Formation of solidification cracks in deep penetration hybrid laser - GMA welding of API 5L X70 pipeline steel plates were studied through metallographic and X-ray images. Experiments were operated with varying laser beam width, laser beam power, filler metal and welding speed and with transverse laser beam oscillation. The resulting molten pool geometry, especially bulges in the laser dominated zone, were observed to have a high influence on the occurrence of solidification cracks. To measure the severity of a bulge, the bulge widening angle was introduced as an indication of how steep the widening of the molten pool from a contraction to a bulge along its depth is. During the experiments a high correlation between the bulge widening angle and the occurrence of solidification cracks was observed.