In the Songliao basin there occurred pre-rift volcanic rocks. Then brittle extension of the upper crust gave rise to a semi-graben rift basin. Pre-rift volcanic rocks are distributed subhorizon-tally on the basement, while the sediments of the rifting stages are distributed in the semi- graben, both belonging to different tectonic layers. Pre-rift volcanic rocks may be the product of thermal thinning of the subcrustal lithosphere before the extension of the upper crust and volcan-ism reached a culmination near the rifting stage. The sediments of the rifting stage are the product of extension of the upper crust. The lithospheric extension is a process of balance between tensile strength and extensional stress. Volcanic eruption predominated when extensional stress was smaller than the tensile strength; when the extensional stress was equal to or greater than the tensile strength, the lithosphere would undergo lateral separation, resulting in extension of the upper crust and development of a rift basin and meanwhile volcanism weakened.