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T GROUNDWORK Furthering the Influence of Earth Science The state of interactions among tectonics, erosion, and climate: A polemic

Peter Molnar, Dept. of Geological Sciences, Cooperative what “controlled” the change in those quantities, or what Institute for Research in Environmental Sciences (CIRES), “drove” them. The occurrence of change, external to “equation University of Colorado, Boulder, Colorado 80309-0399, (1),” transforms questions that concern drivers or controls from USA; [email protected] being ill posed to well posed. In the earth sciences, perhaps no concept illustrates better a state of quasi-equilibrium than isostasy. Isostasy is not a pro- Since Dahlen and Suppe (1988) showed that erosion can cess; it exists. On short time scales (thousands of years), vis- affect the tectonics of the region undergoing that erosion, cous deformation can retard the inexorable trend toward geodynamicists, tectonic geologists, and geomorphologists isostatic equilibrium, as is illustrated well by the rebound fol- have joined in an effort to unravel the interrelationships lowing the melting of Pleistocene ice sheets. Thus, on such among not only erosion and tectonics but also climate, which time scales, one might understand pressure differences in the affects erosion and is affected by tectonics. The fruits of this asthenosphere as “driving” the overlying lithosphere and sur- effort grow continually, as shown in part by the attraction that faces of Canada or Fennoscandia upward. On longer geologic this subject has for students and young scientists. Yet, it seems time scales, however, isostasy is maintained as a state of equi- to me that progress has been limited by a misconception that librium; like the perfect gas law, “isostasy,” therefore, does not has stimulated this polemic. Much effort seems to be expend- drive anything, much as traffic laws do not “drive” cars. ed in trying to understand what “controls” erosion, tectonics, Dahlen and Suppe (1988) considered the state of stress with- or climate, or what “drives” erosion, “rock uplift,” climate, and in a wedge of sediment obeying Coulomb friction and the so forth. Although these questions are not meaningless in all equilibrium cross sectional shape of that wedge. It seems un- cases, enough are. likely that erosion would suddenly excavate a divot from a fold My thesis is that erosion, climate, and tectonics interact with and thrust belt, but it is easy to imagine that erosion rates could one another in a state that is usually and in most regions ap- change relatively quickly on geologic time scales, for instance proximated as quasi-equilibrium. “Usually,” because even if we because of some change in climate (e.g., more typhoons). ignore major storms, which are short-lived on any geologic Dahlen and Suppe’s work predicts that the deformation field time scale, climate changes continually with orbital pacing, and would respond comparably rapidly to maintain the cross sec- tectonic processes also change, though more slowly. “Most re- tional shape of the wedge, so as to maintain a state of quasi- gions,” because there always is a place where the unexpected equilibrium. occurs. “Quasi-equilibrium,” because in a system as nonlinear Suppose we recognize that steep terrain offers its erosive as either climate or erosion, if not tectonics too, equilibrium agents more energy to do their job than gentle terrain, that cannot be sustained for long, and geologic time is long on the more precipitation will remove more material (erode faster) time scales of changing climate and erosion, if not tectonics. than less precipitation, but that the stormier the climate the An equation of state describes equilibrium. A good example faster the erosion, and that tectonics can elevate terrain, which is the perfect gas law: then is subject to erosion. I offer the following mnemonic:

PV = nRT . (1) Precipitation rate × Vertical component of velocity = number of floods per annual precipitation × P is pressure, V is volume, n is the number of moles of a gas, Rock removal rate × Tectonic movement. (2) R is the gas constant, and T is absolute temperature. An equa- tion of state, and likewise an equation of equilibrium, does not Here, the Precipitation rate is some average, such as mean assign cause or effect, and hence does not attribute a control- annual precipitation, a measure that can distinguish wet from ling factor or driver to any of its elements, but simply expresses arid environments. The Vertical component of velocity refers to a balance. Of course, if one changes the pressure, then at least movement of rock relative to base level, and might be called one of volume, temperature, and conceivably the number of “rock uplift rate” except that that term is usually used without moles of gas must change. Here it might make sense to ask a clearly defined reference frame. The number of floods per

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44 JULY 2009, GSA TODAY annual precipitation would characterize the recurrence inter- Most of the preceding words are meant to urge geodynami- val of some kind of typical flood, such that a small number cists, tectonic geologists, and geomorphologists to be more means that most of the precipitation comes during a few floods, careful in how they view the interactions among tectonics, cli- and a large number would imply little variability in precipita- mate, and erosion. Yet, an exciting part of these subjects lies tion. The Rock removal rate is the erosion rate or, perhaps where equilibrium does not apply. For instance, to what extent mnemonically better, the e Rosion rate. To be consistent with does the relief that prevented elephants from reaching Rome or how it is commonly used in geomorphology, Tectonic move- that challenges mountaineers reflect a transient state, and per- ment is intentionally vague, but refers to processes that elevate haps one that is decidedly not in equilibrium, because changes the earth’s surface, either by crustal thickening or by the altera- in erosion rates induced by climate change occur faster than tion of mantle structure. Note that (2) is a mnemonic, not an the landscape can evolve? Moreover, there is a difference be- equation to be solved. tween (1) and (2) that should not be overlooked. The beauty Nevertheless, (2) captures much of the common sense state- of (1) lies in the state that it describes, and in that context, each ments that began the previous paragraph. With no changes to of P, V, n, R (especially), and T need not, individually, make climate (P or n) and to erosion (R), tectonic movements (T) for interesting study. In (2), beauty is in the mind of the be- scale with the rate that rock goes up (V). In the absence of holder, and real progress may come more from understanding tectonics (T) or climate change (P or n), because of isostasy, each of P, V, n, R (again, especially), and T independently of the erosion rate (R) will scale with the rate that rock goes up the others, than from studying the interactions among them. (V). All else being equal, increased precipitation (P) will be proportional to the erosion rate (R). Moreover, if that precipita- Manuscript received 29 January 2009; accepted tion (same P in each case) occurs in rare storms (small n), 2 February 2009. erosion will be faster (big R) than if precipitation is steady (large n and small R). Of course, the mnemonic is imperfect, REFERENCE CITED not just because it ignores glaciers. For instance, according to Dahlen, F. A., and Suppe, J., 1988, Mechanics, growth, and erosion of moun- (2), in the absence of erosion (R) and tectonics (T), precipita- tain belts, in Clark, S.P., Burchfiel, B.C., and Suppe, J., eds., Processes in tion (P) would scale inversely with the rate of vertical move- Continental Lithospheric Deformation: Geological Society of America Special Paper 218, p. 161–178. ment of rock (V), which is nonsense (if, however, increasing V might, via orographic precipitation, call for an increase in P). Anyhow, if (2) were perfect, wouldn’t that be a gas?

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