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Slab pull is generally the largest direct driving contribution for plates attached to substantial cold descending slabs. Cooling makes oceanic lithosphere negatively buoyant; sinking slabs transmit a pulling force to the rest of the tectonic plate. The comparison is a force balance, so the following driving and resisting processes both matter.

The first two gravitational contributions ultimately belong to the same coupled mantle convection system: the plates are its cold upper thermal boundary layer. They are not passive rafts on a prescribed conveyor belt. A plate with little attached descending slab need not have slab pull as its largest local driver, and a large gross slab pull can be partly consumed by bending and interface resistance. Geological observations and force comparisons nonetheless support the general dominance of slab pull for rapidly moving subducting plates.^[ref]

Left: ordinary frictional strengthening with depth followed by thermally activated material creep. Right: a very weak shallow faulted layer above a narrow strong region near 10 km, followed by essentially the same weakening material creep branch. These are the shapes actually plotted in Figure 2. The right curve is not a strong upper crust over a uniformly weak lower crust; neither diagram displays a second mantle-strength peak.

For the left curve, increasing overburden increases the effective normal load on faults. A schematic Coulomb friction law iswhere $C$ is cohesion, $\mu$ the coefficient of friction, ${\sigma }_n$ the compressive normal stress and $P_f$ the pore pressure. With ordinary rock friction and approximately hydrostatic pressure, ${\sigma }_n-P_f$ increases roughly linearly with depth. The stress needed for frictional sliding therefore increases, as in the straight shallow branch. Byerlee's law summarizes the relatively high friction found in many laboratory rocks.^[ref]

At higher temperature, dislocation creep or another ductile mechanism accommodates the imposed deformation at lower stress. For example,Here $\dot{ϵ}$ is strain rate, $Q$ an activation energy, $R$ the molar gas constant and $T$ absolute temperature. At fixed strain rate, increasing $T$ lowers the required stress. The operative envelope is approximately $\min ({\sigma }_f,{\sigma }_d)$, expressed in a consistent stress convention; the peak marks the brittle-ductile transition. This accounts for the left curve's peak near 100 MPa and its decline toward small strength by about 20 km.

For the right curve, normal high-friction sliding with depth-independent fluid-pressure ratio cannot explain the nearly vertical weak shallow branch. Plausible weakening includes high pore pressure tracking the overburden, low-friction fault gouge, or a connected network of pre-existing weak faults. Roughly constant small ${\sigma }_n-P_f$ yields roughly constant small frictional strength. The abrupt increase near the base of this layer requires a change of conditions: for example, the weak fluid-rich fault network terminates or becomes discontinuous, leaving stronger intact or less altered rock below. A change in mineralogy or deformation localization can also give a contrast. Simply saying “there are fluids” does not explain the return to high strength. Below the strong region, thermal material creep again lowers the strength.

The figure alone does not identify the weakening mechanism or a particular lithological boundary. In particular, a 10 km strength maximum is not evidence for a 10 km deep Mohorovičić discontinuity. The strong-fault/weak-fault evidence and the distinction between fault strength and surrounding crustal strength are discussed in Stress orientations at intermediate angles to the San Andreas Fault, California by Jeanne L. Hardebeck and Andrew J. Michael.

A depth-dependent estimate of the differential stress a rock column can sustain at a specified strain rate, composition, thermal structure and fluid state. In a simple competing-mechanism model, failure occurs by whichever of frictional sliding and ductile material creep requires the lower stress, giving the displayed lower envelope.

Effective confining pressure generally increases frictional strength with depth. In contrast, a power-law creep model $\dot{ϵ}=A{\sigma }^n\exp [-(Q+P{V}^{\ast })/(RT)]$ gives ${\sigma }_{\mathrm{creep}}=(\dot{ϵ}/A{)}^{1/n}\exp [(Q+P{V}^{\ast })/(nRT)]$. At fixed strain rate, the rise of temperature along a geothermal gradient generally lowers the required creep stress. Their crossing is a brittle-ductile transition, while composition changes at the Moho can produce additional peaks. A rock strength envelope is not uniquely determined by depth or by the labels “continental” and “oceanic”.