The Earth obelisk is used to cast the Charge Earth Orb spell. The spell is cast on the obelisk in order to charge an unpowered orb into an earth orb. The Obelisk is located in the Wilderness area of Edgeville Dungeon, south of the Black demons in the area where Chronozon is found.
When using this obelisk it is recommended to have a method of curing poison in your bank, as the poison spiders can poison you even if you have Protect from Melee prayer on. If you are a low level and afraid the poison may kill you, you can bring a method of curing poison with you, at the price of having fewer orbs in each inventory. However, an antipoison potion that grants immunity can be sipped at the bank right before you head to the obelisk as well.