What criterion decides when rising magma shatters into ash?
In plain words
As magma rises, gas bubbles grow until the frothy melt may break apart into fragments and gas, turning a lava flow into an explosive jet. Scientists still disagree on which condition sets the break point.
Precise statement
Determine the fragmentation criterion for vesiculating silicic magma: a critical porosity (about 0.6 to 0.8), a critical bubble overpressure relative to melt tensile strength, or a critical strain rate exceeding the inverse Maxwell relaxation time of the melt, including the effects of permeability and crystals. An answer is a single criterion, or a regime map, validated by shock-tube experiments and natural pyroclast textures.
What would settle it
Decompression experiments spanning melt viscosity, porosity and permeability that collapse onto one fragmentation threshold, matched by textures of natural pumice.