One Hundred and One Years of Asthenosphere
We like to talk about the plate tectonics revolution and how that overturned static views of the deformation of the earth. But there was a pretty substantial overturn that dates back to a long series of papers in 1914 to 1915 as World War I was raging in Europe that kind of gets lost in the shuffle and it seems appropriate to celebrate the sort-of hundredth anniversary of this work. Joseph Barrell published what is probably the longest serial publication in the earth sciences, an eleven part paper emerging over all of 1914 and well into 1915 in the Journal of Geology (he actually labeled it as being in 8 parts, but three of those were published in two parts). [One wonders if the bean counters of the day felt he had published but one paper or 11]. It was in part 6, late in 1914, that Barrell finally proposed that there had to be a zone of weakness under the stronger crust and uppermost mantle which he named the asthenosphere (he had presented the terms somewhat earlier in a presentation in April of 1914). Unlike concepts like the crust and mantle that emerged and evolved over time, the asthenosphere emerges full-blown in these papers:
The theory of isostasy shows that below the lithosphere there exists in contradistinction a thick earth-shell marked by a capacity to yield readily to long-enduring strains of limited magnitude. But if such a zone exists it must exercise a fundamental control in terrestrial mechanics, in deformations of both vertical and tangential nature. It is a real zone between the lithosphere above and the centrosphere below, both of which possess the strength to bear, without yielding, large and long-enduring strains. Its reality is not lessened because it blends on the limits into these neighboring spheres,nor because its limits will vary to some degree with the nature of the stresses brought upon it and to a large degree by the awakening and ascent of regional igneous activity. To give proper emphasis and avoid the repetition of descriptive clauses it needs a distinctive name. It may be the generating zone of the pyrosphere; it may be a sphere of unstable state, but this to a larger extent is hypothesis and the reason for choosing a name rests upon the definite part it seems to play in crustal dynamics. Its comparative weakness is in that connection its distinctive feature. It may then be called the sphere of weakness-the asthenosphere…
This massive paper, in addition to naming the asthenosphere, reviewed evidence of many sorts, including early seismological observations, to reconcile the apparent strength of the shallow crust and the transmission of shear waves to the geodetic evidence for isostasy.
Just why was this so important?
Most of us are taught about the origin of isostasy in 1855 when Pratt first documented the discrepancy in geodetic measurements near the Himalaya, Airy interpreted this as due to variations in crustal thickness, and Pratt countered that this could be variations in crustal density. Airy’s description, though, flew in the face of understanding of earth materials, suggesting that the crust floated on a liquid or perhaps “viscid” material, his evidence essentially being the relatively small relief on the surface of the earth as a whole. (Calling this material “lava” no doubt made some geologists cringe). The geological implications of this observation were slow to be appreciated although isostasy was embraced by geodesists because the concept greatly reduced the errors in their measurements.
Arguably one of the most prescient presentations on the geological implications of isostasy was made by G.K. Gilbert in 1890 at the first meeting of the Geological Society of America, where he presented a short paper entitled “The strength of the earth’s crust,” a title Barrell later appropriated out of respect for this earlier work. Gilbert used the recently coined term isostasy (created by Dutton) and showed that it operated at certain time and length scales. Among other things, Gilbert came perilously close to obtaining what we would today call the elastic plate thickness in the western U.S. by considering the scales at which isostasy did and did not operate. The most famous geologic observation he used in this presentation was the deformation of the shorelines of pluvial Lake Bonneville in the Salt Lake Valley and surroundings. He inferred this before there were significant measurements of gravity in the region to show that the geodetic model really applied to the region (he would participate in making some of those measurements in subsequent years).
Despite this presentation, isostasy met with great opposition from (or was simply ignored by) geologists: they simply knew that things got stronger with depth, and the new seismological observations of the transmission of shear waves (transverse waves in these early papers) deep into the earth seemed to show that isostasy was not a useful concept. If you piled up crustal material at the surface, how could it sink into the earth in order to produce the mass deficit isostasy demanded?
Barrell’s monstrous paper disassembled this notion from start to finish; his conclusion that there had to be a zone of weakness provided a major clue in understanding continental tectonics. Although it would take several decades for the concept to fully sink in and become an accepted part of the geological canon, the asthenosphere is now a well regarded part of earth structure.
So when you learn of Pratt and Airy, it is probably worthwhile to contemplate Barrell as well.
Of course, that does’t mean there isn’t some disagreement about exactly what it is and just how weak it really is…but that is another story.