Why can lichens grow on rocks?

Lichens are called pioneer species, and their job is to colonize any newly available piece of land/rock/surface after it has been disturbed by some sort of disaster or it has simply emerged for the first time. For example, when glaciers melt, they leave an area of land or rock that has emerged for the first time in years. The first organisms to colonize that area will usually be lichens or bacteria. They're all floating in the air, and as they travel, they search (not literally) for new areas that are available for colonization; where no organism has ever existed before. Just in case you're wondering just exactly WHY they do that, I'll make it a point to ask God once i go to heaven. I'll keep you posted, i promise. :P