Deep thinking is all in a day’s work for
Dr Wouter Schellart.

They call it the ‘Ring of Fire’, but don’t be misled. This chain of submarine trenches, under-sea volcanoes and volcanic islands stretches for some 40,000 kilometres around the rim of the Pacific basin, yet it’s hardly ring-shaped. Some have described it as a horseshoe, but even that doesn’t quite capture the shape of it. Join the dots, and see if you don’t think there’s something canine about the outline, perhaps reminiscent of a fox. The trench along the coast of Peru and Chile suggests the animal’s back, the Aleutian trench below Alaska evokes ears, while the Sunda trench off Java might be the tip of the creature’s nose. Yes, there is something of the fox about it. It’s a fitting image too, given the fox-like cunning research that demonstrated just how these linked geophysical features – and even the Andes mountain range – came to be.

Most people are familiar with the theory of plate tectonics, which divides the uppermost part of the Earth into a number of rigid plates. The continental and oceanic crusts form the uppermost part of these so-called plates. Travel further down and beneath the massive plates there is thought to be another layer, the inner mantle, which is more viscous. This means that at a geological time-scale, the tectonic plates can shift about on the surface of the planet, either moving apart at divergent plate boundaries, moving past each other at conservative plate boundaries, or moving towards each other at convergent plate boundaries.

It’s these convergent plate boundaries that interest Dr Wouter Schellart from the Research School of Earth Sciences. The Dutch-born scientist describes himself as an interdisciplinarian, drawing on geology and geophysics to explore how tectonic movements relate to volcanic and earthquake activity. In a recently completed project funded by the Australian Research Council, Schellart and his colleagues revealed something new about the behaviour of tectonic boundaries that explains why geological formations like the Ring of Fire and the Andes mountain range exist. The team included Dr Justin Freeman at ANU and Dr Dave Stegman, Professor Louis Moresi and David May at Monash University.

Schellart says that tectonic plates are constantly shifting in tiny, tiny increments. But it’s less commonly known that the boundaries between the plates are also constantly shifting – not just in relative position to one another, but also in shape. “There has been a great deal of uncertainty about why they move at all, and also why some move very fast and some move very slow,” he says. “That’s what this latest project was about, to see if there was any relation between the extent of convergent boundaries and the velocity at which they migrate.”

The researchers wanted to solve another problem too. Although it is called the ‘Ring of Fire’, most of the ocean trenches and volcanic arcs around the Pacific bow inwards towards the centre of the ring, rather than pushing outwards. Could the cause of this curious phenomenon be connected to the tectonic boundaries beneath the trenches and volcanoes?

When two tectonic plates converge, two things could happen. The two plates may collide, forming a mountain belt. Alternatively, one plate will override its neighbour, creating a subduction zone: here one plate is being pulled down into the viscous inner mantle by gravity. Subduction zones are considered by geophysicists to be the main engine of tectonic movement, creating a dynamic system that keeps all the Earth’s tectonic plates and the underlying mantle in a constant state of flux. But that’s not the whole story.

As a subducting plate is drawn downward by gravity, it forces the boundary between the subducting plate and overriding plate to move. This explains why the boundaries between tectonic plates are constantly changing in shape. Using supercomputers at ANU and Monash, the researchers ran long-term models to see how these tectonic boundaries would behave over time frames up to 50 million years.

“We found that the width of the tectonic boundary determines the speed and direction of its migration, which will effect whether a mountain range or an ocean basin forms above the activity,” Schellart explains. “Narrow zones will retreat rapidly, while wide zones will migrate slowly, in particular in the centre.”

“We also found that the width determines the shape of subduction zones, which thereby explains the curvature of deep ocean trenches that mark the surface expression of these subduction zones. Narrow zones are concave towards the overriding plate, while wide zones are convex.”

These findings allowed the researchers to explain the unusual shape of the volcanic arcs and trenches in the Ring of Fire, but it also revealed something that had previously been unclear about the Andes mountain range in South America. It’s commonly understood that large mountain ranges occur when one continent collides with another. This kind of collision is responsible for the Himalayas, which have resulted from the Indian continent pushing up into Asia. But there’s no continent butting up against South America, so how did the Andes come to be?

“In the southwest Pacific, near New Zealand, the tectonic boundary is moving backwards (eastward) very fast. That causes the overriding plate to extend and form a deep basin, the Lau Basin. But along the west coast of South America, the boundary is not moving backward (westward) very fast, and in the centre it’s actually moving forward (eastward) very slowly. The overriding plate is moving toward the boundary (westward) itself. Hence you get compression, and the formation of the Andes. The idea that long, slow-moving subduction zone boundaries could result in compression had not really been explored before.”

These results allowed Schellart and his colleagues to see that the concave tendency of volcanic trenches in the Ring of Fire was brought about by a particular kind of tectonic boundary movement where the boundary of the subducting plate retreats rapidly, leaving an ocean basin in its wake. But because of the sheer length of the tectonic boundary along the coast of South America, and the relatively slow speed at which that boundary migrates, the researchers were able to explain why the Andes mountain range is thrusting up out of the Earth.

Schellart says that one of the things that set the group’s work apart was its use of 3D modelling, made possible by the sheer grunt of the supercomputers involved. Unlike many previous studies, which relied on two-dimensional cross sections, the three-dimensional approach allowed the team to fully understand how the tectonic boundaries were behaving. The modelling also meant that they could peer far into the future to see what’s in store for the world’s longest mountain range.

“The Andes will continue to expand,” Schellart says. “Of course the extent of the mountain range also depends on the rate of erosion and the rate of construction. As it currently stands, the mountain building will continue for several millions of years. Our modelling suggests that it will only stop when you get this boundary to segment into smaller pieces.”

He and his colleagues may possess fox-like cunning, but does Schellart ever feel overwhelmed by the sheer scale of time and space he must comprehend to track tectonic boundary migration?

“Every now and then I step back and think, ‘What I’m doing at the moment – is it actually real or in my imagination?’ It does make you feel small in the big world. These tectonic processes have been going on for at least one billion years and possibly longer. They’ll probably continue for another few billion years, as long as there is energy inside the Earth to drive these motions. It will continue long after the human race has shuffled off.”