(Image: DeAgostini/Getty)
OLIVER JAGOUTZ doesn’t have much room for rocks in his narrow tenth-floor office at the Massachusetts Institute of Technology. But the geologist keeps a couple of samples on hand to show visitors how Earth produces something unique in the solar system: continents.
The rocks come from a landscape half a world away, in the remote, hostile mountains of northern Pakistan. But they are a rare record of goings-on deep below Earth’s surface. Along with three to four tonnes of other rocks from the region that Jagoutz and his colleagues have gathered over the years, they could hold the key to the enduring mystery of our planet’s dry land – and much else besides.
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Earth’s surface is like no other in our solar system. Sitting atop the partially molten mass of the planet’s mantle, like the frothy film on the surface of a simmering pot, are a series of vast slabs of solid rock: the tectonic plates of Earth’s crust. That’s strange enough, but the crustal plates also contain two rather different ingredients, as Jagoutz’s samples show. The first – a heavy, dark rock called gabbro – is typical of the basalts that line the ocean basins. The second, a granite characteristic of Earth’s continents, feels light by comparison.
It’s a small but crucial difference. Oceanic crust floats lower on top of the mantle and sinks back into it at subduction zones, where two tectonic plates collide. The oldest oceanic crust is just 200 million years old. The less dense continental crust, meanwhile, bobs higher like an iceberg on water. Plate collisions tend to push it upwards to form mountain ranges, so it can hang around much longer: the oldest known continental rocks are 4 billion years old.
For most of its history, Earth has had just enough water to lay a thin blue skin over the lower, but not the higher, parts of this surface. The relatively stable proportions of sea and land provided an environment unusually suited for complex life as we know it to develop over billions of years. Small wonder the interest in how this situation came about. “The holy grail of geology is to understand the first continental crust,” says Jagoutz.
In numbers, the difference between oceanic and continental crust is small. Oceanic crust has a composition similar to that of the mantle, consisting of about 50 per cent silicates. The continental crust is the anomaly, with up to 60 per cent of these lighter minerals.
The continental material makes up a tiny part of Earth’s bulk – by mass, it is about 0.5 per cent the size of the mantle – but something, somewhere must have given it this fundamentally different composition. We think we know where this transformation occurs: above oceanic crust that is sinking into the mantle at a subduction zone. Heat and pressure squeeze fluid from the sinking crust, which rises and liquefies mantle rocks above. As this material continues to rise, it begins to separate out into lighter and heavier components. The lighter stuff eventually returns violently to the surface as volcanic magma, where it forms the basis of new continents. As for the heavier stuff, the thought was that it must sink, although where or how, no one could quite tell.
A process like this must have been going on since Earth was very young, and is thought to continue today near largely submarine fault lines where two tectonic plates converge and one subducts. A prominent example is the Izu-Bonin-Mariana ridge, an arc of volcanoes running 2800 kilometres south from Tokyo to the Mariana Islands and Guam, part of the “ring of fire” encircling the Pacific.
Drilling down into such areas could provide evidence to test the theory, but that is expensive and difficult, especially in marine environments. What’s more, penetration depths are limited. “You can be happy if you observe just the top 5 kilometres,” says Jagoutz.
“Drilling down into the crust, you are happy if you observe just the top 5 kilometres”
As an Austrian who grew up in Germany and started researching volcanic arcs in Switzerland, was never particularly keen on life on the high seas anyway. “I don’t like ships, so I don’t go on them,” he says. “I got seasick, and it was just not worth it.”
Fortunately, Earth’s past tectonic convulsions do provide some openings for a landlubber. On occasions, volcanic arcs have collided with continents, and the geometry of the collision has skewed their internal layers, forcing them upwards and spreading them horizontally, to be exposed on the surface following subsequent erosion.
Examples of these prostrated sections are found all along the Pacific coast of North America: in parts of Baja California in Mexico, the core of the Californian Sierra Nevada mountain range and much of Vancouver Island in Canada.
But none of these areas presents a continuous record – rocks from some eras are missing – nor do they extend down to the critical layer for the creation of the continental crust. This lies either side of a line known as the Mohorovicić Discontinuity, or “Moho” for short, which marks the point where the crust meets the mantle. Typically 35 to 40 kilometres under continents and 7 to 10 kilometres beneath the sea floor, the Moho is marked by the change in density – shown in a change of speed in passing seismic waves – between the solid crust and the more mobile, slow-flowing rock below.
A first tantalising window on the deep opened up in 1989, when geologist of Western Washington University in Bellingham was studying the Talkeetna volcanic arc, parts of which now lie exposed in south-central Alaska. The properties of some rocks there showed they must have formed at pressures and temperatures corresponding to depths of 30 to 35 kilometres, just at the line of the Moho. There was also evidence of a very dense sort of gabbro, containing as little as 45 per cent silica, that was heavier than the mantle rocks just beneath.
This looked very much like the heavier rock that would be the by-product of making the material of the continental crust. Its position in the exposed arc seemed to imply it would have gone on to sink down into the mantle, under the influence of gravity, had tectonic events not lifted it up and smeared it across the landscape instead. “That dropping-off at the bottom is really the key to creating continental crust,” says DeBari. But it wasn’t a clincher: only a few hundred metres of rock below the Moho were exposed, not enough to show what was actually happening at the bottom of the arc.
(Image: Pierre Bouilhol)
A decade or so earlier, geologists had identified that formations in Kohistan, in the north-east of Pakistan, and the neighbouring Ladakh province in India were also remnants of an ancient volcanic arc. This had formed some 150 million years ago near the equator, close to a subduction zone in the now-vanished Tethys Ocean between Eurasia and what is now India. Subduction of the edge of the plate carrying India pulled the continent northward until it collided with the volcanic arc about 50 million years ago and began to bulldoze it northward. Then, about 40 million years ago, India collided with Eurasia – with the volcanic arc squashed between.
This great continental train wreck, which also threw up the Himalayas, scooped a huge vertical section of the arc onto the top of the Eurasian continental crust, leaving chunks exposed horizontally in an eye-shaped region some 400 by 200 kilometres. In the millions of years since, continuing pressure crumpled it into mountains, resulting in Kohistan: a geological landscape unique on Earth.
“A great continental train wreck scooped a chunk of Earth out onto the surface”
When Jagoutz, then at the Swiss Federal Institute of Technology in Zurich, started investigating the Kohistan deposits in 2000, they “were described in a few different places, but nobody really studied them in great detail”, he says. In the following years, he and a few colleagues went back to the region repeatedly, spending up to three months at a time mapping and studying rock formations, hiring jeeps or donkeys to reach sites and camping in the mountains. At the end of each season, they would haul a tonne or more of rock samples to the airport at Islamabad to ship them back to Europe.
In his office at MIT, Jagoutz opens an old paper map and traces the arc deposits with his finger, showing how the geometry of the continental collision bent the formation and spread it across the surface, and pointing to the thin line marking the suture between the arc and Eurasia. The ability to do fieldwork over such a large area was essential to get the big picture of the processes going on under the surface. “With square kilometres of outcrops, we can wander around and see what is representative and what is not,” he says.
The rocks of Kohistan could hold the key to how Earth made its continents (Image: Jagoutz et al)
Sketching out that big picture has taken years of painstaking microscope work, analysing thin slices of the samples to identify their crystalline structure and chemical composition, revealing the depth at which they formed. Each sample was then carefully mapped back to the location where it was found. In this way, Jagoutz determined that the Kohistan rocks formed at a range of depths up to 50 kilometres down. Those further to the north came from shallower depths, while those further to the south originated deeper. “We have the whole sequence of the arc exposed,” he says. “We can walk through the entire crust, essentially just by walking from north to south.” The sequence in Kohistan goes all the way down to rocks that crystallised at the Moho – and even a little deeper.
The details proved complex, but it was clear that the Moho, at the time it got scooped to the surface, was shedding rock like nobody’s business. About 70 per cent of molten rock in the zone of transformation was in the process of dropping off back into the mantle, forming a tail of heavy material. Dangling about a dozen kilometres down into the mantle, this stuff consisted of just 45 per cent silica and was enriched in heavy metals such as lead. Further up, lighter, high-silica rock was left to rise – and, had the continents not collided, some of it would eventually have erupted through volcanic openings on to the surface.
A mathematical model showed that chunks must have dropped off the base of the Kohistan arc as regularly as every few hundred thousand years. “In geological terms, something that happens in a hundred thousand years is momentary,” says Jagoutz. “It rains rocks all the time.”
What makes Jagoutz’s results revolutionary, says of the Lamont-Doherty Earth Observatory in Palisades, New York, is that they show how continental crust can be formed in a single step, not the several stages of chemical refinement previously assumed. Rock rising from the mantle mixes with fluid from subducted ocean crust and is distilled as it ascends, forming light continental crust, as well as a heavy slag that sinks back down (see diagram). “Oli’s result is definitive, really cool,” says Kelemen.
That’s not all. The high lead content of the heavy rock exposed in Kohistan could shed fresh light on Earth’s origin (see “Mystery of the missing lead“). Analysis of the rocks of Kohistan is allowing the plate-tectonic forces that spread the volcanic arc across Kohistan to be reconstructed. The results could also explain the tremendous, puzzling force with which India slammed into Eurasia to throw up what is now the world’s highest mountain range. A single subduction zone could only have tugged the two land masses together at a rate of 8 to 10 centimetres a year. India was travelling much faster than this – probably because the volcanic arc squashed in between the landmasses meant not one, but two subduction zones were doing the pulling.
It is already an impressive haul from a few tonnes of rock. The sting in the tail is that there might be a limit to how much we can continue to refine these ideas at present. Jagoutz’s last trip to Kohistan was in 2007, since when unrest has made the region less safe to travel to. The hope is that the samples he has already collected hold enough detail to continue to unpick the mystery of beneath. At least the landlubber Jagoutz can be sure he won’t have to get on a boat.
Mystery of the missing lead
The geological formations of Kohistan have already revealed chunks of heavy rock dropping off Earth’s crust and into the mantle (see main story). But the region’s unique geography could also answer a perennially thorny question: why Earth’s composition doesn’t seem to match that of any meteorites. Meteorites are made of the raw material left over from the solar system’s construction phase that should also have gone into making our planet.
Taking an average of all known terrestrial rocks gives an unusual ratio of two kinds of lead isotope formed by the decay of radioactive uranium, compared with “primitive” lead that has been around since Earth formed. For decades, geologists have searched for a missing reservoir of rocks with high levels of primitive lead. “It has to be stored somewhere. It hasn’t left the Earth,” says Oliver Jagoutz of Massachusetts Institute of Technology.
The falling chunks might be just that missing reservoir. Together with Max Schmidt of the Swiss Federal Institute of Technology in Zurich, Jagoutz examined the rocks from Kohistan and found that the material dropping back into the mantle contains between 6 and 40 times as much primitive lead as previously known upper mantle material brought to the surface through volcanic eruptions ().
With such high levels of primitive lead, these sinking rocks need only make up a small percentage of the mantle to potentially explain the discrepancy. Totting up the balance sheet, terrestrial rocks would then be close to matching the elemental composition of a particular sort of meteorite known as a chondrite.
This could be a decisive piece of evidence in a . Without having seen such rocks in the mantle directly, it’s still far from an open-and-shut case, but Jagoutz is confident. “These are the rocks that were hidden in the mantle,” he says – so heavy that they almost never reach the surface for geologists to find.
This article appeared in print under the headline “Rise of the upper crust”
