Uhm you got the elements wrong. It's Earth, Water, Ice vs. Lightning, Wind, Fire.
Here's a tip: all the dark elementals you could hold in your hands.
Actually, you got it wrong. Thanks to Lux Lex for pointing out this can be found in "The Call to Adventure" at any aetheryte.
Earth grounds lightning. Lightning boils water. Water erodes Earth.
I forget the exact relationships between Fire/Ice/Wind, but that's the other triangle.
I believe the first triangle is 'Umbral' and the second, 'Astral', but I'm not certain.
As pointed out above, though, certain THM/CON spells have a general Astral/Umbral affinity. It's unclear to me whether or not casting Dia and then Fire will amp fire damage as opposed to generic Astral damage like Radiance.
EDIT: I'm sorry, we're both wrong! All 6 elements have both Astral and Umbral affinities: http://www.eorzeapedia.com/wiki/en/Astral
The triangles I listed, though, are correct. Edited, Feb 4th 2011 12:22pm by volta1