New research demonstrates a time delayed relationship between the sinking of ocean floor in to the Earths mantle and magnetic pole shifts.
Sinking of cooler slabs in to the Earth can effect the motion of hot lava directly beneath our crust. Over time changes in the movement of this lava mantle can produce earthquakes, mountain chains and upwellings of hot rock that lead to volcanic activity which of course is responsible for several mass extinctions.
Because of this it is important to gain a more accurate understanding of how long it takes a cooler surface slab to sink down in to the core. Not only is it important for the anticipation of pole shifts but also for the more accurate prediction of important geological events that effect life on Earth.
The crust of the Earth is divided in to 7 large plates of solid rock as well as many smaller ones that form underwater when volcanoes erupt – spewing lava on to the ocean floor. As volcanoes continue to erupt over aeons more and more of this molten lava dries out creating plates of rock that expand over millions of years.
As they continue to expand separate plates on the surface will eventually come in to contact smashing in to each other. During this violent event one plate slides under the other and as it gradually sinks downward it can begin to effect geological activity. Since the mantle is mostly composed of molten lava that spreads out as the Earth rotates, a cooler plate entering the area will harden surrounding lava and speed rotation.
These colder slabs can still be identified even up to 300 million years after they’ve descended 2890 kilometers down in to the Earths mantle. The running theory is that eventually they will make it to the core where they can really start to effect geomagnetism. According to numerical models, this extra motion caused by colder slabs cooling down lava at the core should also speed up the rate of reversals (pole shift).
Estimates range from between 50m to 250m years for slabs to sink that far down, but scientists need to know for sure in order to predict a pole shift because the rate at which these reversals occur has varied strongly throughout history. For example they have occurred at a faster rate over the past million years than in the previous 100m.
Scientists examined records of subduction and geomagnetic polarity reversal rate.
Magnetic minerals in each layer of rock align to the direction of the magnetic field during their formation (when they hardened out from lava) so that creates a “paleomagnetic record” that scientists can use to figure out how the magnetic field has transformed over the past 500 million years.
For subduction they used a global model of plate tectonics covering the past 410 million years.
Scientists also recorded the age of zircon deposits around the world (a type of mineral that forms above a subducting slab), because their age would be indicative of how long a particular slab has been underground.
What they discovered is that there is a significant time delayed correlation between subduction and the reversal rate of pole shifts – “a “surface to core” time delay of around 120m years”. Although the correlation is not perfect and does not imply causation scientists have hypothesized that the reversal rate of pole shifts should begin to slow down since subduction has decreased over the past 120 m years
The next step for scientists will be to figure out how the mantle actually moves around. In so doing we should be able to more easily predict volcanic eruptions and the formation of mountains over time. All of which have practical implications for life on Earth.