The driving density difference between fluids that produces intrusion motion could simply be due to chemical composition.
[6] Arguably the most striking of all gravitational intrusions, is the atmospheric gravity current generated from a large, 'Plinean' volcanic eruption.
In which case the volcano's overhanging 'umbrella' is an example of an intrusion laterally intruding into the stratified Troposphere.
As of 2012 the most recent rigorous analytical analysis, designed to determine the propagation speed of a classically extending intrusion, was performed by Flynn and Linden.
The basic structure of a gravity intrusion is approximate to that of a classic current with a roughly elliptical 'head' followed by a tail which stretches with increased current length, it is within the rear half of the intrusion head that the majority of mixing with ambient fluids takes place.