10-gallon fish tank, was 17 centimeters (6.7 inches)
To create rising plumes like those in the mantle, they
stuck a heater at the bottom of the vat and cooled the top.
The heat, simulating that from the Earth's hot outer core,
generated convective plumes and buoyant bubbles like those
in a lava lamp on in a pot of water on the stove, though
they moved around and tended to be short lived.
The two researchers injected a thin layer of less viscous
oil at the bottom, simulating the dense, low-viscosity rock
at the base of the mantle. This oil, tinted red to distinguish
it from the oil above, formed a layer about 1/5-inch thick.
They tried oils of different viscosity, ranging from walnut
oil and various vegetable oils to specialized lubricants,
until they found one - soybean oil - with the right viscosity
to create oil clumps at the bottom of the vat with long-lasting
plumes rising above them. Soybean oil is about 110 times
less viscous than polybutene.
Apparently, Manga said, as a plume rises, it draws up the
less viscous oil to create a peak or ridge, which makes
it easier for the more viscous plume to rise above the clump.
This coupled flow lets plumes stabilize themselves for long
periods. While each plume in the tank lasted about an hour
- the duration of the experiment - this time period scales
up to about 3.6 billion Earth years.
"We now have a mechanism by which to make hot spots last
a long time and fix them," Manga said.
The experiments also predict how closely plumes would be
spaced in the mantle.
"Under the Pacific Ocean, we would expect the spacing to
be about 1,000 kilometers (630 miles), which is about what
we see," Jellinek said. That would mean there could be about
20 hot spots around the globe.
This number accords with some estimates, which identify
as many as 15 hot spots under the oceans and continents.
Continental hot spots are difficult to confirm, however,
because the topography - mountains and crustal deformities
- deflect upwelling rock. Some geologists claim to see evidence
of nearly 100 hotspots - a count that Jellinek and Manga
The experiments clearly demonstrate what happens during
the lifetime of a plume, which can take 30-50 million years
to reach the surface after it starts to rise, Manga said.
The mantle rock, which has a viscosity comparable to glass,
is heated by the hotter core and begins to rise. As it does
so, the head balloons out, trailing a thin conduit, like
a soda straw, that connects it to the bottom of the mantle.
This conduit, perhaps only six miles (10 km) across, sucks
up the dense, low viscosity material through its center,
drawing it to the surface along with hot mantle rock.
Above a certain depth, the head of the plume begins to
melt and spread out under the crust, until it finds a crack
and pours through. The theory of mantle plumes predicts
an initial flood of lava lasting tens of millions of years,
which means that each hot spot should have an associated
flood basalt. The initial burst of lava from the Hawaiian
hot spot has been slurped back into the Earth at a subduction
zone, but other major flood basalts, such as the Siberian
and India's Deccan Traps, are at the end of an arc of volcanoes
and thus fit this model.
Lava can flow for several million years and create flood
basalts as large as 10 million cubic kilometers - an outflow
rate of about 10 cubic kilometers per year. That volume
would submerge San Francisco each year beneath 270 feet
of lava, with only the top of City Hall's dome peeking out.
After the plume head has dissipated, the conduit remains
in place indefinitely, periodically erupting and dotting
the surface of the plates with a chain of volcanoes.
Jellinek and Manga hope to find evidence to support their
"We hope to see if we can find evidence in the geochemical
record of the entrapped material from the dense layer,"
Manga said. "This also provides a way to predict what the
material is like where the hot spot came from at the core-mantle
There are two reigning hypotheses about the origin of this
dense material. One is that the core has partially melted
some of the mantle rock, even at the high pressures near
the core. The partial melt would be less viscous than the
An alternative theory is that bits of the core have become
mixed with mantle rock, forming an iron and silicon slurry
that would be less dense than the mantle material.
"Because the plume should drag up bits of the dense layer,
we should be able to find evidence of its composition in
the flood basalts associated with hot spots," Jellinek said.
The two scientists plan to continue their experiments and
have built an even larger tank to hold nearly two tons of
corn syrup for another simulation of the mantle, including
its interaction with the constantly moving crust.
The research is supported by the National Science Foundation
and UC Berkeley's Miller Institute for Basic Research in