At times, crust communities become dominated by lichens, mosses or liverworts. Soil crusts are not a single biological category, but are defined by communities of various life forms. Thus, there are algal crusts, lichen crusts or moss crusts. Complex organic molecules that act like glue and stabilize the soil surface result from crust growths. These function to control or reduce erosion.
Crusts color the earth black, apple-green, blue, and tones of red.
Crusts, like earth's scabs, are vulnerable.