Another Look at Plate Tectonics

Research suggests major changes to geology textbooks

Research suggests major changes to geology textbooks

A new research of the Earth’s crust and upper-mantle suggests that ancient geologic events may have left deep ‘scars’ that can come to life to play a role in earthquakes, mountain formation, and other ongoing processes on our planet.

This changes the widespread view that only interactions at the boundaries between continent-sized tectonic plates could be responsible for such events.

A team of researchers from the University of Toronto and the University of Aberdeen used super-computers to create models that indicate that former plate boundaries may stay hidden deep beneath the Earth’s surface. These multi-million-year-old structures, situated at sites away from existing plate boundaries, may trigger changes in the structure and properties at the surface in the interior regions of continents.

“This is a potentially major revision to the fundamental idea of plate tectonics,” says lead author Philip Heron, a postdoctoral fellow in Russell Pysklywec’s research group in U of T’s department of Earth sciences.

A new map of Earth’s ancient geology

Heron and Pysklywec, together with University of Aberdeen geologist Randell Stephenson have even proposed a ‘perennial plate tectonic map’ of the Earth to help illustrate how ancient processes may have present-day implications.

“It’s based on the familiar global tectonic map that is taught starting in elementary school,” says Pysklywec, who is also chair of U of T’s department of Earth sciences. “What our models redefine and show on the map are dormant, hidden, ancient plate boundaries that could also be enduring or “perennial” sites of past and active plate tectonic activity.”

To demonstrate the dominating effects that anomalies below the Earth’s crust can have on shallow geological features, the researchers used U of T’s SciNet – home to Canada’s most powerful computer and one of the most powerful in the world – to make numerical models of the crust and upper-mantle  into which they could introduce these scar-like anomalies.

A proposed perennial plate tectonic map. Present-day plate boundaries (white lines), with hidden ancient plate boundaries that may reactivate to control plate tectonics (yellow lines). Image credit: Russell Pysklywec, Philip Heron, Randell Stephenson.

Simulating yesterday’s continents

The team essentially created an evolving “virtual Earth” to explore how such geodynamic models develop under different conditions.

“For these sorts of simulations, you need to go to a pretty high-resolution to understand what’s going on beneath the surface,” says Heron. “We modeled 1 500 kilometers across and 600 kilometers deep, but some parts of these structures could be just two or three kilometers wide. It is important to accurately resolve the smaller-scale stresses and strains.”

Using these models, the team found that different parts of the mantle below the Earth’s crust may control the folding, breaking, or flowing of the Earth’s crust within plates — in the form of mountain-building and seismic activity – when under compression.

In this way, the mantle structures dominate over shallower structures in the crust that had previously been seen as the main cause of such deformation within plates.

“The mantle is like the thermal engine of the planet and the crust is an eggshell above,” says Pysklywec. “We’re looking at the enigmatic and largely unexplored realm in the Earth where these two regions meet.”

An Earth in hibernation

“Most of the really big plate tectonic activity happens on the plate boundaries, like when India rammed into Asia to create the Himalayas or how the Atlantic opened to split North America from Europe,” says Heron. “But there are lots of things we couldn’t explain, like seismic activity and mountain-building away from plate boundaries in continent interiors.”

The research team believes their simulations show that these mantle anomalies are generated through ancient plate tectonic processes, such as the closing of ancient oceans, and can remain hidden at sites away from normal plate boundaries until reactivation generates tectonic folding, breaking, or flowing in plate interiors.

“Future exploration of what lies in the mantle beneath the crust may lead to further such discoveries on how our planet works, generating a greater understanding of how the past may affect our geologic future,” says Heron.

The research carries on the legacy of J. Tuzo Wilson, also a U of T scientist, and a legendary figure in geosciences who pioneered the idea of plate tectonics in the 1960’s.

“Plate tectonics is really the cornerstone of all geoscience,” says Pysklywec. “Ultimately, this information could even lead to ways to help better predict how and when earthquakes happen. It’s a key building block.”

Source: University of Toronto


Plate Tectonics & Earth’s Magnetic Field

Plate Tectonics May Control Reversals in Earth’s Magnetic Field

ScienceDaily (Oct. 21, 2011) — Earth’s magnetic field has reversed many times at an irregular rate throughout its history. Long periods without reversal have been interspersed with eras of frequent reversals. What is the reason for these reversals and their irregularity? Researchers from CNRS and the Institut de Physique du Globe(*) have shed new light on the issue by demonstrating that, over the last 300 million years, reversal frequency has depended on the distribution of tectonic plates on the surface of the globe. This result does not imply that terrestrial plates themselves trigger the switch over of the magnetic field. Instead, it establishes that although the reversal phenomenon takes place, in fine, within Earth’s liquid core, it is nevertheless sensitive to what happens outside the core and more specifically in Earth’s mantle.

This work is published on 16 October 2011 in Geophysical Research Letters.

Earth’s magnetic field is produced by the flow of liquid iron within its core, three thousand kilometers below our feet. What made researchers think of a link between plate tectonics and the magnetic field? The discovery that convective liquid iron flows play a role in magnetic reversals: experiments and modeling work carried out over the last five years have in fact shown that a reversal occurs when the movements of molten metal are no longer symmetric with respect to the equatorial plane. This “symmetry breaking” could take place progressively, starting in an area located at the core-mantle boundary (the mantle separates Earth’s liquid core from its crust), before spreading to the whole core (made of molten iron).

Extending this research, the authors of the article asked themselves whether some trace of initial symmetry breakings behind the geomagnetic reversals that have marked Earth’s history, could be found in the only records of large-scale geological shifts in our possession, in other words the movements of continents (or plate tectonics). Some 200 million years ago, Pangaea, the name given to the supercontinent that encompassed almost all of Earth’s land masses, began to break up into a multitude of smaller pieces that have shaped Earth as we know it today. By assessing the surface area of continents situated in the Northern hemisphere and those in the Southern hemisphere, the researchers were able to calculate a degree of asymmetry (with respect to the equator) in the distribution of the continents during that period.

In conclusion, the degree of asymmetry has varied at the same rhythm as the magnetic reversal rate (number of reversals per million years). The two curves have evolved in parallel to such an extent that they can almost be superimposed. In other words, the further the centre of gravity of the continents moved away from the equator, the faster the rate of reversals (up to eight per million years for a maximum degree of asymmetry).

What does this suggest about the mechanism behind geomagnetic reversals? The scientists envisage two scenarios. In the first, terrestrial plates could be directly responsible for variations in the frequency of reversals: after plunging into Earth’s crust at subduction zones, the plates could descend until they reach the core, where they could modify the flow of iron. In the second, the movements of the plates may only reflect the mixing of the material taking place in the mantle and particularly at its base. In both cases, the movements of rocks outside the core would cause flow asymmetry in the liquid core and determine reversal frequency.

* — Laboratoire de Physique Statistique of ENS (Ecole Normale Supérieure/CNRS/UPMC/Université Paris Diderot) and the Institut de Physique du Globe de Paris (CNRS/IPGP/Université Paris Diderot)