Very good idea indeed. I was always thinking that the "burrowed" status is currently underused in contrast to "frozen" and "delayed". And the Luciferin-like effect fits the theme very well.
Two additional thoughts, however:
any creature without skill gains earth
The enemy's creatures without skill as well, as I take it? Maybe the upped variant could change that behaviour then instead of just being much cheaper (similar to Pandemonium).
Sand dune: 
burrow the targeted creature
I see some problems with Growth and Ablaze creatures: Does burrowing a creature change its active skill to 0
: unburrow? Then your card would be overpowered CC against Golems and Spirits. Or do they keep their active skill? Then I'd summon a Lava Golem, immediately Sand Dune it and watch it grow while being near-immortal. Or is the ATK value calculated as if the creature was unburrowed every time but halved afterwards, making a Golem gain effectively +1|+2 instead of +2|+2 per skill use? This could balance it a little bit but would require some major changes in the game coding, I suspect (see also here (
http://elementscommunity.org/forum/index.php/topic,368.html) and here (
http://elementscommunity.org/forum/index.php/topic,3069.html)).
EDIT: Just saw that this was no regularly submitted card idea. Are forum users expected to reply as usual to a competition submission? If not, please kindly delete my post.
