Lost Souls & Midian

In general, Elementals of greater power will defeat those of lesser power. For example, a major earth Elemental will crush (literally) a minor earth Elemental. For Elementals of equal power but of different types, the following mnemonic guidelines apply:

Water washes away earth. Earth smothers fire. Fire consumes air. Wind blows over waves.

This can also apply to magical effects, e.g. fire spells are effective against air Elementals.

When a fire and water Elemental of equal power meet in combat, they completely destroy one another in a dense cloud of steam. For major and greater Elementals, this steam cloud can prove lethal to mortal life. Minor Elemental destruction is very painful, but not deadly, and lesser Elementals that obliterate one another produce a small cloud that is more of an annoyance than anything. Air and earth Elementals cannot affect each other at all, and even seem to have some difficulty understanding that the other even exists. In practice however, greater Elementals won't fight one another--they aren't interested and nothing you can do or say will convince them otherwise.

Elementals can be spontaneously created in the presence of their respective elements, especially if there is an energy source available. As examples, hurricanes can generate air Elementals, or raging bonfires may spontaneously create fire Elementals.