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1 Seafloor spreading is one of the two major processes of plate tectonics, the other being subduction. 2  Seafloor spreading is the continuous process of forming new igneous rock at midocean ridges by injection of magma that forms new seafloor. Seafloor spreading is one of the two major processes of plate tectonics, the other being subduction. 2  Seafloor spreading is the continuous process of forming new igneous rock at midocean ridges by injection of magma that forms new seafloor.
Plate tectonics is the theory that Earthâs lithosphere is divided into tectonic plates that move around on top of the asthenosphere. In this section, you will learn what causes tectonic plates to move. Tectonic Plates Pieces of the lithosphere that move around on top of the asthenosphere are called tectonic plates. Earthâs major tectonic plates are shown on the map in Figure 1. Note that tectonic plates differ in size. Some tectonic plates contain both continental and oceanic crust.