I have been set the task of researching plate tectonics, specifically: "Why was its discovery so important in its time and controversial?" I do not need a long answer, although more detail is always appreciated.
One of the key aspects of plate tectonics is continental drift.
The person who came up up with the theory of continental drift was Alfred Wegener. He published his theory in 1912.
One of the issues with the theory that geologists at the time had, was that Wegener was not a geologist, but a meteorologist. He was publishing a theory that wasn't associated with his field of science. The other issue the geologists had was based on the commonly held opinion:
that the oceanic crust was too firm for the continents to "simply plough through".
When he published his theory, Wegener did not propose a means by which the different land masses could break away from each other.
Initially, some geologists could only conceive the idea that ocean waves might be responsible for breaking up land masses, but they couldn't reconcile the fact that the lack of sediments and the clean breaks in land masses would not support Wegener's theory. Without knowing about plate tectonics the theory of continental drift was difficult to support.
Another reason why Wegener's idea was not initially accepted was because of the way he proposed that continents used to fit together. This was because of the assumption most people had was that the continents split along the lines of coast lines and not the 200 m isobath proposed by Wegener.
Wegener came up with the idea of continental drift by noticing that all the major land masses appeared to fit together like a jigsaw puzzle.
It wasn't until the early 1950s when data from paleomagnetic studies of India showed that India had once been in the southern hemisphere that data started to support Wegener's theory. Also, it wasn't until the 1960s that sea floor spreading data was available to support the theory.
Why was the initial theory of plate tectonics so controversial?
Plate tectonics was anything but controversial. A mere four or five years expired between its original proposal in 1963 by J.T. Wilson and its almost universal acceptance in 1967 or 1968.
What was controversial was Wegener's theory of continental drift. His theory was not plate tectonics. Plate tectonics had answers to almost all of the questions that Wegener's theory did not and could not address.
Wegener's theory was based partially on the shapes of the eastern coasts of the Americas versus the shapes of the western coasts of Europe and Africa. Wegener was hardly the first to notice this similarity. What Wegener added was fossil evidence; there appeared to be something much deeper than a mere random similarity of coastlines. What Wegener didn't add was a mechanism.
What he did add was exactly what geologists had been fighting for about a hundred years, which was religion. Nineteenth century geology was a battle between those who believed in the Bible (literally) and those who looked at the geological evidence. Early nineteenth century geologists explained the diversity of the Earth's geology via catastrophism, with Noah's flood playing a predominant role. Later geologists looked at the evidence and saw no signs of catastrophes. They instead adopted a theory of uniformitarianism.
One big problem was the 1920s Scopes trial in the US. Scientists in the US were hard pressed at the time. One counter reaction amongst the American scientific community to the Scope trial was a rejection of anything and everything that remotely hinted of religious catastrophism. This included Wegener's continental drift.
Plate tectonics is not Wegener's continental drift. Unlike Wegener's continental drift, plate tectonics has a mechanism. Plate tectonics is founded on a lot more than a mere similarity of continental outlines. A mountain of evidence had accumulated in the time that intervened between Wegoner's unfounded claims and J. Tuzo Wilson's 1963 article that proposed plate tectonics.
The evidence that turned plate tectonics into the accepted science in five short years included
- The recognition of a single, globe-spanning mid-ocean ridge.
Scientists had discovered bits and pieces of this ridge long before, but recognizing that it was continuous and globe-spanning didn't happen until after World War II. (Wars are good for developing technology).
- The recognition of very deep oceanic trenches.
These are the flip side of the mid-ocean ridges, and these (along with the mid-ocean ridge) were a key to understanding seafloor spreading. Technologies developed during World War II were once again the key.
- The recognition of magnetic field reversals.
Yet again, technologies developed during World War II were key. This showed the oceanic crust originated from the mid-ocean ridges, and died in the deep trenches.
- The recognition of the Ring of Fire.
Here it was post-WWII technologies that came into play. While seismographs existed during Wegener's time, they were rather crude devices compared to those needed to determine whether ones enemies (or for that matter, whether ones supposed allies) were conducting illicit atomic tests.
Plate tectonic theory integrated data from many sub-disciplines of geoscience including the new area of paleomagnatism as noted above, but also seismology, structural geology, bathymetry, stratigraphy, and others. I have a book on earthquakes from the early 1960's that clearly shows the earthquakes along subduction zones but are presented as a curious observation, since the concept of subduction hadn't been developed. Plate tectonics upset some previously widely held models like geosynclinal structures or at least required that they be modified greatly. Much of the observation and testing of the theory happened over time so initially there were inherently questionable aspects. That's science.
One could speculate on controversy due to people ferreting around in their own area of expertise and developing explanations that are at odds with data from other areas but much is due to the development of extensive data sets to show the big picture. But certainly people have a tendency to be personally vested in their own theories and that, too, is needed to demonstrate the robustness of new ideas.
I'm not sure the discovery is so important 'in its time' - it made its own importance and would have been so whenever it was developed. It did have importance in providing models for resource exploration to meet the rapidly increasing demands of the time. It also provided tangible benefit from the large government and private investment in science and technology of the period.
I'm a bit late noticing this question, but if the OP is interested, here are a few additional thoughts which I hope may compliment the excellent answers already produced.
The distinction has already been made, above, between Wegener's 1920s Continental Drift (he actually called it Continental Displacement, the more whimsical term was created later by others to disparage Wegener's idea) and Plate Tectonics which was developed 50 years ago, during the mid-1960s. It took several years for the plate tectonics model to be accepted, but as far as 'revolutionary' scientific theories are concerned, it actually caught on relatively quickly. Marie Tharp discovered mid-ocean rifts (1956); Harry Hess explained how they work and invoked the idea of subduction zones (1961); Morley, Vine, and Matthews proved seafloor spreading by using paleomagnetism (1963); Tuzo Wilson proposed hot spots and transform faults (1963); Isacks, Oliver, and Sykes mapped a subduction zone with seismic (1967); Jason Morgan divided the planet into plates (1968). Of course many others were also involved, but by 1968 only a few stubborn holdouts opposed plate tectonics theory.
To the point of the original question: It took massive paleomagnetic, seismic, and thermal readings to convince most geologists that the crust moves laterally. It took a while before technology reached a level to prove tectonics. Further – and this is important – scientific theories need to be testable and measurable. With advanced technology, it became possible to predict plate movement and with accurate measurement (especially GPS), it finally became indisputable that the crust is moving in the ways the theory predicted.
Fine answers above. I think a historian of science would propose the "psychology of scientists" and the "sociology of scientific institutions" had a greater contribution to the controversy than any question about the veracity of the observations. You may be interested in viewing this interview with the science historian William Glen about PARADIGM INERTIA. His book about the history of plate tectonics (The Road to Jaramillo) is based on direct interviews.