The density of vegetation on the flows depends on the availability of soil. Where basalt rock is very young, the only soil available is what blows into cracks and fractures. As soil develops within these cracks, vegetation can take hold. The depth of crevices, cracks, and depressions fix the amount of soil and moisture that can be held. The size of the crack also determines the degree of protection from the harsh climate conditions, such as air temperature and exposure to wind. These factors in turn determine what plants are capable of living in the different size cracks.