Building the Rockies

I recently re-read John McPhee's wonderful quartet of books on geology, Basin and Range, Rising from the Plains, In Suspect Terrain, and Assembling California.  His lucid prose and capacity for focusing on the human stories connected with the subject while teaching us some fascinating science brought me back to these books, which I first read perhaps twenty-five years ago.

The first two, in particular, describe something that is quite surprising -- or at least was to me when I first learned about it.  The biggest mountain range in the United States, the Rockies, is actually quite poorly understood, and contains some features that are still yet to be satisfactorily explained.  A good part of the Rocky Mountain range is non-volcanic, and although there are some areas that have igneous rocks the vast majority is made up of sedimentary and metamorphosed sedimentary rock -- sandstone, limestone, shale, slate, quartzite, and marble.  Even some of the igneous rocks only show at the surface because the overlayment of sedimentary rock that once was present has eroded away.

[Image licensed under the Creative Commons Self, Rocky Mountain National Park, CC BY-SA 2.5]

As McPhee describes it, the current thought is that most of what is west of Colorado and Wyoming is probably the result of accretion -- the huge North American Plate overriding smaller plates to the west and gathering up microcontinents and island arcs they carried, cementing them onto the coastline.  It's certain that this is how California formed -- the boundaries between the different "suspect terranes" (the alternate spelling is used when referring to these chunks of land that end up in a very different place from where they were formed) are pretty well established.  Also, the subduction process that brought them to North America is still ongoing, as the small Explorer, Juan de Fuca, and Gorda Plates (in order from north to south) are pulled underneath -- giving rise to the Cascade Volcanoes such as Mount Lassen, Mount Hood, Mount Rainier, and Mount Saint Helens.

We got another piece added to the puzzle with a paper in Nature, out of the University of Alberta, by Yunfeng Chen, Yu Jeffrey Gu, Claire A. Currie, Stephen T. Johnston, Shu-Huei Hung, Andrew J. Schaeffer, and Pascal Audet.  Entitled, "Seismic Evidence for a Mantle Suture and Implications for the Origin of the Canadian Cordillera," the paper describes research that found a sharp boundary in the mantle of the Earth between the "craton" -- the central, oldest piece of the North American continent, encompassing what is now the Midwest -- and a long, narrow microcontinent that slammed into the North American Plate as a primordial sea closed -- moving the coastline hundreds of miles further west.

"This research provides new evidence that the Canadian section of this mountain range was formed by two continents colliding," said Jeffrey Gu, professor in the Department of Physics and co-author on the study, in an interview with Science Daily.  "The proposed mechanism for mountain building may not apply to other parts of the Rocky Mountains due to highly variable boundary geometries and characteristics from north to south."

The cool part is that the research was done by looking deep into the Earth's mantle -- not just by studying the surface features.  And this collision, which is estimated to have occurred a hundred million years ago, has left a scar that is still detectable.  "This study highlights how deep Earth images from geophysical methods can help us to understand the evolution of mountains, one of the most magnificent processes of plate tectonics observed at the Earth's surface," said study co-author Yunfeng Chen.

And this technique could be applied elsewhere, as the Rockies are far from the only mountain range in the world that were created by accretion rather than volcanism.  (The obvious examples are the Alps and the Himalayas -- the latter of which are still rising as the Indian Plate continues to plow into the Eurasian Plate.)  "There are other mountain belts around the world where a similar model may apply," said Claire Currie, associate professor of physics and co-author on the study.  "Our data could be important for understanding mountain belts elsewhere, as well as building our understanding of the evolution of western North America."

So we're piecing together the picture of how the Rockies formed -- ironic, as they seem to have been assembled from pieces themselves.  In the process, we're learning more about the processes that move the tectonic plates, and create the landscape we see around us.  It reminds me of the haunting lines from Alfred, Lord Tennyson, which seem like a fitting way to end:

There rolls the deep where grew the tree.
O Earth, what changes hast thou seen?
There where the long road roars has been
The stillness of the central sea.
The hills are shadows, and they flow
From form to form, and nothing stands,
They melt like mists, the solid lands,
Like clouds, they shape themselves, and go.

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A fracture beneath Tibet

If there's one thing I've learned from my forty-plus years of dabbling in science, it's that the universe is a weird and complex place.

It's why I frequently heard the complaint from my students that "in every science class, the first thing the teacher tells us is that everything we learned in the previous science class is wrong."  This, of course, is inaccurate and not particularly fair; it's not that the earlier tier of information was untrue so much as it was incomplete.  After all, you need a basic grasp of the underlying principles before you can understand the twists, complications, and exceptions.

Take, for example, the paper that appeared last week in Science about a strange phenomenon involving the plate tectonics under the Himalayas.

The simple model of plate tectonics is that there are three types of boundaries between plates: (1) a divergent zone or rift, where two plates are moving apart; (2) a convergent zone or thrust fault, where two plates are coming together, and one plunges beneath the other; and (3) a strike-slip fault or transform boundary, where two plates move in opposite directions alongside each other.  This broad-brush depiction can have an additional layer of complication added right away, when you consider the relative directions of motion (two colliding plates aren't necessarily, or even usually, going to be moving at right angles to the boundary, for example), and whether the plates in question are thin, dense, brittle oceanic plates or thick, lightweight, rigid continental plates.

To narrow in on the location in question, the junction between the Indian Plate and the Eurasian Plate is a convergent zone between two chunks of continental crust.  When this happens, the conventional wisdom is that the two big blocks of rock are too cold and thick to subduct, so they basically just ram into each other and crumple, forming a mountain range.  (Besides the Himalayas, another place this is happening is the Alps.)

[Image is in the Public Domain courtesy of the United States Geological Survey]

But it turns out that this picture of what's happening under Tibet is neither complete nor all that accurate.

A study out of Utrecht University looked at the seismic waves produced by earthquakes in the region, and found that they were consistent with a bizarre scenario; as it crashed into Eurasia, beginning about sixty million years ago, India has delaminated.  The bottom slice of the Indian Plate has peeled apart from the top, and that lower, denser piece is subducting, while the rest has simply smashed against the larger mass of the Eurasian Plate, creating two focal points for earthquakes, one shallow and one deep.

The real tipoff came when the researchers analyzed the gas bubbles in hot springs in the region.  Helium comes in two isotopes -- a light isotope, helium-3, and a heavier one, helium-4.  Helium-3, being less dense, tends to offgas more quickly in surface rocks, soils, and water, so a high He-3/He-4 ratio indicates a source lower in the mantle.  And springs in the southern parts of the Himalayas are depleted in helium-3, whereas northern parts have a higher than expected amount of the lighter isotope -- indicating that the bubbles coming from southern parts of the fault zone have a shallower source, but when you cross into the northern parts, suddenly the bubbles are originating from much deeper mantle material that has flowed in over the split section of the fractured plate.

A cross-section of the Himalayas, from south (left) to north (right)

So once again, we have a situation way more complex than the model you were taught in high school.  But that's the way it goes, you know?  Every time we think we have things figured out, the universe turns around and astonishes us.

And those of us who love science wouldn't have it any other way.

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