Complex sentence: The theory builds on the concepts of continental drift, developed during the first decades of the 20th century, and accepted by the majority of the geoscientific community when the concepts of seafloor spreading were developed in the late 1950s and early 1960s.
Split into simpler sentences: The theory builds on the concepts of continental drift, developed during the first decades of the 20th century. It was accepted by the geoscientific community after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.