This work investigates the hot tearing behavior of Mg-Al and Mg-Zn alloys quantitatively based on the measurement of contraction force using an experimental setup which was developed at MagIC. An emphasis is given on the analysis of typical hot tearing curves to understand the corresponding metallurgical phenomena shown by these curves. The recorded hot tearing curves, i.e. the contraction force vs. temperature or time, contain valuable metallurgical information. They give not only the precise temperature at which the hot tearing initiates, but also depict information about the propagation of hot cracks. Combined with thermodynamic calculations, the critical solid fraction corresponding to the occurrence of hot tearing can be calculated. Further microstructural observations were performed. Several phenomena such as the crack propagation and refilling of cracks, which influence the slope of hot tearing curves, are also discussed.