Convection causes large masses of rock to rise from and sink into Earth's mantel. The main heat source for this movement is 1.Earth's maritle and core.
2.Friction produced by plate motions.
3.Radiation from the sun
4.Magma in Earth's crust.

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

Since wxyz is 4, halfof that would be 2.