It is a synthesis of the other three elements. As I have pointed out above, the five is the first number that allows the tracing of a polygon in more than one way. You can trace a regular pentagon or a pentagram. The pentagram, of course, is a pentagon with triangles extended from it. This alone shows the synthesis of fire and water.
Earth is structured, or cold.
Earth is the element of the material planes. You may say that things that are conscious are things material. This means, of course, that you bring the material planes with yourself when you explore the planes that are not material. It is your penetrating consciousness that makes these planes material. Everything is an infinite complexity of the elements that constitute the phenomena.