The carcass of a radial tyre is maintained by the nylon and steel belts. The soft inner core is bonded to the tread surface. Cracks that form on the surface indicate that the tyre is dry rotting and losing its elasticity.
Dry rot is usually caused by exposure to ozone, sunlight, high heat, lack of use and age.
The dry rot may be superficial but is a precursor of things to come. Eventually the cracks will permeate the inner carcass and allow the air to excape.
A worse case senario would be tread separation or moisture and dirt contaminating the nylon and steel carcass allowing for oxidation and weakening of the carcass leading to a potential blow out.
Assuming the enviromental factors causing the dry rot are addressed, it can be slowed dramatically leaving the tyre to not get any worse, the tyre is probably okay to use but understand that the full potential and longevity of the tyre speed rating and load rating will be diminished.
A dry rotten tire is essentially on borrowed time, until the cracks become too deep as they would represent a safety hazard. With cracks on the sidewall representing the greatest risk.
