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How Tectonic Plates Take the Plunge

Mt Etna

The formation of Europe’s largest volcano, Mt Etna, cannot be explained directly by the theory of plate tectonics. Credit: Sebastien Litrico

By Wouter P. Schellart

New evidence shows that the speed of the Earth’s tectonic plates and their boundaries, as well as the formation and destruction of mountain ranges, is controlled by the size of plate boundaries.

In the 16th century a Dutch cartographer named Abraham Ortelius noticed the jigsaw fit between the east coast of the Americas and the west coast of Europe and Africa. He argued that the continents were once joined together and subsequently separated.

This idea was largely forgotten in the following centuries, although several others proposed a similar jigsaw fit. It wasn’t until 1912, when the German geophysicist Alfred Wegener proposed his theory of continental drift, that a wealth of geological data was presented to support the hypothesis of a jigsaw fit of the continents and of the former existence of an ancient supercontinent, which Wegener called Pangea (meaning “all Earth”). Wegener argued that the supercontinent was subsequently destroyed when the individual continental masses, including Eurasia, Africa, North America, South America, India, Antarctica and Australia, separated from each other due to divergent motion between them. Wegener compiled much of the pre-drift geological data to show that the continuity of older structures, rock formations, glacial deposits, fossil floras and faunas located along the shorelines of many continents could be explained with a pre-drift reconstruction of Pangea.

The new theory of continental drift was mostly received with disbelief, since it broke away from the accepted view of a static Earth, because many geoscientists found faults in the details of the geological data presented in Wegener’s work and because Wegener, as he himself realised, did not propose a reasonable mechanism for continental movements. In the late 1920s the Englishman Arthur Holmes proposed that convection currents deep within the Earth’s interior could provide the driving force for the lateral motion of the continents, but in the following three decades not much was written about continental drift.

During this period, however, a wealth of new geographical, geological and geophysical data was acquired, including bathymetric maps and magnetic data of the ocean floor, palaeo-magnetic data from the continents, and earthquake data from the Earth’s interior. These provided a whole new perspective on the origin of the ocean floor, the movements of the continents and the structure of the Earth’s interior.

The Theory of Plate Tectonics
A grand unifying theory to explain the new observations from the ocean floor, and reconcile them with the earthquake data and the geological observations from the continents, was only developed in the 1960s. This theory of plate tectonics revolutionised the earth sciences, and describes the Earth as being covered by 15 large plates and more than 30 smaller ones. The plates have an average thickness of 100 km (referred to as the lithosphere) and move only a few centimetres per year over the underlying, more fluid-like sub-lithospheric mantle.

Most of the action takes place at the plate boundaries, leading to the occurrence of earthquakes and volcanoes, the formation of the ocean basin and the birth of mountain ranges.

There are three fundamental plate boundary types. Divergent plate boundaries are where two plates move away from each other; transform boundaries are where plates move parallel to the plate boundary; and convergent boundaries are where two plates move towards each other. Examples of the last include the Himalayas, which separate the Indian plate in the south from the Eurasian plate in the north, and the South American deep-sea trench, which separates the Nazca plate in the west from the South American plate in the east.

The Himalayas formed in response to the collision of the Indian continent and the Eurasian continent, and are an example of a collisional convergent plate boundary, while the South American deep sea trench formed as a result of the sinking of the Nazca plate below the South American plate into the Earth’s interior at a subduction zone. At collision zones, the lithospheric plates that collide have a continent on top of them while at subduction zones the downgoing lithosphere is oceanic.

Driver of Plate Tectonics
The theory of plate tectonics describes the motions of the plates in great detail but it does not explain the driving forces for such plate motions. Nor does it explain why some plates move slower than others, and why plate boundaries move at velocities comparable to those of the plates. In the past four decades, scientists have worked to solve these fundamental questions in Earth dynamics.

In the 1970s and 1980s, geologists and geophysicists started to realise that the forces required to move the tectonic plates primarily originate from the subduction zones, where cold and dense oceanic plates like the Nazca plate and the Pacific plate sink into the Earth’s interior at deep-sea trenches. Indeed, the lithosphere of the oceanic plates is denser than the underlying mantle because it is much colder, and is thus negatively buoyant. The oceanic lithosphere thus stores gravitational potential energy that is released as the lithosphere progressively sinks into the mantle.

Subduction Zones: Size Matters
The size of subduction zones varies dramatically on Earth, from only a few hundred kilometres across for a number of miniature subduction zones in the Mediterranean to many thousands of kilometres for several subduction zones in the Pacific region. Surprisingly, only in the past decade have geodynamicists started to investigate how the size of a subduction zone influences its behaviour.

A breakthrough came in 2007, when a paper published in Nature illustrated how the size of subduction zones determines the “rollback” velocity of these plate boundaries and their curvature. (Rollback refers to the backward motion of a sinking slab.) Advanced computer simulations of subduction and observations from active subduction zones on Earth showed that small subduction zone plate boundaries, such as those in the Mediterranean, retreat rapidly and have a concave curvature, whereas large subduction zone plate boundaries, such as those along the west coast of South America and the east coast of Asia, move slowly and have an overall convex geometry with concave edges.

A more recent paper published in Science last year showed that the size of subduction zone plate boundaries controls not only their velocity and curvature, but it also controls the velocities of the tectonic plates. A plate with a very wide subducting slab attached to it will move rapidly towards the subduction zone (e.g. Australia moving 6 cm/year towards the north-east) while a plate with a narrow subducting slab will move much slower (e.g. the African plate moving only about 2 cm/year).

These seemingly unrelated phenomena of plate velocity, plate boundary velocity and plate boundary curvature are explicable when considering the type of mantle flow patterns that are induced during slab sinking. Narrow slabs experience little resistance to move laterally, because mantle material can easily move sideways around the lateral slab edges, thereby inducing curvature at the edges, rapid slab rollback, slow plate motion, and a concave trench curvature. Conversely, wide slabs experience large resistance to move laterally because mantle material experiences large resistance to move sideways around the lateral slab edges, resulting in a stationary centre, rollback at the edges, rapid plate motion and a convex curvature.

Mountain Ranges Form ...

As an oceanic plate sinks at a subduction zone, it produces a deep-sea trench and causes arc volcanism at the overriding plate on the other side of the trench. Most overriding plates on Earth show either deep basins rimmed by volcanoes, such as the North Fiji basin bordering the Vanuatu islands, or relatively flat-lying continental margins such as Sumatra and Java.

In stark contrast to these examples is the west coast of South America, which is home to the Andes mountains, the longest and second highest mountain chain in the world with peaks reaching almost 7000 metres. The Andes are located at a subduction zone, which makes them unusual because most mountain belts are located at continental collision zones. For instance, the Himalayas formed after a collision between India and Asia.

So why are the Andes where they are, located above a subduction zone, and why do they exist in the first place? It appears that subduction zone size again plays a crucial role.

An important observation is that the Andes span the entire west coast of South America, running parallel to the South American subduction zone that stretches from Colombia in the north to Patagonia in the south. The total width of this subduction zone is a massive 7400 km, making it the widest subduction zone in the world.

Three-dimensional computer simulations indicate that such wide subduction zone plate boundaries are relatively immobile because of the large resistance that the subducted slab experiences when moving laterally through the mantle. Because of this large mantle resistance, the subduction zone plate boundary is capable of supporting large compressive stresses, in particular in the centre near Bolivia. Narrow subduction zones would not be able to support such stresses.

These compressive stresses are generated by the westward motion of the South American plate, resulting in squashing, squeezing and shortening of the western edge of the plate and ultimately leading to the formation of the Andes mountains. The research on subduction zone size thus solves a paradox of how such a large mountain range can exist at a subduction zone, with sinking of an oceanic plate below a continental plate.

... and Mountain Ranges Crumble
Geologists who have studied the structure of the Andes mountains have found that it started forming roughly 50 million years ago, and since then the crust in the centre (Bolivia region) has experienced 350 km of east–west shortening. It is ultimately this process of crustal shortening and thickening that leads to the formation of mountain ranges.

Interestingly, geologists working along the western edge of the North American continent have discovered that this region was in a similar tectonic environment in the Late Cretaceous and Early Cenozoic as the Andes is now. The geological data indicate that there existed a massive mountain range stretching from Alaska in the north to Mexico in the south, which was characterised by up to 300 km of east–west crustal shortening in the centre – comparable to the present-day central Andes.

Surprisingly, this mountain range started disappearing as it was torn apart some 50 million years ago and was replaced by the Basin and Range province, a 2 million km2 area in western North America of narrow, elongated basins and ridges that formed due to stretching of the continental crust.

Geologists and geophysicists have long wondered why this mountain range disappeared at this time and was replaced by the Basin and Range province. The answer lies in what happened with the subduction zone bordering the west coast of North America. This subduction zone decreased dramatically in size from some 14,000 km across 55 million years ago to only 1400 km now. As the subduction zone decreased in size, the subducting plate slowed down and the subducted slab started to rollback westward on its own. This reduced the compressive stresses along the west coast of North America, resulting in the destruction of the mountain range, stretching of the crust and formation of the Basin and Range province.

Subduction, Mantle Flow and Mount Etna
As models and observations show, the size of subduction zones provides a strong control on plate and plate boundary velocities, and the formation and destruction of mountain ranges and ocean basins. This is ultimately related to the presence of lateral edges of subduction zones, which allow for mantle flow to occur around slabs. As subducted slabs sink into the mantle, the lateral component of motion induces a mantle flow from one side of the slab to the other side, and this occurs around the lateral slab edges. The existence of this flow was first demonstrated in the late 1990s, and was thought to be mostly horizontal.

More recent modelling work, however, indicates that this flow has a strong vertical component as well. Next to lateral slab edges, the flow has a strong component of upwelling, which is important for it might provide an explanation for puzzling volcanism that cannot be explained directly by the theory of plate tectonics. One example is Mount Etna, the largest volcano in Europe that is located on the island of Sicily in the central Mediterranean.

Mount Etna’s existence is puzzling because it is located close to, but south of, the volcanoes in the Tyrrhenian Sea. The volcanoes in the Tyrrhenian Sea, such as Vulcano and Stromboli, are typical arc volcanoes that formed above the subducted African plate, similar to the arc volcanoes found in the Pacific “Ring of Fire” that also formed above subducted plates.

Mount Etna is not situated vertically above the subducted African plate but is located 70 km south of it. This indicates that Mount Etna is not the result of arc volcanism, but its close spatial association with the subduction zone suggests that it might still be related to the subduction zone.

Detailed laboratory models investigating the flow patterns near lateral slab edges show how the subduction zone is indeed responsible for the volcanism. As the subducted African plate sinks and at the same time moves eastward, it induces flow in the mantle that goes westward around the southern edge of the subducted African plate, but at the same time it also goes upward. This upward motion decreases the pressure on the rocks, inducing decompression melting of the mantle. Because these melts are very buoyant, they rise to the surface and extrude as lava and volcanic ash, thereby forming Mount Etna.

Future Directions
Although the new theory of subduction zone dynamics indicates that subduction zone size provides a dominant control on plate motions, plate boundary motions, mountain formation and destruction, and flow in the Earth’s mantle, many questions remain. For example, why did the Andes only start to form some 50 million years ago, while the South American subduction zone has been active for more than 200 million years? And why did the mountain range that characterised the east coast of Australia, New Zealand and Antarctica until the Early Cretaceous disappear in the Late Cretaceous to make way for the Tasman Sea region, a region full of ocean basins and submerged continental ribbons and plateaus?

In the next decade geologists, geophysicists and geochemists might make much progress towards solving these questions.

Wouter P. Schellart is an Australian Research Council QE II Fellow and a Monash Fellow with the School of Geosciences at Monash University.