The four 'elements' are actually the four states of matter. Earth represents solids. Water represents liquids. Air represents gases. Fire represents energy. The elementals are beings composed of matter in one of these four states.
SOLIDS: The plane of solids in an endless cave system, with many different caverns giving space to the multiple solids.
Stone/earth- the most common solid, these elementals watch over stone and rock. They dabble in dirt as well, but are more focused on rock.
Ice- ice falls under the domain of these elementals. Snow is more the territory of the particulates, but can be handled by ice elementals as well. They exude a constant aura of cold, freezing anything nearby.
Metal- representing the various metals, these elementals are linked to ore.
Radioactive- usually found with large pieces of radioactive material embedded in them, these are one of the most dangerous elementals. They have the power to detonate in a nuclear explosion, without dying. They must recharge after detonation, but are still alive. They do take precautions around fleshier beings however, covering themselves in materials such as lead when dealing with species vulnerable to radiation.
Crystal- similar to the metal elementals, these guard crystalline solids. This includes precious gems, and other regular structures.
Particulate- These elementals cover particulate solids such as sand and sugar. They are often found working with gaseous elementals, due to the nature of their portfolio.
LIQUIDS: an unending sea, with no top or bottom. The majority of the plane is water, but other liquids are seen in bubbles.
Water- The most common liquid, it makes up the majority of the plane. Due to it's prevalence, it is easy to mistake their home for a plane of water
Lava- easily mistaken for fire elementals, lava elementals are part of the Liquid state.
Acid- Capable of eating away at other forms of mater, acids are one of the most feared soldiers in the liquid world.
Blood- one of the scarier elementals, these elementals monitor the fluids that flow through living creatures. This means they can also control these fluids, allowing them to kill or even control creatures with their own blood.
Solutions- This group is vast, and covers the many combinations of chemicals that result in liquids. The solid and gaseous states also have a sect dedicated to compounds but they are easiest to find in the liquid plane.
GASES: infinite sky, with no floor. Clouds of various gases float by, some dense enough to walk on.
Air- representing breathable atmosphere, these elemental rule the skies.
Poisonous gas- Various gases are toxic, and so these elementals keep them in line.
Flammable gas- Similar to poisonous, but volatile instead of toxic.
Cloud- water is one of the substances that goes through all three states of matter, and so each state has elementals to deal with it. Vapor or Cloud elementals represent water vapor. They are responsible for most walk-ably dense clouds in the plane as well.
Cosmic- These control the celestial clouds and nebulae found in space.
Plasma- it is argued whether these should be classified as gaseous or energy, but they live on the gas plane. This is due to the nature of their charge. When gas super heats (such as in a star), it becomes plasma. It is very hot gas, but still gas. However, the super heating and differing physical properties have made some question whether they are still gaseous, or whether they should be reclassified.
Fire- The fire elementals handle thermal energy. They are one of the most common forms of energy.
Lightning- Handling electric energy, these elementals are made of pure lightning.
Electromagnetic- This category can be subdivided and transferred between with ease. They control light and magnetism, as well as the rest of the spectrum.
Kinetic- their form is constantly moving, giving them a blurred appearance. They can give or take speed at will.
Sound- These elementals command sound waves, amplifying or muting them as they wish. They can unleash sonic booms against their foes, making them powerful allies. They can also generate music, inspiring and empowering their friends like D&D bards.