The paraelemental planes are the result of the cardinal Elemental+Planes blending together.
Between the plane+of+Fire and plane+of+Air is the plane+of+Smoke, between the plane+of+Air and the plane+of+Water is the plane+of+Ice, between the plane+of+Water and the plane+of+Earth is the plane+of+Ooze, and between the plane+of+Earth and the plane+of+Fire is the plane+of+Magma.
Some claim the paraelemental planes were not always as they are today. Eons ago, they were Dust, Steam, Magma, and Ice, but they have shifted over time, Dust and Steam merging with the Quasielemental Planes and Water and Fire, Earth and Air becoming the new opposition. This is just a theory, mind you.
Additional Resources:
3.5 conversion of the Paraelemental Planes
Sources:
The Inner Planes
Planescape Monstrous Compendium Appendix III
Manual of the Planes (D&D 3.0)
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