Relationship between mantle convection currents and plate motions?

Content:

Introduction: Plate tectonics has been regarded as a scientific revolution. If so one might argue that it is incomplete. While we have a detailed understanding of how the plates behave, the relationship between plate motions and convection in the mantle is still vigorously debated. This is evident in the ongoing discussion about mantle plumes and hot spot trails. New information from seismic tomography and compute modeling capacities promise to provide insight in the near future. As we expand our knowledge in science we also expand the boundaries of the unknown. Convection is also a fundamentally important process that controls much of the dynamics of atmospheric, ocean, mantle and outer core systems, and thus well worth understanding better.

Image to right depicting top to bottom mantle convection from http://geomag.usgs.gov/about.php .


Two basic simple end-member models

coupled: currents in the mantle raft the overlying plates around. Traction stresses at the base of the plates would be critical. This could be akin to sea ice being moved by underlying currents in the sea water.

decoupled: plates move due to internal body forces, and influence the shallow convection current pattern in the mantle.

locally exclusive, but not globally.

Schematic diagram of coupled (left) and decoupled (right) models. In the first case the motion in the asthenosphere rafts the plate above along. The plate would move slower than the underlying asthenosphere, and the relative sense of motion between the two would be as the blue arrows show. In the other case some other force such as trench pull would pull the lithosphere and drag would induce movement in the asthenosphere below, which would diminish downwards, and the relative sense of motion between the two would be the opposite. The second possibility ignores the possibility that some other motion could exist in the asthenosphere due to mantle convection, and treats the asthenosphere as passive. In other words, the arrows in the decoupled case only show the movement component in the asthenosphere that would be created by drag as the over riding plate moves and not movements due to other causes.


Possible driving forces for plate tectonics

Here is a list to start with:

Lava lake videos that demonstrate possibilities on a small scale:


Convection basics

Conduction vs. convection vs. advection. Advection is often assumed to be absent in the mantle, but is it (partial melt migration)? We won't have time to really address that here. Realize that the term advection is used in different ways.

Buoyancy is driven by gravity acting on density contrasts caused by thermal differences and by phase changes. Both are important in earth's mantle.

Viscosity of the convecting medium is obviously a crucial parameter. What is the nature of viscosity?

Significance of the Rayleigh number in convection behavior:

Geometries of convection:


Basics of mantle convection: flowage in response to buoyancy forces

Importance of Clapeyron slope: For most mineral transformations the transformation pressure increases with increasing T (i.e. a positive Clapeyron slope). This means that in a colder mantle region the transformation can occur at a shallower level and in a hotter region it occurs at a deeper level. We already considered this in the context of the olivine to spinel transition in a subducting slab and the mechanism of trench pull. We can also consider thermal plumes. If there is a density increase in a colder region of convective downwelling, the phase transition will occur at a elevated level producing a negative buoyancy and enhancement of downwelling would be expected. By the same line of reasoning a hot spot, a positive buoyancy force would exist. In this case a plume may be self perpetuating once it forms. Which is consistent with the long history of some mantle plumes (see below). In fact you could ask why would a mantle plume ever die? What is the slope for the 670 km, spinel to perovskite boundary?

Mantle viscosity: a most crucial parameter!!

Shallow mantle viscosity dominated by olivine rheology.

Mantle material is of varying viscosity: lithosphere vs. asthenosphere, and upper vs. lower mantle. Is there a viscosity contrast across the 370 km phase change boundary?

Estimations of mantle viscosity from glacio-isostatic rebound:


Insight into mantle convection from seismic anisotropy

Basic reasoning: Remember that mantle convection involves the flow of a crystalline solid - i.e. deformation. Such solid-state flow induces a strong preferred direction of crystallographic axes of the deforming minerals. In addition to hardness, seismic velocity varies with crystallographic direction. Seismic velocity anisotropy is observed in the mantle, and can be interpreted to reflect the flow/movement directions.

IRIS and traveling array project.

Some results:

Map showing azimuthal seismic anisotropy (fast directions) at two different depths in the mantle. Small green circles are hot spot locations. Source of image: Debayle et al. - IRIS website - https://www.iris.edu/gallery3/research/2010proposal/upper_mantle/debayle_fig1 .

 


Hot spots and mantle plumes

Well known examples: Hawaiian island chain, Iceland, Yellowstone.


Stratified vs. whole mantle convection

What is the significance of the upper-lower mantle boundary?

Viewing present patterns of convection:

IRISsource

S- wave velocity anomalies at different levels in the mantle. What patterns do you note. From IRIS site - http://www.iris.edu/hq/gallery/photo/8935. Credit: Sergei Lebedev, Dublin Institute for Advanced Studies, Rob D. van der Hilst, MIT/IRIS Consortium

IRIS source

Shear wave splitting interpreted as due to the direction of mantle flow (and alignment of olivine crystals) beneath western U.S.. Image source: http://www.iris.edu/hq/gallery/photo/8935, Credit: Matthew J. Fouch, Arizona State University, John D. West, Arizona State University/IRIS Consortium


Model simulations of mantle convection

Computer models are only as good as the input and underlying assumptions, but can still inform about possibilities.

 


Parting thoughts on the mechanisms


Some References:


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