Earth Magic
is about the secondary skill. For earth magic spells, see School of Earth Magic.
Earth magic is a secondary skill, that increases the effectiveness and reduces costs of spells from the School of Earth Magic.
Discussion
The main reasons for choosing earth magic are town portal and slow spells. The first one is one of the most powerful adventure map spells with fly and dimension door. Expert earth magic causes slow to be cast at mass spell, which makes it worth the secondary skill slot.