  Materials typically fail under complex stress states, essentially involving
dilatational (volumetric) components that eventually lead to material
decohesion/separation. It is therefore important to understand dilatational
irreversible deformation -- i.e., dilatational plasticity -- en route to
failure. In the context of glasses, much focus has been given to shear
(volume-preserving) plasticity, both in terms of the stress states considered
and the corresponding material response. Here, using a recently-developed
methodology and extensive computer simulations, we shed basic light on the
elementary processes mediating dilatational plasticity in glasses. We show that
plastic instabilities, corresponding to singularities of the glass Hessian,
generically feature both dilatational and shear irreversible strain components.
The relative magnitude and statistics of the strain components depend both on
the symmetry of the driving stress (e.g., shear vs.~hydrostatic tension) and on
the cohesive (attractive) part of the interatomic interaction. We further show
that the tensorial shear component of the plastic strain is generally
non-planar and also extract the characteristic volume of plastic instabilities.
Elucidating the fundamental properties of the elementary micro-mechanical
building blocks of plasticity in glasses sets the stage for addressing
larger-scale, collective phenomena in dilatational plasticity such as
topological changes in the form of cavitation and ductile-to-brittle
transitions. As a first step in this direction, we show that the elastic moduli
markedly soften during dilatational plastic deformation approaching cavitation.
