  Under normal conditions in a neutron-star crust, ions are locked in place in
the crustal lattice and only electrons are mobile, and magnetic-field evolution
is thus directly related to the electron velocity. The evolution, however,
builds magnetic stresses that can become sufficiently large for the crust to
exceed its elastic limit, and to flow plastically. We consider the nature of
this plastic flow and the back-reaction on the crustal magnetic field
evolution. We formulate a plane-parallel model for the local failure, showing
that surface motions are inevitable once the crust yields, in the absence of
extra dissipative mechanisms. We perform numerical evolutions of the crustal
magnetic field under the joint effect of Hall drift and Ohmic decay, tracking
the build-up of magnetic stresses, and diagnosing crustal failure with the von
Mises criterion. Beyond this point we solve for the coupled evolution of the
plastic velocity and magnetic field. Our results suggest that to have a
coexistence of a magnetar corona with small-scale magnetic features, the
viscosity of the plastic flow must be roughly $10^{36}-10^{37}\ \textrm{g
cm}^{-1}\textrm{s}^{-1}$. We find significant motion at the surface at a rate
of $10-100$ centimetres per year, and that the localised magnetic field is
weaker than in evolutions without plastic flow. We discuss astrophysical
implications, and how our local simulations could be used to build a global
model of field evolution in the neutron-star crust.
