From the publisher's website:
Of the classic elements—air, earth, fire, and water—earth is often the hardest to work into the magic of a fantasy setting. The core roles of other elements are easy to see—fire burns your foes and grants light, air allows for flight and control of the weather, and water allows ships to move and wells to bring life to farms, plus is the secret domain of sailors and merfolk. The easy uses for these elements in magic are often quickly covered, leading to the exploration of alternative uses. This results in broader magic options that only loosely relate to the most basic functions of air, fire, and water. Scrying pools, holy flames, and killing vapors are time-honored examples of drawing on those three elements without having much to do with real water, fire, or air. This kind of development is a good thing, giving characters with strong elemental themes a wide range of powers to choose from.
Because it is generally less dynamic in its natural state, earth seems to have been given short shrift when magic powers were being developed. While the basic ideas of creating walls of metal and rock or borrowing the durability of stone for personal protection are common, few campaigns have the expanded selection needed to make a rock-themed spellcaster a viable and interesting option. The Pathfinder Roleplaying Game expands the range of earth magic some by adding a connection to acid (see “Acidic Earth”, below), but this still leaves earth with significantly fewer options than the other elements. The Genius Guide to Earth Magic looks to develop earth and stone magic, both by expanding the applications for “typical” acidic earth magic and by adding a new [stone] descriptor for spells that deal damage using rocks as weapons.