What is the best indication the planet may have plate tectonics?

Answer 1

Alfred Wegener proposed in 1912 that the continents were slowly drifting apart over time. He called this "continental drift" and based his theory on the evidence that the continents fit together like jigsaw puzzle pieces, and where the joined there was evidence that they had once actually been one large continent (called Pangaea). The evidence he found included fossils records of plants and animals that lived in specific regions, and also mountain formations that spanned between continents and stopped at the edge of one only to start again at the matching location of the other one.

Unfortunate for Wegener, he could not explain in his lifetime why, or what could cause the continents to drift on such a scale.

Starting in the 1940's during World War 2 (WWII) the US government started doing research of the seafloor to locate places where German and Japanese submarines could hide off the coasts of America and wait in ambush on US ships. What they discovered was that the seafloor was not a flat landscape like they predicted, but had deep rifts and mountain ranges. What they discovered that relates to this question is that there are ridges (called Mid-Oceanic Ridges or MORs) that are continually erupting from volcanic activity and growing outwards from that ridge or line under the sea, this is known as "seafloor spreading". This continued buildup of magma explained the force that could cause the continental plates to move. As the MORs expanded outward they pushed the continental plates in an outward direction.

Between these two understandings you get "plate tectonics."