Older glaciers have thinner shorter longer ice crystals than younger glaciers.

Older glaciers have "longer" ice crystals than younger glaciers. Glacier from where there is more snow falls than snow melts, over a period of years. It is then compacted into ice. The longer the glacier is the older it is.

So  if  the sum of the enthalpies of the reactants is greater than the products, the  reaction  will be  exothermic.  if  the products side has a larger enthalpy, the  reaction  is  endothermic. you may wonder why  endothermic reactions, which soak up energy or enthalpy from the environment, even happen