Why did it take so long to discover the volcanic nature of mid-oceanic ridges?

Question

I just read this article and was astonished to see the following:

Oceanographers stumbled on [the mid-oceanic ridges'] volcanic nature in 1973.

What I find surprising is how recent that is.

Considering that

• the history of geology in general goes back at least 3 or 4 centuries
• the study of plate tectonics goes back to 1912 (even though it took a few more decades to gain wide acceptance)
• oceanic ridges were known at least as early as 1872
• the correlation between seismic activity and volcanoes has been known for some time (I don't know exactly how long, but certainly the Romans felt the ground shaking when Vesuvius' erupted)

Then my question is, with regards to the (seemingly unusually) recent date quoted in the above article:

Why did the discovery of volcanic activity at sub-oceanic plate boundaries take so long?

Answer 1

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!

Answer 2

I think the U.S. Geological Survey has the gist of your answer -- prior to the mid-1900s, there wasn't enough seismological data to draw any conclusions:

During the 20th century, improvements in seismic instrumentation and greater use of earthquake-recording instruments (seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40-60° from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration worldwide.

This evidence led to geophysicist Harry H. Hess hypothesizing seafloor spreading in 1960. Researchers from Scripps Institution of Oceanography and Woods Hole Oceanographic Institute, among others, would spend years gathering evidence from the world's seafloors to test that theory, using oceanographic vessels, equipment, and analytical techniques that were not available in previous decades.