The day the Earth moved
Tectonic plate theory revolutionised the way we look at our planet, writes Andrew Gleadow.
The revolution began on 7 September 1963, although at the time I didn’t know it. On that day, 50 years ago this year, a paper published in the journal Nature triggered one of the greatest paradigm shifts in the history of science.
In our Year 10 science class at a suburban Melbourne high school, my schoolmates and I didn’t feel the tremor. We were still being taught what geologists had long believed – that the Earth’s crust was quite literally set in stone. Apart from allowing that parts of it could move up and down in the same place, the crust under both continents and oceans was believed to be fixed firmly in place. The paper in Nature suggested otherwise. Beneath the mild-mannered heading “Magnetic anomalies over oceanic ridges” lay irrefutable evidence that, far from being stationary, the ocean floor was behaving more like a huge conveyer belt creeping across the face of the Earth. The paper by University of Cambridge PhD student Fred Vine and his supervisor, Drummond Matthews, became known as the “Vine-Matthews Hypothesis”, and it ignited what is known as the plate tectonics revolution.
The word “tectonics” is derived from the Greek word, tektonikos, “of a builder”. Plate tectonics describes the way the Earth’s crust is built. A complete understanding of the Theory of Plate Tectonics emerged rapidly within a few years of Vine and Matthews’ work. This was the beginning of our understanding of the true nature of our planet: the Earth’s surface is broken up like a jigsaw puzzle into large rigid slabs, called plates. Moreover, propelled by the heat of the planet’s interior, these plates are constantly on the move, crashing together, tearing apart or grinding past each other. For the first time, plate tectonics provided a convincing explanation for the formation of great mountain ranges, the distribution of earthquakes and volcanoes, and much more. Fifty years on, the revolution Vine and Matthews started is still delivering new insights into the inner workings of our planet.
Vine and Matthews had not set out to trigger the plate tectonics revolution. They were merely trying to solve an enigma. Following World War II, highly sensitive “magnetometers”, originally used to identify enemy submarines by their magnetic signatures, were re-deployed to study the rocky bottom of the ocean floor. They revealed something bizarre. Unlike anything on land, the ocean floor was laid out in a striped magnetic pattern. Vine and Matthews offered an explanation. The seemingly static ocean floor was actually on the move. Magma rising up from cracks along the mid-ocean ridges was solidifying and adding new material to the ocean floor that spread away from the ridge on either side. As the magma solidified, it recorded the magnetic field of the day and, since that field reversed its direction about every half million years or so, the sea floor ended up with alternating bands of rock with opposite magnetic polarity.
The explanation was so elegant and so powerful that it just had to be true. But it also inadvertently resurrected a long-discarded and highly controversial theory.
In 1915 Alfred Wegener, a brilliant German meteorologist and intrepid polar explorer, proposed that Earth’s land masses had once been joined together in a massive supercontinent that has been called Pangaea before breaking apart and drifting to their current positions. The idea became known as continental drift. As a look at any world atlas reveals, the Earth’s major continents do look like pieces of a strewn jigsaw puzzle. Shrink the South Atlantic Ocean and the elbow of South America fits nicely into the nook of West Africa. Shrink the Indian Ocean and Antarctica and India fit nicely on to the flank of southeast Africa, with Australia snuggling down on the side of Antarctica.
But besides scrutinising maps, Wegener had also amassed an impressive body of geological and palaeontological evidence to support his idea, all published originally in German in a 1915 book entitled The Origin of Continents and Oceans. Wegener pointed to glacial deposits that must have formed around an ancient ice cap but are now found on continents in tropical or temperate zones, suggesting they had travelled considerable distances from their polar origins. While continents that look like matching jigsaw pieces now have vastly different climates and ecosystems, they nevertheless share the same plant and animal fossils, suggesting their ecosystems were once the same. Adding to that picture are ancient mountain ranges that seem to break off at the edge of one continent, only to re-emerge an ocean away on another, such as the Appalachians of North America that continue as the Caledonian chain of northern Europe and east Greenland. As Wegener wrote: “It is just as if we were to refit the torn pieces of a newspaper by matching their edges and then check whether the lines of print ran smoothly across. If they do, there is nothing left but to conclude that the pieces were in fact joined in this way.”
Despite the compelling weight of all this evidence, his theory received a hostile reception, most notably in America, when it was translated into English after World War I. Wegener was seen as an outsider, primarily because he was not, in fact, a geologist but also perhaps because there was little sympathy for new ideas from Germany in the aftermath of the war. While the geological community of Wegener’s day allowed for massive vertical movements of the crust, horizontal movements were heresy.
Antagonism to Wegener’s theory came to a head at a meeting of the American Association of Petroleum Geologists organised to discuss continental drift in New York in 1926. Wegener, present as one of the participants, was forced to endure the brutal rejection of his theory. History records some of the more unbridled responses: “Utter, damned rot!”, “If we are to believe this hypothesis, we must forget everything we have learned in the last 70 years and start all over again” and that anyone who “valued his reputation for scientific sanity” would never dare support such a theory.
Disillusioned, Wegener returned to his other passion: researching the polar climate of Greenland and its great ice cap. He led a mission to establish three meteorological stations from west to east across the ice cap but fell victim to the vagaries of Greenland’s climate. In a classic polar catastrophe, he died heroically just four days after his 50th birthday while mounting a rescue for two scientists trapped at the middle station without supplies. His colleague Fritz Loewe survived (after amputating his own frost-bitten toes with a pocket knife and a can-opener) to set up the Department of Meteorology at the University of Melbourne, which later merged into the School of Earth Sciences of which I became the head many years later.
Following his death, Wegener’s brilliant theory languished. Notwithstanding the politics and prejudices of the day, one of the major reasons was that nobody could convincingly explain just how continents could move across the Earth. Wegener himself had acknowledged the failing. In 1929, in the last revision of his book he wrote: “It is probable the complete solution of the problem of the forces will be a long time coming... The Newton of drift theory has not yet appeared.”
But drift theory never completely died thanks to a small group of doughty supporters scattered around the world. One of the most notable was Samuel Warren Carey, professor of geology at the University of Tasmania, and one of the great characters of Australian geology in the mid-20th century. Carey was an outspoken and passionate advocate for continental drift, keeping the torch alight throughout the long decades when the concept was ridiculed almost universally by mainstream geoscientists. In his booming voice, he would speak at length to anyone who would listen about the merits of drift theory and stare down all opposition with the sheer weight of his personality. To all appearances, this maverick role was one that Carey relished and he passed the baton for drift theory to a widening group of students, one of whom was later to become my geology teacher in my final year of school. It didn’t take much before I too was hooked.
In the late 1950s, scientists started turning their attention to the long-neglected geology of the ocean floor. Because it is largely composed of iron-rich basalt, one way to tackle this difficult problem was to map its magnetic properties. Exquisitely sensitive magnetometers were deployed for reconnaissance of this very alien part of the planet. The magnetism of any particular region of the ocean floor was determined by measuring the total reading on the magnetometer and subtracting the background value of the Earth’s magnetic field. By the early 1960s, magnetometers towed around the surface of the Pacific, Atlantic and Indian oceans, reported something astonishing. What they found was a mysterious wave-like pattern: long ridges where the magnetic signal was slightly stronger than the background field and troughs where the signal was slightly weaker. On a map they looked like zebra stripes roughly 10-20 km wide. The pattern appeared to centre on the then somewhat mysterious mid-ocean ridges, a globe-encircling chain of undersea mountains cleaved by a deep rift along its spine. The magnetic pattern appeared to spread symmetrically from these ridges, splaying out hundreds of kilometres on either side. Nothing like these magnetic zebra stripes had ever been seen before and geologists were at a loss to explain them.
Enter Vine and Matthews. Matthews was one of the scientists collecting magnetometer data from the Indian Ocean. Back in Cambridge, using the clunky computers of the day, his student Fred Vine was given the job of analysing the data.
What could explain the pattern of magnetic stripes splayed out on either side of the ocean ridges? Vine made an inductive leap, linking two strange and nebulous ideas into something rock solid. One was the notion that the Earth’s magnetic field, which despite its wobbles usually orients closely to the geographic poles, can actually flip its poles from time to time. North can become South! Bernhard Brunhes first proposed this startling idea in 1905 when he found magnetically reversed rocks in ancient lava flows in France.
The second idea, referred to as “sea-floor spreading”, held that the mid-ocean ridges were actually long gashes on the skin of the ocean floor where the two sides of the ridge are slowly being pulled apart. Molten magma oozes up from the region below to plug up the ever-widening gap with new lava rocks. Two American scientists had independently published this idea: Robert Dietz in 1961 and Harry Hess in 1962. Sea-floor spreading was consistent with what little was known of the mid-ocean ridges at the time but, like Wegener before them, Hess and Dietz were unable to prove their new theory. Even Hess, one of the most eminent American geologists of his day, self-deprecatingly described his 1962 paper as an “exercise in geopoetry”.
Vine and Matthews found the evidence that would turn geopoetry into geoscience: the ocean-floor magnetic stripes. Just as Dietz and Hess surmised, new ocean floor was continually being created from molten magma and moving away from the ridges like a slow-motion conveyer belt. As the magma cooled to form new basalt rocks, they would take on the magnetic signature of the Earth’s field at that time. The spreading sea floor was like a tape recorder, recording whatever the Earth’s magnetic orientation was at the time it was formed. The central part of the ridge, where the new ocean floor is being formed today, would always show a magnetic signal stronger than the background, the sum of Earth’s present normal field plus the similarly “normal” magnetism of the rocks. The stripes on each side showing a weaker signal mark the position of rock that solidified at the oceanic ridge prior to the last reversal when the earth’s polarity was in the opposite or reversed direction. The weaker magnetism over these rocks is the sum of the current field and the “reversed” field present in the rocks from when they solidified.
How long ago did these magnetic reversals happen? We now know from dating volcanic rock layers on land and measuring their magnetic properties that the most recent magnetic reversal occurred 780,000 years ago. So all of the rocks from the centre of the oceanic ridge to where they show the first magnetic reversal are younger than this. Moving further out from the ridge, steadily older magnetic reversals are recorded in the ocean floor rocks.
By solving the mystery of magnetic reversals on the sea floor, Matthews and Vine found the critical last piece of the puzzle. The spreading sea floor was now a reality. It was part of a system that recycled the sea floor in and out of the mantle beneath, and that ultimately provided the force that would explain the movement of continents. But we shouldn’t just credit Vine and Matthews with delivering the evidence that backed the theory of the spreading sea floor. There was actually a third. In one of the great injustices in the annals of science, Canadian geologist Lawrence Morley came to the same conclusion as Vine and Matthews about sea-floor spreading and the magnetic pattern on the ocean floor, but both Nature and the Journal of Geophysical Research rejected his manuscript earlier that same year, 1963. By rights, we should be celebrating the 50th anniversary of the Morley-Vine-Matthews Hypothesis.
After nearly 40 years of being all but banished from respectable debate, continental drift was now back on the table. Thanks to Morley, Vine and Matthews there was now a mechanism to explain it: sea-floor spreading. Over the next five years or so, sea-floor spreading and continental drift were combined into the grand synthesis now known as plate tectonics. The planet’s surface was broken into so-called lithospheric (or rocky) plates that rode on a weak, partially molten layer known as the asthenosphere, which lay a little over 100 km down in the Earth’s mantle. The lithospheric plates included not only the crust but also the solid rocks of the upper mantle. Some plates, such as the Pacific Plate, included mostly heavy oceanic crust; others, such as the Eurasian Plate, were mostly lighter continental crust; but all of the seven major plates included some of both. Oceanic lithosphere formed at the mid-ocean ridges, and spread out across the sea floor. As it spread, it became cool and dense, eventually sinking back into the mantle under the force of gravity (a process known as subduction). Subduction zones created, for instance, the Pacific’s volcanic zone known as the Ring of Fire. It was this conveyer belt moving across the earth’s mantle, driven by gravity, that created the force to move the plates and with them the raft-like continents that ride on top of them.
Despite the elegance of their explanation, Vine and Matthews’ work didn’t receive instant universal recognition. Four years after their seminal paper, I arrived at the University of Melbourne to start my science degree, freshly inspired by these incredibly exciting ideas. To my surprise many of my lecturers remained opposed to the idea of continental drift and for the most part were resistant to any new-fangled ideas arising from the ocean basins. We students, many of us well aware of the revolution going on around the world, were stunned to find plate tectonic theory absent from our lectures. But resistance was crumbling. A decade after Vine and Matthews’ paper, plate tectonics became the reigning paradigm, the all-embracing explanation of how our planet worked. By the mid-1970s virtually all remaining pockets of opposition had melted away and the revolution was won.
But the repercussions continued. Understanding how the ocean crust forms had another very important consequence. It enabled the rocks of the ocean floor to be dated simply by towing a magnetometer behind a ship and mapping the distribution of the magnetic stripes. What gradually emerged is that there have been more than 170 reversals over the past 180 million years, an average of one just over every million years. In the past 65 million years they have occurred more frequently – on average about once every 500,000 years, with the next one probably overdue. But there is no need to panic. They happen gradually, taking more than a thousand years to complete.
The magnetic surveys, confirmed by a major and on-going program of ocean-floor drilling, revealed yet another astounding finding about the sea floor. Nowhere in the ocean basins of the Earth is the crust more than about 180 million years old, a mere 4% of the age of the Earth. Our ocean floor is recycled at a rapid rate! Only one small remnant of slightly older ocean floor is known, underlying the eastern end of the Mediterranean. That is all that remains of a once-great ocean that has now largely been swallowed back into the mantle. The oceanic crust is therefore very much younger than the average age of the continental crust, and particularly than the ancient cores of the continents, such as are found in Western Australia and Canada, dating back more than 4,000 million years.
With plate tectonic theory in place, focus shifted to trying to apply the relatively simple lessons learnt from the ocean crust to understand the infinitely more complex evolution of the ancient and much thicker continents. The formation of continental crust starts during the recycling of oceanic plates back into the mantle beneath. During subduction, melting forms new lighter magmas that rise to form chains of volcanoes at the surface. Over time these new volcanic rocks accumulate and grow thicker. As plates collide, they may be deformed and buckled, eroded away and deposited again as sedimentary rocks, buried and recrystallised to form metamorphic rocks, and even melted and solidified again to form granites. But once formed, continental rocks behave like Styrofoam in a swimming pool, too buoyant to be recycled on a large scale back into the mantle beneath. Rather they accumulate throughout the Earth’s history to form the continental crust, periodically pushed up into great mountain ranges and torn down again, constantly changing and reforming, but never disappearing. Ocean crust, on the other hand, is not permanent and is eventually absorbed back into the mantle.
the Australian plate is moving north at breakneck rate of 7 cm a year.
We now know that the continents have been through several cycles of clumping into supercontinents, only to break up again and disperse across the planet. Today the Earth is about half-way through a cycle which began with the progressive break-up of Wegener’s Pangaea about 200 million years ago. Africa is the last remnant of a supercontinent that is still in the process of breaking up. The Great Rift Valley today marks an unravelling seam where Africa is splitting apart into two separate plates, the continental equivalent of the oceanic ridge system, of which it is ultimately destined to become a part. On the other hand, Asia is becoming the nucleus of a new supercontinent around which various continents, most recently India, are assembling. On its present course, Australia will also become part of Asia, scraping up the various island chains to our north on its way to an eventual collision with East Asia, in about 50 million years’ time. So we’re not just in for the Asian century; more like the Asian era.
The success of plate tectonics, as with any theory, may be judged by its testable predictions. The past 50 years have vindicated one prediction after another as they are confirmed by increasingly sophisticated observations of our planet. We are now able, for example, to measure horizontal movements of points on the surface of the Earth directly, with a precision not dreamed of 50 years ago. GPS satellite observations mean that plate motions can now be observed precisely over the space of just a few years. From these measurements we know that the plates are moving at velocities of between 1 cm and 7 cm a year, confirming estimates made years ago from the magnetic stripes on the ocean floor. Australia, as part of the larger Indo-Australian plate, is one of the fastest, moving northwards at the breakneck speed of 7 cm per year.
Vine and Matthews’ 1963 paper emerged into a world on the brink of many revolutions. Bob Dylan crooned “the times they are a changing”. Not long after, Neil Armstrong and Buzz Aldrin landed on the Moon. But for me, nothing can compare to the revelation that the surface of the Earth was moving beneath my feet. Plate tectonics has explained much about our planet, yet the Earth still holds secrets that will continue to challenge our ingenuity.