ScienceWise - Jul/Aug 2008

When push comes to shove

New 3D model of the earth’s crust may change theories of continental drift

Most people are aware that the continents of the earth sit on top of various tectonic plates that in turn, slowly drift across the surface of the planet. In places they collide and one plate passes beneath another in a process called subduction.
For decades it has been widely believed that the force of impact between plates drives one plate beneath the other. A new three-dimensional tomographic map of the earths crust developed by Dr Simon Richards and Professors Gordon Lister and Brian Kennet is helping geologists understand the shape and dynamics of subduction zones and how these zones may well define how continents move around the earth.

It’s actually quite difficult to probe the structure of the earth a few hundred kilometres down. The depth is too great for ground penetrating radar and sonar images created by small probe explosions on the surface. To get around this the researchers take advantage of the millions of tiny earth tremors and quakes that occur almost constantly around the globe. The vast majority of these are so small as to be imperceptible but each one sends seismic shock waves through the planet. By having a large network of sensitive recorders across the world and by recording data from them all simultaneously, it is possible to build up a sonar picture of the earth’s interior. Although extraction of tomography from the raw data is a highly complex task, the researchers have been able to build up a large-scale model of much of the earths crust extending down in places by over a thousand kilometres.

Just like sound waves in air, the shock waves that travel through the planet are strongly reflected at places where the density changes abruptly. This means that the technique is particularly suited to the study of subduction zones, where cold dense surface rock protrudes deep in to the hotter mantle.

The rock that makes up the subducting plate (typically oceanic seafloor) contains fluids trapped within and between crystals. When these are plunged into the surrounding mantle - which is often hundreds of degrees hotter - they induce a variety of effects. As the cold rocks heat up and recrystallise, the escaping fluids interact with the overlying mantle which, when combined with stretching of the lithosphere below the overlying plate, induces the generation of magmas at depth. These magmas rise to the earth’s surface where they form volcanic activity at the earth’s surface. Plate subduction is the process causing volcanism around the Pacific rim today. This process also has the side effect of scavenging/concentrating minerals from the mantle/subducting plate and transporting them upwards with the magma flow. The practical upshot of this is that highly concentrated mineral deposits such as gold and copper are often found in the vicinity of subduction zones.

Dr Richards explains that “the detailed models that we are now able to develop show that certain structures found on the subducting plate are frequently associated with a high concentration of earthquakes and whilst it’s very difficult to predict the timing of such quakes, the model does help pinpoint places where these events are highly likely.” Unfortunately for us, one such zone is a couple of thousand kilometres off the north east coast of Australia. A large quake there could have the potential to generate a tsunami similar to the one in the Indian Ocean on Boxing Day 2004, though this time hitting land on Australia’s eastern coast. “The interpreted geometry of the subducted plate there is alarmingly similar to the structure of the slab below Sumatra,” says Dr Richards.

Better identification of areas likely to be affected by large earthquakes and exploitation of mineral assets are two very practical uses for this new model but from a purely scientific perspective, it is showing us just how important subducting slabs are in controlling the motion and shape of plates. Where two plates collide it was once believed that the force of the convecting mantle forced the underlying plate down into the mantle. Whilst Dr Richards acknowledges this is definitely the case in some areas, the majority of subduction zones visible on the model seem to operate quite differently. The edge of a tectonic plate is denser that the underlying mantle so it sinks slightly into it. The forward movement of the plate then causes this leading edge to plane down into the mantle. This has the potential to tear continents apart such as the case when South America broke away from Africa around 130 Million years ago.

Dr Richards believes that as the leading edge of the plate planes down into the underlying mantle, it causes extension in the region immediately in front of it. In effect this creates a suction that draws the overlying plate towards the retreating hinge of the subducting plate. This is the situation with South America and Africa. A subduction zone off the west coats of South America created the vulcanism that formed the Andes. It also has the effect of continually dragging South America to the west, away from the African continent. The Atlantic ocean separating the two continents is being pulled apart and the tearing crust is replaced by the formation of new oceanic crust by up-flow of mantle. So, while the Pacific Ocean is being consumed by the westward motion of South America, the Atlantic ocean is growing larger. In this way, it is subduction that appears to be controlling the drift of continents.

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