What is plate tectonics, and how was it discovered?

Plate tectonics describes how Earth's outermost layer, the lithosphere, is about 100 kilometers thick and broken into a jigsaw of plates. These slabs carry continents and seafloor and slide over a hot, slowly swirling inner layer. They move at roughly 2 to 10 centimeters a year. Some collide, some pull apart and some grind past one another. New seafloor forms in the middle of oceans and is lost as plates sink back into the interior. This cycle produces many of Earth's geologic features and natural hazards, as described by [sciencenews.org](https://www.sciencenews.org/century/earth-history-plate-tectonics-volcanoes-earthquakes). [1]

The idea that continents move came first, as continental drift, and it was viewed with skepticism in the early twentieth century. One piece of support was the observation that the continents fit together like a jigsaw puzzle, according to the [nps.gov](https://www.nps.gov/subjects/geology/plate-tectonics-the-unifying-theory-of-geology.htm) overview. The [pubs.usgs.gov](https://pubs.usgs.gov/gip/dynamic/developing.html) account says the debate continued on and off for decades after Wegener's death, and the idea was largely dismissed as eccentric, preposterous and improbable. It was revived from the 1950s by new evidence. [2] [3]

Wartime technology helped. Sonar mapping revealed a continuous underwater mountain chain with a central crack, a rift system more than 72,000 kilometers long. Magnetometers adapted from devices built to detect submarines showed that seafloor rock is magnetized in a zebra-like pattern of alternating normal and reversed polarity bands, laid out symmetrically on either side of the mid-ocean ridges. This suggested that the bands formed at different times and that Earth's magnetic field has reversed many times. [1] [3]

Around 1961 scientists proposed that ridges are weak zones where the ocean floor is ripped apart and new magma erupts to form crust, a process later called seafloor spreading. Science News credits geologist Harry Hess and geophysicist Robert Dietz, working independently, with assembling this picture, and notes they drew on Arthur Holmes' older idea of circulating currents in hot rock. Rocks were youngest at the ridge crest and older farther away. In 1968 the drilling ship Glomar Challenger took cores across the Mid-Atlantic Ridge, and their ages provided what USGS calls the clinching evidence. [3] [1]

A global network of about 120 seismograph stations in 60 countries, installed by the end of the 1960s partly to monitor underground nuclear tests, showed that earthquakes cluster along ridges and trenches. Lynn Sykes found an earthquake pattern at ridges showing the seafloor pulling apart. A 1966 meeting of perhaps 100 scientists in New York gathered such evidence into one narrative: ridges create seafloor and trenches consume old lithosphere. J. Tuzo Wilson then combined continental drift and seafloor spreading into plate tectonics, as [divediscover.whoi.edu](https://divediscover.whoi.edu/history-of-oceanography/continental-drift-and-seafloor-spreading/) summarizes. [1] [4]

Sykes recalled that the wider community accepted the new view quickly, with remarkably little pushback. Dan McKenzie and Robert Parker, and separately Jason Morgan, then used a theorem by Leonhard Euler to calculate how the plates move, completing the theory. [education.nationalgeographic.org](https://education.nationalgeographic.org/resource/plate-tectonics/) adds that the age of oceanic crust supported Hess's hypothesis, and that fixed hot spots were proposed to explain island chains such as Hawaii. [1] [5]