Most of the lavas appear to originate in fissure vents, under the influence of dykes and tectonic faults. In addition, there are about 150 individual volcanic massifs and more smaller volcanic cones, many of which form rows of cones and sometimes have large craters and which occur mainly in the Al Haruj al Abyad part of Haruj. Craters range from deep pits with steep inward walls to wide and shallow depressions, and the craters are often filled with material eroded from their flanks. Phreatomagmatic processes triggered by groundwater interacting with rising magma have generated some of these large craters, while others formed when lava lakes drained through gaps in their rims. Like the fissure vents, the position of individual cones and massifs is controlled by ground fractures and often reflect the activity of dykes, and some cones appear to have been active more than once.