Plate Tectonics

As with continental drift theory two of the proofs of plate tectonics are based upon the geometric fit of the displaced continents and the similarity of rock ages and Paleozoic fossils in corresponding bands or zones in adjacent or corresponding geographic areas (e.g., between West Africa and the eastern coast of South America).

Ocean topography also provided evidence of plate tectonic theory. Nineteenth century surveys of the oceans indicated that rather than being flat featureless plains, as was previously thought, some ocean areas are mountainous while others plummet to great depths. Contemporary geologic thinking could not easily explain these topographic variations, or "oceanscapes." Surveys in the 1950s and 1960s provided an even more detailed picture of the ocean bottom. Long, continuous mountain chains appeared, as well as numerous ocean deeps shaped like troughs. Geoscientists later identified the mountainous features as the mid-oceanic ridges (MORs) where new plates form, and the deep ocean trenches as subduction zones where plates descend into the subsurface.

Modern understanding of the structure of Earth is derived in large part from the interpretation of seismic studies A section of the San Andreas Fault south of San Francisco is occupied by a reservoir. JLM Visuals. Reproduced by permission.

that measure the reflection of seismic waves off features in Earth's interior. Different materials transmit and reflect seismic shock waves in different ways, and of particular importance to theory of plate tectonics is the fact that liquid does not transmit a particular form of seismic wave known as an S wave. Because the mantle transmits S-waves, it was long thought to be a cooling solid mass. Geologists later discovered that radioactive decay provided a heat source with Earth's interior that made the athenosphere plasticine (semi-solid). Although solid-like with regard to transmission of seismic S-waves, the athenosphere contains very low velocity (inches per year) currents of mafic (magma-like) molten materials.

Another line of evidence in support of plate tectonics came from the long-known existence of ophiolte suites (slivers of oceanic floor with fossils) found in upper levels of mountain chains. The existence of ophiolte suites are consistent with the uplift of crust in collision zones predicted by plate tectonic theory.

As methods of dating improved, one of the most conclusive lines of evidence in support of plate tectonics derived from the dating of rock samples. Highly supportive of the theory of sea floor spreading (the creation of oceanic crust at a divergent plate boundary (e.g., Mid-Atlantic Ridge) was evidence that rock ages are similar in equidistant bands symmetrically centered on the divergent boundary. More importantly, dating studies show that the age of the rocks increases as their distance from the divergent boundary increases. Accordingly, rocks of similar ages are found at similar distances from divergent boundaries, and the rocks near the divergent boundary where crust is being created are younger than the rocks more distant from the boundary. Eventually, radioisotope studies offering improved accuracy and precision in rock dating also showed that rock specimen taken from geographically corresponding areas of South America and Africa showed a very high degree of correspondence, providing strong evidence that at one time these rock formations had once coexisted in an area subsequently separated by movement of lithospheric plates.

Similar to the age of rocks, studies of fossils found in once adjacent geological formations showed a high degree of correspondence. Identical fossils are found in bands and zones equidistant from divergent boundaries. Accordingly, the fossil record provides evidence that a particular band of crust shared a similar history as its corresponding band of crust located on the other side of the divergent boundary.

The line of evidence, however, that firmly convinced modern geologists to accept the arguments in support of plate tectonics derived from studies of the magnetic signatures or magnetic orientations of rocks found on either side of divergent boundaries. Just as similar age and fossil bands exist on either side of a divergent boundary, studies of the magnetic orientations of rocks reveal bands of similar magnetic orientation that were equidistant and on both sides of divergent boundaries. Tremendously persuasive evidence of plate tectonics is also derived from correlation of studies of the magnetic orientation of the rocks to known changes in Earth's magnetic field as predicted by electromagnetic theory. Paleomagnetic studies and discovery of polar wandering, a magnetic orientation of rocks to the historical location and polarity of the magnetic poles as opposed to the present location and polarity, provided a coherent map of continental movement that fit well with the present distribution of the continents.

Paleomagnetic studies are based upon the fact that some hot igneous rocks (formed from volcanic magma) contain varying amounts of ferromagnetic minerals (e.g., Fe₃O₄) that magnetically orient to the prevailing magnetic field of Earth at the time they cool. Geophysical and electromagnetic theory provides clear and convincing evidence of multiple polar reversals or polar flips throughout the course of Earth's history. Where rock formations are uniform—i.e., not grossly disrupted by other geological processes—the magnetic orientation of magnetite-bearing rocks can also be used to determine the approximate latitude the rocks were at when they cooled and took on their particular magnetic orientation. Rocks with a different orientation to the current orientation of the Earth's magnetic field also produce disturbances or unexpected readings (anomalies) when scientists attempt to measure the magnetic field over a particular area.

This overwhelming support for plate tectonics came in the 1960s in the wake of the demonstration of the existence of symmetrical, equidistant magnetic anomalies centered on the Mid-Atlantic Ridge. During magnetic surveys of the deep ocean basins, geologists found areas where numerous magnetic reversals occur in the ocean crust. These look like stripes, oriented roughly parallel to one another and to the MORs. When surveys were run on the other side of the MORs, they showed that the magnetic reversal patterns were remarkably similar on both sides of the MORs. After much debate, scientists concluded that new ocean crust must form at the MORs, recording the current magnetic orientation. This new ocean crust pushes older crust out of the way, away from the MOR. When a magnetic reversal occurs, new ocean crust faithfully records it as a reversed magnetic "stripe" on both sides of the MOR. Older magnetic reversals were likewise recorded; these stripes are now located farther from the MOR.

Geologists were comfortable in accepting these magnetic anomalies located on the sea floor as evidence of sea floor spreading because they were able to correlate these anomalies with equidistant radially distributed magnetic anomalies associated with outflows of lava from land-based volcanoes.

Additional evidence continued to support a growing acceptance of tectonic theory. In addition to increased energy demands requiring enhanced exploration, during the 1950s there was an extensive effort, partly for military reasons related to what was to become an increasing reliance on submarines as a nuclear deterrent force, to map the ocean floor. These studies revealed the prominent undersea ridges with undersea rift valleys that ultimately were understood to be divergent plate boundaries. An ever-growing network of seismic reporting stations, also spurred by the Cold War need to monitor atomic testing, provided substantial data that these areas of divergence were tectonically active sites highly prone to earthquakes. Maps of the global distribution of earthquakes readily identified stressed plate boundaries. Earthquake experts recognized an interesting pattern of earthquake distribution. Most major earthquakes occur in belts rather than being randomly distributed around Earth. Most volcanoes exhibit a similar pattern. This pattern later served as evidence for the location of plate margins, that is, the zones of contact between different crustal plates. Earthquakes result from friction caused by one plate moving against another.

Improved mapping also made it possible to view the retrofit of continents in terms of the fit between the true extent of the continental crust instead of the current coastlines that are much variable to influences of weather and ocean levels.

In his important 1960 publication, "History of Ocean Basins," geologist and U.S. Navy Admiral Harry Hess (1906–1969) provided the missing explanatory mechanism for plate tectonic theory by suggesting that the thermal convection currents in the athenosphere provided the driving force behind plate movements. Subsequent to Hess's book, geologists Drummond Matthews (1931–1997) and Fred Vine (1939–1988) at Cambridge University used magnetometer readings previously collected to correlate the paired bands of varying magnetism and anomalies located on either side of divergent boundaries. Vine and Matthews realized that magnetic data reveling strips of polar reversals symmetrically displaced about a divergent boundary confirmed Hess's assertions regarding seafloor spreading.

In the 1960s ocean research ships began drilling into the sediments and the solid rock below the sediment, called bedrock, in the deeper parts of the ocean. Perhaps Types of plate convergence. (a) Oceanic-continental. (b) Oceanic-oceanic. (c) Continental-continental. The Gale Group. the most striking discovery was the great age difference between the oldest continental bedrock and the oldest oceanic bedrock. Continental bedrock is over a billion years old in many areas of the continents, with a maximum age of 3.6 billion years. Nowhere is the ocean crust older than 180 million years.

Marine geologists discovered another curious relationship as well. The age of the oceanic bedrock and the sediments directly above it increase as you move from the deep ocean basins to the continental margins. That is, the ocean floor is oldest next to the continents and youngest near the center of ocean basins. In addition, ocean crust on opposing sides of MORs show the same pattern of increasing age away from the MORs.

The great age of continental rocks results from their inability to be subducted. Once formed, continental crust becomes a permanent part of Earth's surface. We also know that the increase in age of ocean crust away from ocean basins results from creation of new sea floor at the MORs, with destruction of older sea floor at ocean trenches, which are often located near continental margins.

Plate movement an today be measured by sophisticated GPS and laser-based measuring systems. A much slower but certainly more spectacular proof of plate movement is exemplified by the still-ongoing formation of the Hawaiian Islands. The Pacific plate is moving north over a stationary lava source in the mantle, known as a hot spot. Lava rises upwards from this hot spot to the surface and forms a volcano. After a few million years, that volcano becomes extinct as it moves north, away from the hot spot, and a new volcano begins to form to the south. A new volcano is forming today on the ocean floor south of the island of Hawaii.