Seismologists have detected a discontinuity in the mantle at a depth of 670 kilometers, and experiments in high-pressure mineral physics indicate that a phase transition occurs at this depth as the intense pressure causes rocks to collapse into a denser state. The pressure-temperature relationship of this transition creates buoyancy forces that tend to keep the material from crossing the boundary between the lower and upper mantle. When this complexity is added to the model, a form of partially layered convection appears: downwelling cold material within the mantle accumulates into pools above the phase transition boundary. At periodic intervals, "avalanches" occur that flush great masses of this cold material into the lower mantle (Figure 1). The true picture seems to lie between the extremes of layered and whole-mantle convection, and provides a way to reconcile various pieces of seemingly contradictory geophysical evidence.
The Earth's tectonic plates consist mostly of lithosphere, the cold, strong top part of the mantle. The crust is just a thin layer of buoyant rock embedded in lithospheric plates. Some plates have continents embedded in part of them, so they are part ocean and part continent, while others are entirely oceanic with only a very thin crust--six-kilometers thick.
"In my opinion, self-consistent generation of plate tectonics is the most important issue facing mantle modelers right now," Tackley said. Researchers who have included lithospheric plates in 3-D models have always had to impose them by hand. The effects of temperature-dependent viscosity alone simply result in an immobile, rigid lid--analogous to a single, world-girdling tectonic plate (Figure 2).
Tackley is investigating the possible role of complex stress-dependent deformations and material flows and horizontal compositional variations, such as non-subducting continental material, in giving rise to our planet's three different types of plate boundaries: mid-ocean ridges, subduction zones and transform faults.
In his current strain-rate weakening model, material stress increases with strain rate to a critical point, past which the stress decreases and the material weakens--not unlike a snapping twig. Strain-induced shearing causes the lithosphere to "break" into a number of very high-viscosity plates; these are separated by sharply defined weak zones with viscosity orders of magnitude lower. Broad weak zones with dominant convergent/divergent motion above upwellings and downwellings are interconnected by a network of narrow weak zones with dominant strike-slip motion (Figure 3).
The model results are similar to rigid plates separated by broad weak zones analogous to subduction zones and mid-ocean spreading centers and narrow weak zones that resemble strike-slip faults such as the San Andreas Fault. This is something of a breakthrough, since for the first time separate plates are the result of a 3-D model rather than an initial assumption. "Although the features are not fully realistic, these results show that self-consistent plate generation is a realizable goal in 3-D mantle convection and provide a promising avenue for future research," Tackley said.
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he Earth's outer crust is only a few miles thick--thinner, in proportion, than the shell of an egg. Between the crust and the molten core lies the mantle, nearly 3,000 kilometers of rock at temperatures and pressures so high that it behaves like a thick liquid. Carrying heat from the core to the surface, the mantle creeps like tar on a hot day. The mantle's movement reveals itself through such forces as volcanoes, earthquakes, and continental drift. To better understand the behavior of the mantle, UCLA geophysicist Paul Tackley and colleagues use NPACI computers to perform increasingly complex and realistic 3-D simulations. 
Figure 1. Going Down 
Figure 2. The Original Lava Lamp
Figure 3. Making Plates 