  The fragility of a glassforming liquid characterizes how rapidly its
relaxation dynamics slow down with cooling. The viscosity of strong liquids
follows an Arrhenius law with a temperature-independent barrier height to
rearrangements responsible for relaxation, whereas fragile liquids experience a
much faster increase in their dynamics, suggesting a barrier height that
increases with decreasing temperature. Strong glassformers are typically
network glasses, while fragile glassformers are typically molecular or
hard-sphere-like. As a result of these differences at the microscopic level,
strong and fragile glassformers are usually treated separately from a
theoretical point of view. Silica is the archetypal strong glassformer at low
temperatures, but also exhibits a mysterious strong-to-fragile crossover at
higher temperatures. Here we show that softness, a structure-based machine
learned parameter that has previously been applied to fragile glassformers
provides a useful description of model liquid silica in the strong and fragile
regimes, and through the strong-to-fragile crossover. Just as for fragile
glassformers, the relationship between softness and dynamics is invariant and
Arrhenius in all regimes, but the average softness changes with temperature.
The strong-to-fragile crossover in silica is not due to a sudden, qualitative
change in structure, but can be explained by a simple Arrhenius form with a
continuously and linearly changing local structure. Our results unify the study
of liquid silica under a single simple conceptual picture.
