In the late 1950's, scientists mapped the present-day magnetic field generated by rocks on the floor of the Pacific Ocean. The volcanic rocks which make up the sea floor have magnetization because, as they cool, magnetic minerals within the rock align to the Earth's magnetic field. The intensity of the magnetic field they measured was very different from the intensity they had calculated. Thus, the scientists detected magnetic anomalies, or differences in the magnetic field from place to place. They found positive and negative magnetic anomalies. Positive magnetic anomalies are places where the magnetic field is stronger than expected. Positive magnetic anomalies are induced when the rock cools and solidifies with the Earth's north magnetic pole in the northern geographic hemisphere. The Earth's magnetic field is enhanced by the magnetic field of the rock. Negative magnetic anomalies are magnetic anomalies that are weaker than expected. Negative magnetic anomalies are induced when the rock cools and solidifies with the Earth's north magnetic pole in the southern geographic hemisphere. The resultant magnetic field is less than expected because the Earth's magnetic field is reduced by the magnetic field of the rock.
When mapped, the anomalies produce a zebra-striped pattern of parallel positive and negative bands. The pattern was centered along, and symmetrical to, the mid-ocean ridge.
A hypothesis was presented in 1963 by Fred Vine and Drummond Matthews to explain this pattern. They proposed that lava erupted at different times along the rift at the crest of the mid-ocean ridges preserved different magnetic anomalies.
For example, lava erupted in the geologic past, when the north magnetic pole was in the northern hemisphere, preserved a positive magnetic anomaly.
In contrast, lava erupted in the geologic past, when the north magnetic pole was in the southern hemisphere, preserved a negative magnetic anomaly.
Lava erupting at the present time would preserve a positive magnetic anomaly because the Earth's north magnetic pole is in the northern hemisphere.
Vine and Matthews proposed that lava erupted on the sea floor on both sides of the rift, solidified, and moved away before more lava was erupted. If the Earth's magnetic field had reversed (changed from one geographic pole to the other) between the two eruptions, the lava flows would preserve a set of parallel bands with different magnetic properties. The ability of Vine and Matthews' hypothesis to explain the observed pattern of ocean floor magnetic anomalies provided strong support for sea floor spreading.
If new oceanic lithosphere is created at mid-ocean ridges, where does it go? Geologists had the answer to this question before Vine and Matthews presented their hypothesis. In 1935, K. Wadati, a Japanese seismologist, showed that earthquakes occurred at greater depths towards the interior of the Asian continent. Earthquakes beneath the Pacific Ocean occurred at shallow depths. Earthquakes beneath Siberia and China occurred at greater depths. After World War II, H. Benioff observed the same distribution of earthquakes but could not offer a plausible explanation.
The movement of oceanic lithosphere away from mid-ocean ridges provides an explanation. Convection cells in the mantle help carry the lithosphere away from the ridge. The lithosphere arrives at the edge of a continent, where it is subducted or sinks into the asthenosphere. Thus, oceanic lithosphere is created at mid-ocean ridges and consumed at subduction zones, areas where the lithosphere sinks into the asthenosphere. Earthquakes are generated in the rigid plate as it is subducted into the mantle. The dip of the plate under the continent accounts for the distribution of the earthquakes. Magma generated along the top of the sinking slab rises to the surface to form stratovolcanoes.
The new hypotheses of the early 1960s explained several puzzling sets of observations. All that remained was a synthesis of these hypotheses.
The synthesis began in 1965 when Tuzo Wilson introduced the term plate for the broken pieces of the Earth's lithosphere. In 1967, Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, Xavier Le Pichon published a synthesis showing the location and type of plate boundaries and their direction of movement.
Since the mid-1960s, the plate tectonic model has been rigorously tested. Because the model has been successfully tested by numerous methods, it is now called the plate tectonic theory and is accepted by almost all geologists.
But where are the plates?