'''Plate tectonics''' is a theory of [[geology]] that has been developed to explain the observed evidence for large scale motions of the Earth's crust. The theory encompassed and superseded the older theory of [[continental drift]] from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s. | '''Plate tectonics''' is a theory of [[geology]] that has been developed to explain the observed evidence for large scale motions of the Earth's crust. The theory encompassed and superseded the older theory of [[continental drift]] from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s. |