Alfred Wegener advocated continental drift theory. He found similarities in the coastlines of South America and Africa. He found three pieces of evidence.
All continents at present were split from only one continent: Pangea. This time, he could not show the proper reason of continental drift. Therefore, continental drift theory was denied.
Oceanic ridges and trenches were discovered, and gradually the age of the nature of the rock retreated from oceanic ridge. Because of these reasons, the Seafloor Spreading Theory was advocated by Harry Hammond Hess. It didn’t have enough evidence of Seafloor Spreading. Geomagnetic reversal happening every few million years is recorded symmetrically on both sides of oceanic ridges. It became evidence of creating new plate, Seafloor Spreading, and continental drift.
The US made World-Wide Standard Seismograph Network (WWSSN) to track the underground nuclear experiment during World War Ⅱ. Distribution of the earthquake drawn as a line was found by WWSSN.
The Earth’s surface is covered by more than a dozen plates, and then diastrophism is caused by relative motion of plates. This is Plate Tectonics. We can explain continental drift, seafloor spreading, and distribution of earthquakes because of Plate Tectonics. A plate boundary is decided by line of distribution of earthquakes.