4. The birth of plate tectonics
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During the Cold War there was a need to know what nuclear tests the enemy was carrying out. To do this, a network of seismographs was set up around the globe. These instruments detected any vibration of the ground, including those produced by atomic explosions and natural vibrations, which could be distinguished by analysing their seismic waves.
4.1. The distribution of earthquakes and volcanoes
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All the natural vibrations recorded by the seismographs were mapped and compared to the location of volcanoes. These maps showed something surprising: earthquakes are distributed in thin lines, called seismic belts , and they are abundant in areas where volcanoes are found.
Scientist began to relate the release of magma from the Earth's interior and seismic activity. They could also see that there was a relationship between the location of seismic and volcanic activity and the position of ocean trenches and ridges. They therefore concluded that seismic and volcanic activity must be related to their formation.
Global distribution of major earthquakes and volcanoes
Video 4: Earthquakes on the Pacific 'Ring of fire'
4.2. Tectonic plates
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Solid pieces of lava floating on a lava lake.
Seismic belts showed that the lithosphere was fragmented, similar to pieces of a large jigsaw puzzle.
A tectonic plate , also known as a lithospheric plate, is a fragment of the lithosphere. Each plate is separated by a seismic belt and volcanoes.
Depending on the type of boundary between plates, there are varying levels of risk of earthquakes and volcanic eruptions.
4.2.1. Types of plate
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Lithospheric plates can be classified according to two criteria:
Size . There are eight large plates and many smaller fragments called microplates .
Type of lithosphere . There are oceanic plates , which are composed of oceanic lithosphere, continental plates , which are composed of continental lithosphere, and mixed plates , which are composed of both oceanic and continental lithosphere.
As Wegener discovered 50 years earlier, these plates are mobile. They can increase and reduce in size, fragment or join each other. The study of the dynamic nature of these plates is called plate tectonics .
Lithospheric plates map and their movement
Weblink 4: Tectonic plates
4.3. Sea-floor spreading
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Drilling surveys not only showed that the ocean floor contains many volcanoes, they also showed that the age of rocks formed from their lava flows varies depending on their position relative to the ridge.
The diagram below shows the ocean floor of the Atlantic. You can see that the rock closest to the Mid-Atlantic Ridge is the youngest and that there is no marine sediment, or only a thin layer, in this area. This is because not enough time has passed for the sediment to accumulate.
Ages of rocks in the Atlantic Ocean
Sea-floor spreading during the separation of Africa and North America
The plates at the Mid-Atlantic Ridge move apart, and when they do magma rises up from the mantle and flows out as lava through the rift. As this new lava moves up, it pushes out the older lava deposits. As the lava cools it turns into rock, forming symmetrical bands on both sides of the ridge and creating new ocean floor. This process is known as sea-floor spreading .
As the ocean floor expands by sea-floor spreading, the continents move away from each other. Science was finally able to explain how the continents could move such great distances. So at last, Wegener's theory of continental drift was accepted by the scientific community.
4.4. The subduction of the ocean floor
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The ocean floor continually expands from the ridges but the surface of the Earth does not expand, and no very old rocks were found on the ocean floor, so it was thought that there must be something destroying the crust.
The areas where it was suspected that this could happen were at the trenches. In the trenches, the oldest part of the ocean floor seemed to bend and sink into the mantle. So while oceanic lithosphere is constantly being formed at the ridges, it is also continually being destroyed at the trenches, by a process called subduction.
Subduction is the process in which the oceanic lithosphere moves under the continental lithosphere (or oceanic lithosphere in the case of island arcs) and into the mantel.
Geologist Harry Hess compared the ocean floor to a conveyor belt. It rises up from the ridges, moves outward from the ridge and sinks down into the trenches as it reaches the continental lithosphere.
Video 5: Sea-floor spreading & plate tectonic evidence
Key concepts
Tectonic plates are fragments of the lithosphere, separated by seismic belts and volcanoes.
The ocean floor is continually generated at a very slow rate at ocean ridges, by a process known a sea-floor spreading.
As the process of sea-floor spreading generates new oceanic lithosphere, it pushes the oldest lithosphere under the continental lithosphere, or oceanic lithosphere in the case of island arcs, by a process known as subduction.
Sea-floor spreading occurs at ridges and subduction occurs at trenches.