IM Interview: Earth's Aneurysms

The scoop: Once upon a time, not very long ago, geologists were taught that mountains were built from below. It was either magma from below or tectonic forces squashing or folding the crust that pushed land into the sky and made mountain ranges, especially monster ranges like the Himalayas or Andes. But over the past 20 years geologists have found that just isn't enough to account for the growth of some mountains. Something else is happening. The missing piece turns out to be rain and erosion, which allows the crushing tectonic forces to have their way and squish more crust together by constantly removing the piles of crust above. It's a weird top-down way to grow mountains that goes against earlier thinking. Geodynamicist Peter Koons of the University of Maine remembers how the view of mountains changed.

larryo': Hello. You there?

pokoons: Looks as though I am set up.

larryo': Hope it wasn't too much bother.

pokoons: No, fine ...

larryo': I was hoping to get into the matter of how mountains in collisions zones grow. The roles of tectonics and erosion

pokoons: Be glad to address the subject. I hope that I can help.

larryo': Has there been a lot of other work like that of Mora? connecting extreme precip to mountain growth?

pokoons: It has been a subject pursued fairly intently for at least 20 years, first in regions of very asymmetric precipitation patterns due to orographic effects as in New Zealand, and then in areas with less obvious differences in precipitation.

larryo': Any idea how the hypothesis got started?

pokoons: Not exactly, but I do know where it was when I became interested in the late 1980's. At that time, Plate tectonics had provided a good framework for geophysical behavior, petrological observations and sedimentary records, but geomorphology [the study of landforms and their evolution] was working pretty much outside of the tectonic paradigm. Several groups, often of non-geomorphologists, became interested in the relationships of climate and tectonics. I was influenced by observations of Harold Wellman and his student John Adams in New Zealand who linked the shapes of mountains together with the kinematic description of convergence across the Pacific/Australian plate boundary that runs through New Zealand. I was working on modeling heat-flow of NZ Southern Alps during collision and thought, incredibly naively, that if I could just solve for the upper boundary of the earth, i.e. the topography, that I could finish off the heat-flow problem of conduction and convection in rapidly uplifting mountains; a problem that I was working on with my colleague, Dave Craw. Once I started the topographic modeling, it became apparent that the surface evolution could not be uncoupled from the tectonics, the velocity field. Similarly, the velocity field couldn't be uncoupled from the load of the mountains and their evolution.

larryo': velocity field. Does that mean the collective tectonic movements for the area? Or is that seismic velocity?

pokoons: By velocity field, I mean the particle velocities of material (rocks) caught up in the deformation as measured by geological observations, geodetic observations or model inferences. Basically looking at how relative plate velocities are accommodated within the plate boundaries.

larryo': So, it's sort of how fast things are squishing upwards from the collision of plates?

pokoons: Pretty much. The relationship to precipitation dervies from the influence of load removal on the stresses at depth and therfore on the pattern of rock exhumation in the squishy zones. Generaaly, the faster you erode the surface, the more you focus flow into the eroding region. Sometimes this coupling produces huge relief as in the case of Nanga Parbat and other examples of what we called Tectonic Aneurysms.

larryo': Tectonic Aneurysms! I like that. Very illustrative! So this is what made you aware that something else was going on in New Zealand?

pokoons: The tectonic aneurysm expression is something that Peter Zeitler and I came up with to describe the remarkable focusing of flow into Nanga Parbat [9th highest mountain in the world, in the Western Himalayas, Northern Pakistan] and subsequently into other large massifs that is related to coupling of erosion and tectonics. But getting back to NZ, the Southern Alps are a relatively obvious place to look for these influences because the signals of precipitation and tectonics were so high and other NZ scientists were doing ground-breaking work on many aspects of both tectonics and geomorphic systems. It was a fertile time and it coincided with interest in the problem from other mountain belts, other people.

larryo': Was the idea considered radical? Seems that I first heard bits about this in the 90s, and I found it pretty intriguing -- having been taught to think of mountain building as a bottom-up process entirely.

pokoons: Well it was certainly considered unpublishable. I cherish a particular review of my first surface evolution model, I believe the first solved for the 3D surface as a function of tectonic velocities and surface processes (bloody rough by today's standards) in which the reviewer said that no one in geomorphology was interested in this kind of work. Fortunately, someone was, actually many people were starting to do this kind of work, but it did introduce some surprises to the static view of mountains.

larryo': Ha! So it was a mini scientific revolution of sorts.

pokoons: The paradigm was changing, perhaps caught best by the Tectonics and Topography Chapman Conference organised by Mike Ellis and Dorothy Merrits.

larryo': Looks like my time is up. Would love to dive deeper into this subject at another time. Thanks for your help!

pokoons: I need to go. It was fun, cheers pok

larryo': cheers!