A number of observational, marine geological, geochemical
and geophysical studies suggest that mantle plumes interact
significantly with spreading ridges. One leading model for
plume ridge interaction is that hot buoyant plumes rising off-axis
, or away from the spreading ridge, are deflected along the base
of the lithosphere (much like an inverted sink with material flowing
downgradient to the drain). Material from the distant hotspot
plume is then sampled at the ridge axis. An example of such
a plume-ridge interaction system is St. Helena and the Mid-
Atlantic Ridge.
Shown below is a schematic of the plume ridge interaction model (in a vertically oriented cross section, normal to a spreading ridge) whereby warm plume fluid rises off-axis
and is diverted towards the ridge axis. Also shown are results
from a numerical model developed to test the physics of this
proposed mantle flow model. The frames on the left show the
thermal structure, the frames on the right show chemical fields
(blue material is the plume, green is background mantle).
In this 2-D model we represent
both the plate-driven mantle flow due to diverging plates and
buoyant flow of a plume and monitor plume-to-ridge flux as a
function of plume buoyancy, plate speeds and plume-ridge
separation distance. The numerical models support the feasibility
of the plume-ridge interaction model and highlight which physical
factors are most important in controlling this process.
In the figure below we show a schematic of the laboratory apparatus designed to also test the
physics of the plume-ridge flow model. In the lab we are able to
model the process in 3-D.
Plate flow is modeled in the
lab by dragging sheets of mylar in opposite directions across
the surface of a viscous fluid. Plume flow is modeled by adding
thermal buoyancy to the fluid with a circular disk heater, located
at the base of the fluid. We then monitor the amount of plume fluid
to reach the spreading axis for different 1) plume buoyancies
, 2) plate rates, 3) plate cooling rates and 4) plume-ridge
separation distances.
In the next figure (below) we show a photo from a plume-ridge interaction lab experiment.
Beads are released into the plume source region and these tag
the flow of plume fluid. Here we see a number of beads being
deflected away from the ridge axis, but a couple have been deflected
along the base of the plate towards the spreading axis.
Both
the numerical models and lab models suggest the original plume-ridge
interaction model is physically feasible given reasonable mantle
parameters. Ongoing studies are investigating the role of
transform offsets on plume-ridge interaction and the influence of migrating ridges. These are shown schematically below.
