what discovery gave scientist the evidence for the theory of plate tectonics
Mid ocean ridges and volcanoes. Before that, the similar theory of continental drift was suggested by mountain formation and earthquakes. When mid-ocean ridges were discovered, it was realized how continents moved, they were "floating" on top of the magma underneath the crust, and their movement was influenced by the currents in the magma. Some plates move under other plates, and they move apart.
Alfred Wegener discovered Pangea and the theory that over time they moved apart using tectonic plates. They had plates holding each part of the earth so as they move it pulls the land above apart. I hope this helps.