@greyhud90 That's a fantasy thing. Elemental combinations don't have to make sense from a scientific point of view. It IS magic after all. I've seen this kind of combinations a lot.
Like Earth + Fire = Lava or Magma, Water + Earth = Nature(as in plant-life, trees, etc.), Air + Fire = Lightning (though in Oriental fantasy Lightning is a base element rather than a result of a combination), etc. There can also be multiple different results from the combinations of the same elements, such as Air and Fire also resulting in smoke, steam, etc. Or Water and Air also resulting in fog. Water and Fire could also result in Gaz, Steam, etc. Not every series or table top system do it the same way.