The element planes are the repositories of raw element energy and exist separately from the physical plane. They are vast, swirling cauldrons of a single element and are populated by beings made only of that element, known as elementals. These places are the embodiment of a state of matter: Earth is solid, Air is gaseous, and Water is liquid. These three planes overlap and surround the physical plane, but do not directly interact with it. The gulf between them was thought to be uncrossable except by the Element Guardians. That changed with the advent of direct conveyance technology and the Apocalypse.
These places would seem chaotic and be deadly to an unprepared human who happened to visit them as any and all possibilities for a given state of matter are constantly forming and reforming within their bounds.
The Earth plane is one continuous mass of constantly shifting forms of solid material. Utterly impossible on the material plane, there is no space between particles of one substance and another or itself.
The Air plane consists of only gaseous substances and the swirl of their intermixing. The density of the gasses can change as well, creating masses that seem solid within the vortex.
The Water plane is made up of pure liquids in an endless churn. Like in the Air plane, the density of the liquids varies along with the chemical makeup of the liquids.
How and why these changes in substance within a given state of matter on the element planes are affected is not known. However, it is theorized that the planes are a lesser form of Chaos within the order of this sphere. Just as Chaos contains within itself all energetic potentials but commits to none, the element planes contain within them the potential for all combinations of a given state of matter. The Element Guardians, acting in accord with natural laws and within the systems set in motion by the Steward, commit these potentials to a single ordered form when they enter the physical plane.