This Volatile Earth farming guide will show you the places I use for farming 
Volatile Earth. There were no major changes to Volatile drop rates in Cataclysm zones, so this guide can be used for Classic and Retail WoW.
Volatile Earth
Zul'Gurub - dungeon
This farming place is for Retail WoW players only. If you play Classic, scroll down a bit for the other farming places. But, for retail players, this is the best farming place. They can ignore the rest of the guide. (assuming you have a high-level character)
Zul'Gurub is a heroic dungeon, so you will need a high level character to solo it. Scroll down to the bottom of the page to find alternative farming methods if you don't have a high level character.
The entrance to this dungeon can be found in Northern Stranglethorn at the coordinates 72, 33.
Northern Stranglethorn
First, you have to set the instance difficulty to Heroic (right click your portrait). This dungeon doesn't have a Normal mode, so you can't enter until you set the difficulty to Heroic.
Tor-Tun drops 5-8 x Volatile Earth. Florawing Hive Queen drops 
Hive Queen's Honeycomb which contains some Volatile. (Hive Queen's Honeycomb is daily, so you only have to kill her once)
Don't kill any other bosses or you can't reset the instance!
After you killed one of these bosses, or both of them, you can go outside and reset the instance by right clicking over your own portrait then click the "Reset all instance" button. This restarts the whole instance and all mobs will be back. You can only do 10 instance runs in an hour, so you will have to wait a bit since one runs usually won't take longer than 2 minutes and you will hit the instance cap in like 20 minutes.
Twilight Highlands
You can farm Volatile Earth in Twilight Highlands by killing Obsidian Stoneslave. Do not forget that if you are a miner, you can mine the dead mobs and you will get more Volatile Earth.
Mount Hyjal
There is another good place in Mount Hyjal. Kill Scalding Rock Elemental. These mobs are easy to kill and the respawn rate is pretty fast.
Contact me if you have other great farming spots!