SYDNEY: A new theory explains why tectonic plates move at different rates and solves other long-standing problems in Earth sciences, Australian scientists said.

A diagram of a subduction zone, where one plate is pushed under the other, dragging the rest of the plate behind it. Credit: NASA

The rate at which tectonic plates move depends on the width of its associated subduction zone – a place where two tectonic plates meet, and the thicker plate is pushed under the other.

The theory explains why the Australian Plate moves much faster than other plates, and explains the disintegration of a mountain range in North America, among other puzzles.

Subduction zones affect plate velocity

Previous theories and hypothesises, such as the idea that plate velocity depended on the age of the plate, have been unable to explain simultaneously the velocity of the different plates and the velocity of subduction zones.

But when geoscientist Wouter Schellart from Monash University in Melbourne, Australia, and his team compiled measurements of all the tectonic plates – 15 large tectonic plates and 30 smaller ones – and all the subduction zones on Earth, they noticed a clear pattern:

“The width of the subducted plate that has gone down into the mantle at the subduction zone determines the velocity of both the plate and the subduction zone plate boundary,” said Shellart, whose finding were reported in the journal Science.

Ancient mysteries of tectonic plates

The researchers also ran an advanced four-dimensional computer simulation of the Earth’s plates and their subduction zone boundaries. “We found the same velocities and patterns as observed in the data, thereby confirming our hypothesis.”

This research helps solve some ancient mysteries involving plate tectonics. The ancient Farallon Plate, once called the Juan de Fuca Plate and located where Utah in the U.S. sits today, slowed down from 10 cm per year 50 million years ago to just two centimetres per year today.

Schellart and his team showed that during this period the subduction zone reduced in size from 14,000km to only 1,400km, which was the cause for the slowing of the plate, and the crumbling of an associated mountain range.

Mountains rise … and crumble

When the Farallon Plate was moving faster, North America had an extensive mountain chain running similar to that of the Andes today.

But “as the width of the plate decreased, the slab could no longer support the massive mountain range, and it started to retreat westward by itself, thereby destroying the mountain range and extending the North American continent, forming the Basin and Range Province,” Schellart said.

The research also explains why some plates move much faster than others.

Fast-moving Australian Plate

“The Australian Plate moves rapidly northward at some six centimetres per year, which can be prescribed to the wide subduction zones at its boundary, namely the Sunda [Indonesian] subduction zone and the Melanesia subduction zone, where the plate subducts beneath New Britain, Solomon and Vanuatu islands” Schellart said.

Not every plate moves this quickly – the African plate moves northward at just one or two centimetres per year. “This can be ascribed to the narrow subduction zones that are located at its northern boundary, namely the Calabrian subduction zone, where the plate subducts below southern Italy, and the Hellenic subduction zone, where the plate subducts below the Greek islands,” Schellart said.

Craig O’Neil, a geoscientist at Macquarie University in Sydney describes the research work as critically important, in particular the fact that Schallart is changing the idea of Earth science from a 2-D modelling to a 3-D dynamic system.

“Thinking of the Earth in 3-D is critical if we want to understand the geological record, and why the surface plates are moving the way they do.”

Source: Article by Emma Bastian