PLANETARY MUSINGS

Entries in "Plumes"

The shape of a plume

Farnetani, CG and H Samuel, Beyond the thermal plume paradigm, GRL, 32, L07311, doi:10.1029/2005GL022360, (2005).

The issue of mantle plumes (i.e., whether they exist) is a long-running problem that just keeps going back and forth on the teeter-totter. I've always thought that with as complex a planet as we have here that they yes or no approach was a bit limiting given that the fluid physics of thermal plumes is well-established... but I digress.

Farnetani and Samuel tackle the formation of plumes from a more general framework than is usually taken. The canonical view of plumes is that a mushroom-like head and thin conduit tail plume structure is created at a deep thermal boundary layer and traverses the mantle to the surface with modest entrainment of surrounding mantle. The head impacts the surface, creates a large igneous province and the conduit creates a following chain of volcanic centers as a plate moves over it. That's the broad framework, first-order and all. The Earth is more complex and this paper shows what happens if chemical buoyancy effects, mantle wind (caused by imposed plate motion at the surface), phase transitions, and heterogeneities are considered (if memory serves, many of these effects have been considered before, though maybe not all at once with the spatial resolution of this study). And in this more "relaxed" study of the relevant parameters, the authors have discovered that the plume head-tail structure is but one possible structure of a plume as it reaches the upper mantle. A plume head isn't even necessary, and a concentrically zoned plume tail isn't even necessary either. Basically, models of the Earth can be messy - consistent with the geochemical and seismological view.

Mantle plumes are just plain hard to avoid - there is heat coming out of the core, the boundary layer down at the core-mantle boundary is going to get unstable in some places every once in a while, and there are going to be plumes. The real questions are: what do they look like, how do they sample the mantle, and what happens when (if?) they reach the surface? These questions are still open. But this paper shows us that not every plume looks alike.

But... this view of plumes that a plume head is possible not necessary is interesting when extended to other planets. Now the "mantle wind" may not be as extensive on a planet without plate tectonics, but what is the driver behind the headless-plume? On Venus there are all the coronae and volcanic rises, many of which seem consistent with a thermal plume or diapir source (e.g., recent conceptual framework of Johnson and Richards, 2003). If even some of those, e.g. coroane, require head-like features, what does that mean for the mantle? Maybe it is just a question of scale... but the questions about mantles and their plumes seem to abound.