  Early-type stars show a bimodal distribution of magnetic field strengths,
with some showing very strong fields ($\gtrsim 1\,\mathrm{kG}$) and others very
weak fields ($\lesssim 10\,\mathrm{G}$). Recently, we proposed that this
reflects the processing or lackthereof of fossil fields by subsurface
convection zones. Stars with weak fossil fields process these at the surface
into even weaker dynamo-generated fields, while in stars with stronger fossil
fields magnetism inhibits convection, allowing the fossil field to remain
as-is. We now expand on this theory and explore the time-scales involved in the
evolution of near-surface magnetic fields. We find that mass loss strips
near-surface regions faster than magnetic fields can diffuse through them. As a
result, observations of surface magnetism directly probe the frozen-in remains
of the convective dynamo. This explains the slow evolution of magnetism in
stars with very weak fields: these dynamo-\emph{generated} magnetic fields
evolve on the time-scale of the mass loss, not that of the dynamo.
