  Solar active regions (ARs) are the main sources of large solar flares and
coronal mass ejections. It is found that the ARs producing large eruptions
usually show compact, highly-sheared polarity inversion lines (PILs). A
scenario named as collisional-shearing is proposed to explain the formation of
this type of PILs and the subsequent eruptions, which stresses the role of
collision and shearing induced by relative motions of different bipoles in
their emergence. However, in observations, if not considering the evolution
stage of the ARs, about one third of the ARs that produce large solar eruptions
govern a spot-spot type configuration. In this work, we studied the full
evolution of an emerging AR, which owned a spot-spot type configuration when
producing a major eruption, to explore the possible evolution gap between
collisional shearing process in flux emergence and the formation of the
spot-spot type, eruption-producing AR. It was found that the AR was formed
through three bipoles emerged sequentially. The bipoles were arranged in
parallel on the photosphere, so that the AR exhibited an overall large bipole
configuration. In the fast emergence phase of the AR, the shearing gradually
occurred due to the proper motions of the polarities, but no significant
collision occurred due to the parallel arrangement of the bipoles. Nor did the
large eruption occur. After the fast emergence, one large positive polarity
started decay. Its dispersion led to the collision to a negative polarity which
belonged to another bipole. A huge hot channel was formed through precursor
flarings around the collision region. The hot channel erupted later,
accompanied by an M7.3-class flare. The results suggest that in the spot-spot
type AR, along with the shearing induced by the proper motions of the
polarities, a decay process may lead to the collision of the polarities,
driving the subsequent eruptions.
