Numerical ages for boundaries between time units on the geologic time scale primarily resulted from the study of in conjunction with relative age data.

Radiometric dating of igneous rocks

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

B  maybe the answer hoped it