@Phabiohost my guess is that the first tier of magic controls what is readily available, while the advanced tier is more creating something from scratch? So earth users can readily move the ground to create pit traps and walls as the materials are already there. Metal would be summoning or creating swords and other objects from "thin air" and there would be enough iron, steel, or what have you in the immediate vicinity to draw upon and reshape. As Mercedes has knowledge of the concept of gravity, being generated by the mass of the earth, she doesn't need to "create" it, just manipulate what is already there.
On the other hand, magic is somewhat arbitrary, broken up into 4 elements, 6 elements (light and dark), or in this case 8 elements. Sometimes healing is classified as water magic. I doubt the people of this world are familiar with spatial magic, yet modern day physics associates gravity with the manipulation of space, so perhaps spatial magic will be "assigned" as a new advanced class of earth magic. When she does unlock metal magic, will she also be able to call upon the earths molten core? Essentially giving her access to fire magic?