Montmorillonite is a group of clay minerals. The decomposition behavior and phase transition of montmorillonite are studied on STA $409$ PC using simultaneous DSC$-$TG analysis under an oxygen atmosphere with a heating rate of $10\frac{K}{m}in.$ The following graph shows the DSC and TG curves. Please analyze the entire reaction process, including the reaction type at each stage, the onset and terminal temperature ranges of the reaction, weight loss, peak temperature, etc.