The most prominent feature of ocean topography discovered in the 1960s was:

1) volcanic activity at the ocean bottom
2) a submerged mountain ridge
3) seamounts
4) subduction zones

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