Plate tectonics and the mantle
You may feel that the grounds you play football on every evening is unchanging as ever. However, nothing can be further from the truth. The earth’s crust too undergoes a cycle of birth and death that have changed the way our planet looks over millions of years. In 2011, scientists found evidence that this cycle was quicker than thought: 500 million years instead of 2 billion.
The tectonic plates that constitute the crust are pushing, pulling or brushing past one another, and are in constant motion. For plate tectonics to be functional, the lithospheric material that moves down the subduction zones needs to be recycled through the mantle – to ultimately emerge as new lithosphere created along the spreading margins of the mid-ocean ridges.
The deepest boundary has been measured at about 2900 km below earth where it sharply divides the mantle from the core. The iron-rich liquid below this core-mantle boundary is much denser than the solid rocks of the mantle, preventing any significant exchange of material between the two layers. We can thus imagine a system of whole-mantle convection in which the material from the plates circulates all the way through the mantle, down as far as the core-mantle boundary (Figure 1a).
Figure 1: Stratified Mantle Convective System
Source: Grotzinger J., Jordon T. H., 2014, Understanding Earth
When plate tectonics theory was studied initially, scientists were of the view that plate recycling took place at shallow depths in the mantle. They sought evidence from deep earthquakes that were believed to mark the entering of lithospheric slabs into subduction zones.
The depth of these earthquakes was varied depending upon the different subduction zones. But, geologists could not find any evidence of earthquakes below 700 km. Moreover, when these earthquakes at great depths were studied it showed that the slabs were touching something more rigid that somehow blocked their downward progress.
Thus, scientists hypothesized that the mantle may be divided into two layers – an upper mantle in the outer 700 km, where the recycling of lithosphere was taking place, and a lower mantle zone, from 700 km deep to the core-mantle boundary. A theory for a stratified convection system was put forward with lighter rocks in the upper layer and denser ones in the lower mantle – in the same way as the mantle floats on the core (Figure 1b).
Now, to test these two competing hypotheses and to find lithospheric graveyards below old subducted plates earthquake waves were used. From past plate motions, geologists estimated that, since the breakup of Pangaea, lithosphere equivalent to the surface area of Earth has been recycled back into the mantle.
Scientists found regions of colder material in the deep mantle under North and South America, eastern Asia, and other sites adjacent to plate collision boundaries. This evidence leads most scientists to conclude that plate recycling takes place through the entire mantle rather than through stratified convection.
Nearly all of the world’s oceanic islands are volcanoes. Many of them, for example the Hawaiian Islands, originate from mantle plumes that emerge from the lowest part of the mantle. As it comes near the surface, the pressure is considerably reduced forming volcanoes.
With a special laser, scientists at the Max Planck Institute for Chemistry in Mainz, Germany, investigated samples of Hawaiian lava, which suggested that the rock was less than 500 million years old, indicating that recycling of material from the entire mantle probably took less time than previously understood.