Although glass has been used for thousands of years (figure 1) and is nowadays of great indus- trial importance, questions associated to what is sometimes refered to naively as a fourth state of matter, in addition to solid, liquid and gaz, are still puzzling scientists despite a large effort in the condensed and soft matter community over the last decades. In this short document I will try and show how the recent progress in the understanding of the physics of glassy materials can shed new light on the deformation processes that appear in a large variety of natural complex systems.
Figure 1: Venitian glass.
Glass formation.    The first great challenge is to understand the for- mation of the glass itself, in other words why when a liquid is cooled below a certain temperature, refered to as glass transition temperature, at fast rate most substances rather than reaching a cristalline ordered final state remain in a metastable disordered configuration. Even more puzzling is the fact that this transition from liquid to glass does not seem to be associated to any simple structural change of the system. To the bare eye a snapshot of a glass looks exactly similar to a snapshot of the corresponding liquid. Yet one can somehow arbitrary define a glass transition temperature TG as the temperature at which the viscos- ity of the liquid has increased by 13 orders of magnitude with respect to the viscosity of the high temperature liquid. Therefore this transition does not seem to be a structural one (in a first approximation as the structure still undergoes smooth and weak changes with T) but rather a dynamical transition, refered to as dynamical arrest. Despite this ap- parent lack of a growing static structural length scale as temperature is lowered towards Tg (as would be the case in a ‘normal phase transition’), evidences of such clusters or domains have been identified dynamically in several experiments and numerical simulations over the last decades. In recent years these observations have lead to the generalization of the term glass not only to the hard, brittle, transparent silicate glasses
used in windows and bottles, but to an entire class of materials that exhibit this type of dynam- ical arrest. Taking this definition allows to include in the class of glassy materials, materials as diverse as hard glasses s uch as silica glass, amorphous silicium, metallic glasses and polymeric glasses, soft glasses such as pastes, colloids, cells, foams, different complex fluids such as ketchup, mayonaise, but also granulars. In section 3 we will even extend this definition to the Arctic sea ice.