Cooperative Institute for Research in Environmental Sciences

Erosion

Erosion

We are involved in a number of studies to understand the affect of climate change on erosion rates, and consequently to separate the role that geodynamics (tectonics) plays from that of climate change. The motivating thought (prejudice?) is that erosion rates seem to have increased abruptly at 2-4 Ma. This inference is suggested by accumulation rates of sediment [e.g., Molnar, 2004a; Molnar and England, 1990;Zhang et al., 2001], but more importantly by assertions that mountain ranges around the world rose rapidly since ~2-4 Ma [e.g., Molnar and England, 1990]. Because plate motions have changed little in the past 5 Myr, and no process within the earth could coordinate a global rise of mountain ranges, some other global process must be responsible for the increased sedimentation and the perception of globally synchronous mountain building. Because of its concurrence, global climate change, from warm and equable to rapidly oscillating ice ages and interglacial periods, seems the likely culprit for stimulating erosion and deceiving tectonic geologists. If so, the scientific question becomes, "How did climate change affect erosion rates?"

Several possible answers to that question present themselves, such as an Increase in glaciation, a stormier climate, or increased variability, at least if we reject the idea that tectonics played a role. It follows that to understand how climate affected erosion, we must understand how rivers and glaciers erode, and what processes, including tectonic processes, affect erosion. Our efforts have pursued a potpourri of separate questions that are focused on the general question of what influences rates of erosion.

Glaciation: We exploited the simple balance between accumulation and ablation of ice with the assumption that glacial erosion varies monotonically with the flux of ice to derive simple forms for longitudinal profiles of glacially eroded valleys [Anderson et al., 2006]. A simple result is that the relatively flat longitudinal profiles imply that glacial erosion is faster than fluvial erosion, for which longitudinal profiles tend be steeper below terminal moraines than the glacially eroded parts above them. One reason that rivers in such settings erode slowly is that glaciers provide them with so much sediment that little stream power is left to transport additional material from the bottom of the river.

Most fluvial erosion (as distinguished from sediment transport) occurs in rare large floods. The ratio of the number large floods to that of smaller ones tends to be higher in arid than humid regions. Thus, one might suspect that a shift to more arid conditions would increase erosion rates [e.g., Molnar, 2001]. Yet, a shift to a more arid climate would mean less discharge in rivers, and hence less sediment transport, if not slower erosion. We examined the distribution of daily discharges along rivers in climatically different parts of the USA fitting such distributions to expressions of the form: N(Q) ~ Q-a , where N(Q) is the number of days per year with discharge greater than Q, anda is an empirical constant. We find that 1 < a < 6 , and that a is small in arid regions and larger in more temperate climates [Molnar et al., 2006]. For river incision to occur more rapidly in arid than humid environments, we found that incision would have to occur only in very rare events, those with recurrence intervals of many hundreds to thousands of years. Hence a shift to more arid conditions does not seem to facilitate more rapid erosion.

In the 19th Century, Gilbert and Dutton noted that erosion consists of two processes: disintegration and transport, and they recognized that rock can disintegrate at different rates depending on different conditions. The fracturing of rock can play a key role in the disintegration. As Miller and Dunne pointed out, such fracturing can result from stressed required to support topography, and we examined how static fatigue might accelerate erosion where valleys erode rapidly [Molnar, 2004b]. We also imagine that tectonics can facilitate erosion by fracturing rock. (Most imagine that tectonics plays it most important role in lifting rock relative to base level ("rock uplift") and therefore increasing potential energy of rivers and glaciers to erode, but, because at least on large scales, isostasy accounts for 80% of "rock uplift," tectonics itself plays only a minor role.) Fracturing might be tectonics’s most important contribution to high erosion rates in steep terrain. [Molnar et al., 2007] Molnar is not currently focused on erosion, not from lack of interest, but from lack of time or imagination.

 

References:

Anderson, R. S., P. Molnar, and M. A. Kessler (2006), Features of glacial valley profiles simply explained, J. Geophys Res., 111, F01004, doi:10.1029/2005JF000344.

Champagnae, JD; Molnar, P; Anderson, RS; Sue, C; Delacou, B (2007), Quaternary erosion-induced isostatic rebound in the western Alps. Geology, 35 (3) 195- 198, doi: 10.1130/G23053A.1.

Molnar, P. (2001), Climate change, flooding in arid environments, and erosion rates, Geology, 29, 1071-1074.

Molnar, P (2007), An examination of evidence used to infer late cenozoic "Uplift" of mountain belts and other high terrain: What scientific question does such evidence pose?. J. Geol. Soc. India, 70 (3) 395-410.

Molnar, P. (2009), The state of interactions among tectonics, erosion, and climate: A polemic. GSA Today, 19 (7) 44-45, doi: 10.1130/GSATG00GW.1

Molnar, P. (2004), Late Cenozoic increase in accumulation rates of terrestrial sediment: How might climate change have affected erosion rates?, Ann. Rev. Earth Planet. Sci., 32, 67-89.

Molnar, P. (2004), Interactions among topographically induced elastic stress, static fatigue, and valley incision, J. Geophys. Res., 109, F02010, doi:10.1029/2003JF000097.

Molnar, P., and P. England (1990), Late Cenozoic uplift of mountain ranges and global climate change: chicken or egg?, Nature, 346, 29-34.

Molnar, P., R. S. Anderson, G. Kier, and J. Rose (2006), Relationships among probability distributions of stream discharges in floods, climate, bed load transport, and river incision, J. Geophys. Res., 111, F02001, doi:10.1029/2005JF000310.

Zhang Peizhen, P. Molnar, and W. R. Downs (2001), Increased sedimentation rates and grain sizes 2–4 Myr ago due to the influence of climate change on erosion rates, Nature, 410, 891-897.

Research Group

Participants

Robert Anderson and Suzanne Anderson, University of Colorado
Jean-Daniel Champagnac, Swiss Federal Institute of Technology, Zurich, Switzerland