The theory of plate tectonics has been in the mainstream of science thinking for the past 50 years. At such a milestone it is appropriate to see how far things have come and indeed what is still left to discover.
The theory of plate tectonics was born out of an understanding of our oceans. A map showing the age of oceanic lithosphere highlights the discontinuity between land and sea — the oldest rocks on our continents are billions of years old, yet the oldest part of our ocean is less than 200 million years old (Fig 1). The birth of the oceans at ridges in the mid-Atlantic and the Pacific means that there is a constant death of oceans at subduction plate boundaries (such as in western South America). But where do these oceans go?
As the mantle is shielded from the surface by the crust, it is almost inaccessible. Despite this, we are able to image inside the Earth in a similar way to an ultrasound. During an ultrasound, energy is transmitted and then the reflections of that energy are recorded. For the Earth, there are natural energy makers in the form of earthquakes. By placing devices that can monitor these Earth motions, called seismometers, we can capture the energy of the seismic waves given off by the earthquake. By interpreting the energy at the surface related to many earthquakes at the various monitors around the world, a ‘ultrasound’ of the Earth’s interior can be generated. The scan of the Earth reveals a more complicated setup than the basic layering of crust and homogenous mantle — with old oceans being destroyed at the surface but living a subterranean life, sinking to the bottom of the mantle.
Aside from the colder regions in the mantle thought to be old oceans, the data from the seismometers also reveal hotter structures. Since the early 2000s, the geophysics community has been postulating findings that have the potential to change the fundamental thinking of our planet. The discovery of ‘blobs’ of material at the bottom of the mantle are thought to be distinctive from their surroundings, forming an unusual shape. These blobs are two separate regions below the Pacific and under Africa (Fig 2) and are, in places, 100 times larger than Mt Everest. In seismological terms, they are described as Large Low Shear Velocity Provinces (LLSVPs).
A strong debate rages on pretty much every aspect of the character of these LLSVPs. One thing that seems to be clear is that some super volcanoes from Earth’s past appear to form from these blob regions. An example of which was the Central Atlantic Magmatic Province — a large igneous province that started 201 million years ago and lasted for 600,000 years, producing 4 separate eruption sites. The lava flow spanned large portions of Northern Africa and South America, with the super volcano begin attributed to a ~40% extinction of all species. The fundamental understanding of how and why these super volcanoes occur is clearly important — and these deep blobs may play a role in their genesis.
Mantle convection drives the tectonics plates at the surface to produce oceanic ridges and subduction. However, the interaction of these surface processes with the deep mantle blobs is not clear. There may be a dynamic system between the oceanic subduction shaping the blobs to produce huge super volcanoes, which may in turn influence plate tectonic motion to produces ridges and subduction — this feedback system may be integral to our understanding of plate tectonics (Fig 3). The future of plate tectonics may lie within these deep mantle structures, and whether they are fundamentally important in the way our planet evolves.