Why did it take so long to discover the volcanic nature of mid-oceanic ridges?
If it weren't for WW2 and the search for German submarines, the discovery of mid-ocean ridges might have waited until satellite based gravimetric mapping in the 1980s. It was the use of fine magnetometers (if I remember correctly) used to detect submarines that discovered the banding of magnetic stripes on the Atlantic ocean floor, followed by it's mirror image after a certain mid-point in the ocean basin. When superposed with bathymetric maps, it was further discovered that these magnetic stripes had their counterpoint in the same topographic region on the other side of a high point.

The analysis of thousands of these ship transect records (released by the Navy following WW2)  led to the creation of partial ocean floor maps revealed very long parallel strips of rocks containing magnetic reversals reflected in mirror symmetry on the opposite of a mid-ocean rise. Ocean drilling then confirmed that only a spreading volcanic ridge, spewing magma over tens of millions of years could create such features. Following that, precise seismological mapping also confirmed this developing "theory" of plate tectonics, that laid to rest Alfred Wegener's model.

These were heady times for the geologic community in the 60s and early 70s! All thanks to the Navy and WW2!