Editing Plate tectonics (section)

=== Floating continents, paleomagnetism, and seismicity zones ===
[[File:Quake epicenters 1963-98.png|thumb|upright=1.35|Global earthquake [[epicenter]]s, 1963–1998. Most earthquakes occur in narrow belts that correspond to the locations of lithospheric plate boundaries.]]
[[File:Map of earthquakes in 2016.svg|thumb|upright=1.35|right|Map of earthquakes in 2016]]

As it was observed early that although [[granite]] existed on continents, seafloor seemed to be composed of denser [[basalt]], the prevailing concept during the first half of the twentieth century was that there were two types of crust, named "sial" (continental type crust) and "sima" (oceanic type crust). Furthermore, it was supposed that a static shell of strata was present under the continents. It therefore looked apparent that a layer of basalt (sial) underlies the continental rocks.

However, based on abnormalities in [[plumb line deflection]] by the [[Andes]] in Peru, [[Pierre Bouguer]] had deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had "roots" was confirmed by [[George B. Airy]] a hundred years later, during study of [[Himalaya]]n gravitation, and seismic studies detected corresponding density variations. Therefore, by the mid-1950s, the question remained unresolved as to whether mountain roots were clenched in surrounding basalt or were floating on it like an iceberg.

During the 20th century, improvements in and greater use of seismic instruments such as [[seismograph]]s enabled scientists to learn that earthquakes tend to be concentrated in specific 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 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){{sfn|Stein|Wysession|2009|p=26}} 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.

Meanwhile, debates developed around the phenomenon of polar wander. Since the early debates of continental drift, scientists had discussed and used evidence that polar drift had occurred because continents seemed to have moved through different climatic zones during the past. Furthermore, paleomagnetic data had shown that the magnetic pole had also shifted during time. Reasoning in an opposite way, the continents might have shifted and rotated, while the pole remained relatively fixed. The first time the evidence of magnetic polar wander was used to support the movements of continents was in a paper by [[Keith Runcorn]] in 1956,{{sfn|Runcorn|1956}} and successive papers by him and his students [[Ted Irving]] (who was actually the first to be convinced of the fact that paleomagnetism supported continental drift) and Ken Creer.

This was immediately followed by a symposium on continental drift in [[Tasmania]] in March 1956 organised by [[Samuel Warren Carey|S. Warren Carey]] who had been one of the supporters and promotors of Continental Drift since the thirties<ref>{{Harvnb|Carey|1958}}; see also {{Harvnb|Quilty|Banks|2003}}.</ref> During this symposium, some of the participants used the evidence in the theory of an [[expanding Earth|expansion of the global crust]], a theory which had been proposed by other workers decades earlier. In this hypothesis, the shifting of the continents is explained by a large increase in the size of Earth since its formation. However, although the theory still has supporters in science, this is generally regarded as unsatisfactory because there is no convincing mechanism to produce a significant expansion of Earth. Other work during the following years would soon show that the evidence was equally in support of continental drift on a globe with a stable radius.

During the 1930s up to the late 1950s, works by [[Felix Andries Vening Meinesz|Vening-Meinesz]], Holmes, [[Johannes Herman Frederik Umbgrove|Umbgrove]], and numerous others outlined concepts that were close or nearly identical to modern plate tectonics theory. In particular, the English geologist [[Arthur Holmes]] proposed in 1920 that plate junctions might lie beneath the [[sea]], and in 1928 that convection currents within the mantle might be the driving force.<ref>{{Harvnb|Holmes|1928}}; see also {{Harvnb|Holmes|1978}}, {{Harvnb|Frankel|1978}}.</ref> Often, these contributions are forgotten because:
* At the time, continental drift was not accepted.
* Some of these ideas were discussed in the context of abandoned fixist ideas of a deforming globe without continental drift or an expanding Earth.
* They were published during an episode of extreme political and economic instability that hampered scientific communication.
* Many were published by European scientists and at first not mentioned or given little credit in the papers on sea floor spreading published by the American researchers in the 1960s.