Channelers that choose Earth as their primary sphere will receive Split Earth as their prime specific weave. Split Earth is an attack weave which targets everything in the room that is not a part of your group. This includes not only mobs, but pets, mounts, and other players.
Earth prime channelers will have any of the remaining four spheres available to choose as their secondary sphere. The choice of secondary sphere strength, along with primary sphere strength, will determine the strength of weaves and stamina needed to cast.
For example, it is likely that a channeler choosing Earth/Water will be able to cast the weave Identify using less stamina than a channeler choosing Air/Fire, though not as little stamina as one choosing Spirit/Earth. Sphere strength and its effect on weave strength, duration, and stamina is complicated, and still being researched by the most experienced players.
Please check the weaves page for the sphere strength of each weave.