Long-range lateral dyke propagation is independent of the level of neutral buoyancy

Lateral dyke propagation is a fundamental process in magmatic plumbing systems and a key process in crustal construction at mid-ocean ridges and large igneous provinces. Classical models attribute the tendency for magma to move laterally to the level of neutral buoyancy (LNB), where magma density equals host-rock density, and vertical ascent stalls, forcing lateral propagation. While this concept explains many observations in shallow magmatic systems, its applicability to long-range dyke swarms remains uncertain. The Mull Dyke Swarm (MDS) provides a critical test case. Extending over 600 km from its source on Mull in northwest Britain, the swarm exhibits a consistently shallow upper tip line, sill emplacement near the surface, and no evidence of eruption along its trajectory. Pressure reconstructions based on associated sill complexes indicate that magma pressures were sufficient to permit eruption over the first 150 km, yet lateral propagation continued. Moreover, calculated LNB depths diverge markedly from observed intrusion geometries. These mismatches suggest that the LNB concept is neither a necessary nor sufficient control on long-range lateral dyke propagation. Instead, propagation is better described as a fracture mechanics problem governed by the competition among magma overpressure, host-rock strength, and confining stress. The MDS thus exemplifies a broader class of magmatic systems in which buoyancy plays a minor role in fracture propagation. We argue that lateral dyke emplacement on Earth and other planets should be reframed within the physics of large-aspect-ratio hydraulic fractures, rather than buoyancy-driven ascent.