Author: The Conversation / Source: The Next Web
Underneath our feet, deep down in the Earth, liquid iron is producing the magnetic field that we all take for granted. But every now and then that magnetic field reverses or flips its polarity. What was once magnetic north becomes south – and vice versa.
When these reversals take place – and why they do so – has been an enduring mystery.But our new research shows that there is a relationship between the Earth’s magnetic field and the amount of ancient ocean floor that descends from the surface into the hot ductile mantle beneath, through a process known as subduction.
Not only might this relationship give us some idea of how many magnetic field reverses occur over any time period, it also enables us to understand how quickly the mantle (the layer of earth between the crust and the core) moves. This is important because mantle motion is ultimately responsible for producing nearly all earthquakes, volcanoes and mountain chains. Hot plumes from the mantle may also be responsible for earths major extinctions. If we can understand the workings of the mantle we can have better insight into long timescale geological phenomena that affect our species.
Graveyard for surface plates
Plate tectonics is the scientific theory that the Earth’s “lithosphere” (the cold, uppermost mantle and the crust, which are welded together) is split into seven large plates and many smaller ones. Plates are formed by volcanism at mid-ocean spreading centers such as the Mid-Atlantic Rift. Once at the surface, new lithosphere moves away from spreading centers and cools over millions of years. Over time it becomes denser and eventually the lithosphere sinks back into the hot mantle at subduction zones, such as that found just west of the Andes.
At this point, the plates disappear from the earth’s surface. But seismologists claim that colder “slabs” of lithosphere can be identified deep down in the mantle up to 300 million years after they have disappeared from the surface. The slabs of lithosphere have descended thousands of kilometers downwards, displacing vast volumes of solid mantle in the process and forming a “slab graveyard” just above the much denser liquid iron outer core. This means that the slabs of lithosphere descend as much as 2,890 km and it is there that they might influence the motion of iron liquid in…
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