What was unknown to scientists during Wegener’s time?

Question

Earth’s crust is made of plates that move.
Fossils were found in many places.
Mountain ranges of some continents lined up.
The coasts of some continents fit together.

Answer 1

During Wegener's time, the primary unknown to scientists was the mechanism that explained how Earth's crust is made of plates that move. While he presented compelling evidence for continental drift, such as the alignment of mountain ranges, the presence of similar fossils across continents, and the fit of coastlines, Wegener lacked a plausible explanation for the process driving the movement of these continents. It wasn't until the development of the theory of plate tectonics in the mid-20th century that scientists understood the dynamic processes underlying plate movement, such as mantle convection and subduction.