Question

Passage

[1] For much of the twentieth century, geological orthodoxy maintained that Earth's tectonic plates moved at an almost glacial, immutable pace, driven solely by deep mantle convection currents. This view, known as classic mantle drag theory, treated the lithospheric plates as passive passengers floating atop a churning asthenosphere. The paradigm offered a neat, mathematically elegant explanation for continental drift, aligning perfectly with the mid-century urge to find unifying, macro-level physical laws governing geological systems.

[2] However, this elegant consensus began to fray with the introduction of the "slab pull" model, which posits that the cold, dense subducting edges of plates themselves pull the rest of the plate behind them as they sink into the mantle under the influence of gravity. Far from being passive cargo, plates are now understood to be the primary drivers of their own motion. This paradigm shift was not merely a substitution of one mechanical driver for another; it fundamentally altered how geophysicists calculate thermal budgets and model mantle convection, rendering old simulations obsolete.

[3] To appreciate the impact of this shift, one must examine the Pacific Plate, where subduction zones are particularly active. Recent seismic tomography has revealed that the descending lithospheric slabs do not merely sink passively; they actively deform the surrounding mantle, creating localized, high-velocity currents that accelerate plate velocity. These findings have forced a reevaluation of the role of boundary forces, suggesting that localized edge dynamics play a far greater role in global tectonics than previously assumed.

[4] Consequently, the debate has moved beyond identifying the primary motor of plate motion to exploring the complex feedback loops between slab pull and mantle drag. Geophysicists now recognize that while slab pull initiates and dominates movement, mantle drag is not entirely passive; it acts as a stabilizing governor, preventing runaway plate acceleration. The contemporary model is thus not a rejection of classic theory, but a synthesis that accommodates both macro-convective forces and micro-boundary dynamics.

Based on the passage, which of the following best describes the relationship between the third paragraph (Paragraph 3) and the fourth paragraph (Paragraph 4)?