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Traction transmitted between the base of a tectonic plate and the mantle. Its component parallel to the plate can drive or resist plate motion, depending on the relative motion and surrounding mantle flow. Its direction should not be inferred from the word “drag” alone.

Thermal or chemical modification that changes an older lithosphere's properties, potentially reducing its strength or effective thickness. Magma transport, mantle plumes and other mantle flow can contribute. A younger thermal signature does not automatically imply complete replacement of the old crust or lithospheric mantle.

Yes: substantial along-strike segmentation is expected and observed in the Andes, but its geometry and dominant controls need not match Cascadia. The older Nazca Plate has a thicker cooled lithosphere and a larger thermal density excess relative to hot mantle than the young Juan de Fuca system. In a half-space cooling model, thermal thickness scales as $\sqrt{\kappa t}$, so the age comparison alone gives a thickness ratio $\sqrt{50/5}=\sqrt{10}\simeq 3.16$. This changes the balance of slab buoyancy, bending resistance and mantle flow; it does not make the margin laterally uniform.

Changes in incoming ridge and plateau buoyancy, hydration, fracture zones, convergence obliquity, upper-plate thickness and motion can all alter slab dip and coupling. The Peruvian and Pampean flat-slab regions illustrate this: long nearly horizontal slab sections contrast with steeper subduction beneath active arc sections. Suppression or displacement of a hot mantle wedge explains why flat-slab subduction can coincide with volcanic gaps and deformation farther inland. The steeply dipping slab beneath much of the central Andes should not be conflated with every Andean segment.

The Juan Fernández Ridge is associated with the Chilean/Pampean transition, while the Nazca Ridge intersects the Peruvian system. However, association is not proof that crustal thickness alone supplies all the buoyancy. Wide-angle seismic observations at the Juan Fernández system found only moderate crustal thickening and motivated consideration of hydration as well as ridge emplacement.^[ref] The same combination of slab imaging, geodesy, volcanic chronology and incoming-plate characterization is needed to test causes. Thus the transferable idea is lateral variation in forces and material properties, not an identical three-segment pattern or one age-controlled mechanism.

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]

The asthenosphere exists because the mantle's ability to deform is strongly dependent on temperature, pressure, composition, water and deformation mechanism. Cooling from the surface creates a strong lithosphere. Beneath it, hotter mantle deforms by material creep at much lower stress on geological timescales. The asthenosphere is predominantly solid weak rock, not a global ocean of magma. On the much shorter timescale of a seismic wave, it still supports S waves.

The hot shallow mantle can be close to its solidus. Water weakening of rocks and, in some settings, a small connected melt fraction can further lower effective viscosity. At greater depths, changes in pressure, homologous temperature, mineral structure and water storage modify the deformation laws; increasing depth does not imply indefinitely decreasing viscosity. Explaining a weak upper-mantle interval does not require every location to contain the same amount of melt.

There are several physically distinct boundaries to test. The thermal lithosphere-asthenosphere boundary describes a thermal transition, the mechanical boundary a contrast in long-term strength or viscosity, and the seismic lithosphere-asthenosphere boundary a change in seismic velocity or other wave properties. They need not have identical depths. A smooth geothermal gradient can explain broad thermal weakening, but a very sharp decrease of seismic velocity may require changes in melt fraction, hydration, composition or seismic anisotropy. A sharp shallow reflector within a craton might instead be an internal lithospheric boundary.^[ref]

Begin with a thermal null model. A half-space cooling model givesIf a thermal boundary corresponds to a fixed temperature $T_{\ast }$, its depth isso it deepens as the square root of plate age. Test that prediction against independently determined seafloor ages, bathymetry and heat flux. An oceanic plate cooling model adds finite thickness and basal heat supply, allowing the old-age depth to approach a finite value. For continents, include radiogenic heating, crustal thickness, the history of continental rifting or tectonic thickening, and advective heating by magma or mantle flow. A cold, dry craton and a recently extended volcanic region should not share the same thermal profile.

Then calculate the observable consequences of competing mechanisms rather than merely assigning a boundary to a map:

Compare models against the observed boundary depth and its sharpness, including uncertainty and resolution. An age–depth trend with matching heat flux favours thermal control; a sharp discontinuity that fails the thermal prediction but coincides with an electrical anomaly can support a melt- or hydration-related transition. Neither observation alone proves a unique cause. The spatially varying boundary is best understood as the result of thermal evolution interacting with composition, fluids and deformation history.

In the interpretation of Christopher Kincaid and colleagues, slab rollback deforms and splits a mantle plume's thermal anomaly: one branch is swept toward the trench beneath the High Lava Plains, whereas the Snake River Plain records the overriding plate moving over a deformed plume conduit. Two opposite age progressions need not represent two stationary hotspots.

In the supplied map, ages decrease toward Yellowstone in the northeast and toward Newberry Volcano in the northwest. Southwestward North American Plate motion leaves older Snake River Plain volcanism to the southwest of the younger source expression. Meanwhile, rollback-driven, three-dimensional mantle flow, including toroidal mantle flow around the slab, carries a passive thermal anomaly trenchward beneath the High Lava Plains. The associated migrating heating and melting front produces the opposite younging toward Newberry Volcano.

The original experiment used a viscous syrup mantle analogue, a kinematically moved rigid slab, and an injected buoyant thermal plume. The result is advection of deformed plume heat by subduction-driven flow, rather than simple radial spreading of an intact plume head under a stationary plate. The mapped systematic progression principally concerns rhyolitic volcanism; basaltic eruptions are more diffuse in time. This laboratory result establishes a physically plausible explanation, not a unique observational proof of the deep source geometry.^[ref]

Migration of a subduction trench relative to the surrounding mantle as a slab retreats; the associated mantle flow can advect thermal anomalies.

The effect of slab-induced mantle flow, pressure and traction on the motion of neighbouring plates and their boundary. It is part of the coupled response to a sinking slab, rather than an unrelated energy source that can always be added independently to slab pull.

The component of three-dimensional mantle flow that circulates around a sinking slab's lateral edges.