Guild Wars 2

I really enjoy playing as an elementalist. However, I just don’t know when or how can I unlock the skill called ‘impale’, I have used skills from the other 3 elements (Fire, water, and thunder) but have never used the last one(may be called ‘earth’?). So how will that element be available to me?
Thanks!

Impale is the dagger mainhand auto attack in the earth attunement, which you unlock at level 6.

Sources:
http://wiki.guildwars2.com/wiki/Earth_Attunement