Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Tuesday, January 16, 2024

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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