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. Our planet is yet to reveal all it’s secrets… especially regarding the Earth deep below the surface. Often the understanding about the Earth is shown in Fig 1, with the interior split into layers: the outer crust where we stand, the mantle below it extends down to the outer core (made of liquid iron) and then the inner core (made of solid iron).
This is all true, but in reality the Earth is a far more complicated canvass than as described in textbooks – this is particularly the case for the mantle. Plate tectonic activity occurs at the surface – however, the influence of these processes do not stop there. The impact of plate tectonics continues deep beneath our feet, interacting with complex layering and structures that exist below the surface.
What is plate tectonics? Well, put simply, it is the movement of the Earth’s surface which is divided into rigid sections – called plates (Fig 2). The movement of these plates is driven by convection currents beneath them – the pattern of convection being such that the plates can move in different directions. Radioactive heating inside the Earth helps to move the convection currents, as well as heat coming from a cooling core into the Earth’s mantle.
An analogy to the convection that is happening in the mantle is of a pan of water on a stove. As the water at the bottom warms, it becomes less dense and rises to the surface, where it cools and falls back to the bottom of the pan again. This convection current is happening below the tectonic plates and is helping move the surface – just like a piece of wood floating on the top of a pan of boiling water would move (Fig 3).
It is appropriate here to define what we mean by a tectonic plate, and by doing so we’ll start to change this simple setup of the Earth as shown in Fig 1. A tectonic plate can be defined as the crust, on which we stand, and the uppermost part of the mantle called the mantle lithosphere (Fig 4). Collectively, these two layers are called the lithosphere and make up the tectonic plate. Below the mantle lithosphere is another part of the mantle called the asthenosphere. Confusingly, these two layers are made out of the same mantle material, but they behave different from one another as you go deeper in the planet. The temperature increase at depth changes the mechanics of the mantle, which creates a ductile asthenosphere in which a stronger mantle lithosphere slides over (despite them having the same olivine composition).
This lithosphere layer can be ~200 km thick in places, with the largest component being the mantle lithosphere, yet not as much is known about the mantle lithosphere as compared to the crust. The crust is a different composition to the mantle, yet it forms part of the tectonic plate as a layer within a layer. There are two types of crust – continental and oceanic – which have different compositions. The continental crust is not as dense as the oceanic crust, which means it can stay on the surface for a lot longer. Oceanic crust disappears beneath continental crust in a place tectonic process called subduction (Fig 4).
A theory from the sea
The slow process of the acceptance of plate tectonics came from a better understanding of the life cycle of an ocean (also known as the Wilson Cycle, named after the Canadian Earth Scientist J. Tuzo Wilson), which also highlights different plate tectonic processes (Fig 5).
Before the birth of an ocean there would be a stable craton (i.e. an old continent), for example North American today. Then, for an ocean to be born, the convective processes beneath the plate would allow for the continent to split, and an ocean start to spread and move the continental pieces away from each other. An example of this occurring today is in the mid-Atlantic, where new ocean is being born as fast as your finger nails grow. Over the course of 180 million years, that slow fingernail growth has split North America and Europe in the North Atlantic and South America and Africa in the South Atlantic away from each other.
Eventually though, the ocean will start to close, and the ocean will disappear beneath a continent in a process called subduction. The Pacific ocean has a lot of oceanic subduction around it’s edges – in South America the oceans are being subducted under the continent, causing earthquakes and volcanoes.
Finally, the life of an ocean comes to pass, and all of the remaining ocean disappears beneath the surface, allowing for one final collision between two continents. This tectonic event is a dramatic one, with an example of which being India colliding into Asia – which generated the incredible Himalayan mountain belt, and the world’s highest mountain, Everest.
Plate tectonics is 50 years old (ish)
The acceptance of plate tectonics as a theory was pushed by data. The rise of sonar technology during World War II allowed for the mapping of the seafloor, which, when coupled with the use of magnetic data, produced an understanding of new oceans being formed and then being destroyed. An increase in earthquake data also allowed for these oceans to be slowly tracked as they moved under the continents. The greater understanding of the ocean life cycle meant that the Earth’s surface could move. Yet, plate tectonics was not a ‘Eureka!’ moment – it was a slow burn of ideas over many decades driven by data.
Alfred Wegener in 1912 highlighted that the continents could have previously been all together in one supercontinent (called Pangea). He noted that the Earth’s current landmasses could fit together like a jigsaw puzzle. After analyzing fossil records that showed similar species once lived in now geographically remote locations, meteorologist Wegener proposed that the continents had once been fused. His “amateur” status, combined with anti-German sentiment in the period after World War I, did not help his cause. However, Wegener’s hypothesis was mainly dismissed by geologists because there was no mechanism to explain how the continents could actually “drift.”
In 1944, Arthur Holmes promoted the theory of continents moving which had been outlined by Wegener, and even added a missing piece of the puzzle. Holmes, born in Hebburn and raised in Low Fell, Gateshead, proposed convection currents within the mantle could move the crust. Yet, it wasn’t until the 1960s and the availability of this new data that the plate tectonic theory could become mainstream. In the period 1961 – 1968 there were a number of important papers that came out that seemed to build towards this acceptance. There were a number of pivotal moments, but no well defined Eureka. However, over the course of the past few years we have celebrated the 50th anniversary of plate tectonics – at least three times in fact – with a 2016 birthday commemorating the famous American paper of Walter Pitman and James Heirtzler in 1966, the 2017 British anniversary celebrating the 1967 paper of Dan McKenzie and Robert Parker, and the 2018 French celebration of the 1968 paper Xavier Le Pichon.
The evolution of our understanding of the Earth is quite extraordinary. Barack Obama was born in 1961 (Fig 6). It seems incredible to me in 2018 to think that when Barack Obama was in school, he would not have been taught that the continents move over time.
Where do our oceans go?
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 7). 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. 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 for a pregnant mother. During an ultrasound, energy is transmitted and then the reflections of that energy are recorded. For the Earth, there is a natural energy maker in the form of an earthquake (Fig 8). Every earthquake emits energy in the form of seismic waves – we can feel these waves at the surface through tremors. 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. These monitors produce wiggles of data that are the motion of the waves produced by the earthquake (Fig 8).
Seismic waves do not just travel along the surface, however. These waves pass through the whole Earth, and reflect off different layers and structures as they go. By interpreting the different wiggles related to many earthquake 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 mantle – under North America there exists an anomalous region where seismic waves pass more quickly than if the region was just a heterogeneous mantle (Fig 9). This fast region is interpreted to be cold, and appears to be contiguous with the oceanic subduction region in the Pacific – could this be an ocean sinking from the surface to the core-mantle boundary 3000 km away?
There are many examples of structures within the Earth’s mantle that are interpreted as sinking oceans. However, these slabs of cold material behave differently around the world. In some areas, the old oceans sink uniformly to the bottom of the mantle. In other areas, it appears that the oceans stall at different layers on the journey down (Fig 10). At 660 km depth, there is a change in the mineralogy of the mantle which impacts the mechanics and viscosity of the layer and impedes the slabs. Another layer around 1000 km depth also appears to allow the old ocean to rest upon it before falling into the deeper mantle. However, the complexity of these mantle layers is confounded by the interactions with subducting oceans not occurring consistently – questions remain as to whether the layers are local, rather than global, phenomenon and to how the sinking rate of oceans plays a role?
Hot Blobs and super volcanoes
Aside from the colder regions in the mantle, the wiggles 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 11) and are, in places, 100 times larger than Mt Everest. In seismological terms, they are described as Large Low Shear Velocity Provinces (LLSPVs). Here, however, they will be referred to as ‘blobs’.
Due to the distance at which these blobs are situated, nearly 3000 km below the surface, there is a difficultly in interpreting them. A strong debate rages on pretty much every aspect of their character. One thing that seems to be clear is that super volcanoes from Earth’s past appear to form from these blob regions. Such super volcanoes, which are referred to in geological terms as Large Igneous Provinces, are very much distinctive from a normal volcano. As an example, the 2010 eruptions of Eyjafjallajökull in Iceland can be described as a ‘normal’ volcano. Even though actual lava flow from the volcano was very local, the ash cloud from the eruptions caused major travel woes all across Europe – as a result, the volcano was a significant tectonic event of the 21st Century (Fig 12). A large igneous province super volcano is a very different prospect.
The Central Atlantic Magmatic Province was 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 (Fig 12), with the super volcano begin attributed to a ~40% extinction of all species. Over the past 300 million years, there have been around a dozen of these super volcanoes, with three of them being linked to large scale extinction events (Fig 13). 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.
Blobs – the future of plate tectonics?
The ultrasound imaging of the present-day mantle reveals that these blobs may be surrounded by old oceans (Fig 14). But, what are these blobs?
An excellent summary of the different interpretations of the blobs of material has been given in the review paper by Prof Ed Garnero and co-authors, which I’ll explain here. The first interpretation is that these blobs are not the structures we perceive them to be. Indeed, we are looking at something that is thousands of kilometres away and require earthquake waves to image – it may be that our resolution of the mantle at depth is not as it should be and we ‘see’ a smudged picture of what is really there. There is a strong case that these blobs are just thin, thermal plumes that are smeared in their resolution at depth (Fig 15b). The second interpretation is that these blobs are huge super plumes or domes of hot material that lie beneath the Pacific and Africa. These hot structures rise and fall over time, and interact with the cold oceans that surround them (Fig 15c).
At the other end of the scale, it has been put forth that these blobs are more rigid structures that are made of a dense material that is distinctive from the surrounding mantle (Fig 15d). The stable structures interact with the old oceans, which butt up against the edges of the piles, but the blobs remain fixed in their position for hundreds of millions of years. The stability of the piles of material produces super volcanoes mainly from their edges, and are a dominant force in mantle dynamics.
The fourth interpretation is a softer version of the latter – that these blobs are distinctive piles of dense material, but are able to be moved around on the bottom of the mantle (Fig 15e). Here, the interpretation is that the oceanic subduction acts as a broom to push the blob material into piles over time – this interaction produces super volcanoes through the mantle motion. In this case, the landscape of the deep mantle evolves over time similar to the plate tectonic evolution at the surface.
As mentioned, what these structures are and where they have come from is up for debate. Two theories on their origin prevail – whether they are a primordial leftover from the formation of our planet that has been shifted over time (Fig 16a), or a growing layer that was not present in the early Earth but has accumulated over its history (possibly related to subducting oceans reaching the bottom of the mantle) (Fig 16b).
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 17). The future of plate tectonics lies within these deep mantle structures, and whether they are fundamentally important in the way our planet evolves.
Why is this important for us and beyond…
Why is plate tectonics important? Well, these processes of ocean birth in the oceans and subduction are ways of regulating our atmosphere – the plate tectonic processes produce a Carbon Cycle by which CO2 is put in and taken out of the air (Fig 18). The pleasant nature of our atmosphere can be attributed, in part, to plate tectonic processes. Too much CO2 in the atmosphere can turn our planet into something like Venus – a dense haze which is over 400 C at the surface. Whereas, if we had no CO2 being released into our atmosphere, our surface would be -30 C and our planet not as hospitable to life like ours.
At present, we still do not know when plate tectonics began. There are a number of proposed start dates of plate tectonics as we know it. However, they are over a large time range – highlighting the difficulty that exists in understanding our past (Fig 19). A crucial part may be the way in which the temperature of the mantle has evolved over time – although little data is available in the first billion years of Earth’s history, the mantle temperature seems to have been cooling over the past 2.5 billion years. This cooling may have been due to some form of plate tectonics allowing for more heat to be released from the mantle. By cooling the interior, the outer surface may have been able to interact more and produce a primitive form of plate tectonics followed by the dynamics we see today.
One aspect of plate tectonics that is rarely mentioned is ‘when will it will end’. Craig O’Neill and co-authors speculated on this recently, and they described how the continued cooling of the mantle may end up turning off plate tectonics. What we have now in the present day may just be ‘modern’ plate tectonics – with surface motion before and after being much different than today. It is thought that the earliest Earth had a stagnant surface that did not move, similar to Venus today, which is what O’Neill and others proposed would happen once the mantle cools sufficiently. Their thoughts were that plate tectonics as we know it today would only exist as a window of opportunity on our planet (Fig 20) – and therefore may only exist in short spurt of time on exoplanets, too. It is therefore difficult to get excited about an Earth-like exoplanet and its plate tectonic potential, when we struggle to fully understand our own planet’s thermal evolution and indeed the mega structures beneath our feet.
Data will produce the next big discovery… but without Eureka
Although plate tectonics happens near the surface, the impact continues below the surface. Due to plate tectonics, and in particular subduction, the oceans disappear below the surface over the course of 200 million years. The subducting oceanic plate interacts with the different mantle layers (660 km, 1000 km) during the journey to the bottom of the mantle. As they reach the lower mantle, the old oceans could be interacting with deep blobs of material to modify the mantle flow and trigger upwellings (in special cases super volcanoes – large igneous provinces). However, we only see a snapshot in time – things may have been very different in the mantle in the past (and may be different in the future). What is becoming clear is that Earth’s plate tectonic history can leave a lasting impression on the crust and 3000 km away at the bottom of the mantle.
The remaining mysteries of our planet will no doubt eek out a lot slower than the 1960s revolution – 50 years ago new data arrived after giant technological advances, but presently the data improvements will be more incremental and at the whims of funding agencies which drive science. The understanding of the interior of the planet has changed dramatically over the past 100 years (Fig 21), but there are a number of processes that we do not fully understand. The Earth has yet to reveal all it’s secrets, but every day a new earthquake shines a light on the deep to help us understand our planet… and beyond.
In October 2018, I was invited to give a couple of talks on “The Future of Plate Tectonics” as part of the Geological Society’s Earth Science Week. The talks were given to the general public and as a result were accessible to non-Earth Science professionals – I’ve outlined the information given in these talks in this document. As well as two events at Burlington House in London, the talks were live streamed and a recording is available on YouTube. Special thanks go to the European Union Horizons 2020 program for ongoing support, the Geological Society for the invitation, ComputeCanada for the numerical modelling resources, Prof Russell Pysklywec, and Prof Ed Garnero (whose work and talks on the LLSVPs I have heavily borrowed from). The talk was dedicated to Joseph Housam (1987 – 2017).
Dr Phil Heron is a Marie Skłodowska-Curie Research Fellow at Durham University. He gained a Masters of Geophysics from Leeds University and a PhD in Geophysics from the University of Toronto. Phil has been using high performance computing for over a decade to understand the workings of Earth’s crust and mantle across various scales – contributing to the field of supercontinent formation and dispersal through large-scale mantle convection, through to understanding triggers in volcanic eruption from small-scale magma flow. Recently, Phil has been studying the mantle lithosphere – the main component part of a tectonic plate – as well as the inner workings of the deep mantle over time. Prior to his PhD in Toronto, he worked for two years as an educator in Paris (France) teaching English to business people. At Durham University, Phil is working on a prison educational program at HMP Low Newton and HMP Frankland, alongside pursuing his research on numerical modelling of plate tectonic processes.