Mountain building, erosion, and climate
The purpose of this assignment is two-fold. First, it will give you a chance to cement your
understanding of isostasy and tectonics, and secondly, it will introduce you to the primary
literature.
I.
First, read the article from Scientific American, “How Erosion Builds Mountains”, by
Pinter and Brandon. This article will introduce you to the topic and should be easy to read
and understand.
II.
Second, tackle the article from the journal Geology, “Climate, tectonics, and the
morphology of the Andes”. This article is short, but dense.
After you have read both articles, then you should answer the questions below.
As you read, keep a list of words or phrases you don’t understand. Write them down, and attempt
to write a definition. Also, indicate how important that word/phrase/concept seems to be to the
paper (this should help you determine whether or not to spend your time figuring it out). Here’s
one giveaway, and one to start with:
a. An orogeny is a mountain-building process. Therefore, an orogen or orogenic belt
(which are perfectly synonymous words) is a mountain range, and so is an orocline
(this has a slightly different definition, but it’s not critical to understand).
b. Geomorphometric (beg. of 2nd ¶ of intro to Montgomery, et al paper):
Answer questions 1 and 2 based on the Pinter and Brandon article, “How Erosion Builds
Mountains”.
1. Describe why mountain ranges are best viewed as a system. Be specific and give at least
two examples. Summarize in 3 to 4 sentences.
2. Describe the new model of mountain development discussed by the authors. How is it
different from old models? Why is it better? Describe in 3 to 5 sentences.
Answer questions 3 through 9 based on the Montgomery, et. al article, “Climate tectonics and
the morphology of the Andes”.
3. Why did the authors do this study – what research questions were they addressing? (1-2
sentences) Specifically, why did they choose to look at the Andes? (1-2 sentences)
4. What techniques were used to collect data for this paper? There are four – describe the
four techniques in one sentence each using your own words.
5. What were the results? Summarize in 2-3 sentences.
6. What do the authors conclude? What is the new thing that they have added to our
knowledge? Summarize in 2-3 sentences.
7. What are the unanswered questions that remain, or what new questions have been
generated by this work? 1-2 sentences
8. How do the figures fit in? Summarize the role of each figure in one sentence each (there
are three figures, so that should be three sentences, though the first one might be kind of
long…)
9. Obviously, the audiences for the two papers are quite different. Who is the audience for
Scientific American? Who is the audience for Geology? Based on your understanding at
this point, how does the Scientific American article do at translating complex science for
its audience?
Climate, tectonics, and the morphology of the Andes
David R. Montgomery
Greg Balco
Sean D. Willett
Department of Geological Sciences, University of Washington, Seattle 98195-1310, USA
ABSTRACT
Large-scale topographic analyses show that hemisphere-scale climate variations are a
first-order control on the morphology of the Andes. Zonal atmospheric circulation in the
Southern Hemisphere creates strong latitudinal precipitation gradients that, when incorporated in a generalized index of erosion intensity, predict strong gradients in erosion
rates both along and across the Andes. Cross-range asymmetry, width, hypsometry, and
maximum elevation reflect gradients in both the erosion index and the relative dominance
of fluvial, glacial, and tectonic processes, and show that major morphologic features correlate with climatic regimes. Latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential, indicating that,
like tectonics, nonuniform erosion due to large-scale climate patterns is a first-order control on the topographic evolution of the Andes.
Keywords: geomorphology, erosion, tectonics, climate, Andes.
INTRODUCTION
The presence or absence of mountain ranges at the global scale is determined by the location and type of plate boundaries. Other factors become important in the evolution of
individual mountain systems. In particular,
spatially variable erosion resulting from climate gradients may localize exhumation and
deformation in orogens and thereby influence
the geologic structure and morphology of
mountain ranges (Beaumont et al., 1991; Zeitler et al., 1993; Avouac and Burov, 1996).
Earlier studies of climatic geomorphology
have limited relevance to this issue because
they simply classify Earth into normal (fluvial), glacial, and arid zones and generally depict an alpine area as a single morphoclimatic
zone that crosscuts multiple low-elevation
morphoclimatic zones (Tricart and Cailleux,
1972). Even though the large-scale morphology of mountain belts must record the combined effects of climatic and tectonic processes, only a few studies explore climatic factors
(Willett et al., 1993; Brozovic et al., 1997).
Here we show that geomorphometric parameters such as cross-range asymmetry, hypsometry, and maximum elevation of the Andes reflect the influence of zonal climate
regimes on the nature and intensity of erosional processes. In addition, we show that
consequent latitudinal gradients in erosion potential are correlated with the crustal mass distribution and inferred orogenic shortening of
the range, suggesting an ambiguity in the current interpretation of crustal mass distribution
as the result of variations in the tectonic environment. On the basis of these observations
we argue for the first-order importance of
large-scale climate zonations and resulting differences in geomorphic processes to the morphology of mountain ranges.
TECTONIC AND CLIMATIC SETTING
OF THE ANDES
The influences of climate, erosional processes, and tectonics on orogen morphology
may be deconvolved in the Andean orogen because it is a hemisphere-scale, north-south–
oriented range with large gradients in temperature and rainfall across a single convergent
margin. Uplift of the Andes began ca. 25 Ma,
concomitant with accelerated convergence between the Nazca and South America plates
(Allmendinger et al., 1997). Early theories of
formation of the Andes emphasized crustal
growth by magmatic processes, but estimates
of structural shortening and evidence for symmetric paleomagnetically defined rotation on
the northern and southern flanks of the Altiplano gave rise to the hypothesis that the variable size and thickness of the range result
from nonuniform crustal shortening, with
maximum shortening and consequent thickening at the center of the Andean orocline (Isacks, 1988; Gregory-Wodzicki, 2000). However, direct structural shortening estimates are
limited to the Eastern Cordillera and Subandean fold and thrust belt. In the Altiplano and
Western Cordillera, crustal structures are obscured by sedimentation or volcanism, and
global positioning system measurements (Norabuena et al., 1998; Kendrick et al., 1999)
may be influenced by short-term strain accumulation associated with the subduction-zone
earthquake cycle. Some studies have attributed local variations in structural, metamorphic,
and geomorphic characteristics of the central
Andes to erosion (Gephart, 1994; Masek et al.,
1994; Horton, 1999), but none has considered
variations in erosional mass removal at the
scale of the entire mountain range.
The highly variable climate of the Andes
reflects its position transverse to hemispherescale, Hadley cell-driven precipitation regimes
(Fig. 1). In the Intertropical convergence zone
(108N–38S), both sides of the range receive
annual rainfall exceeding 2 m·yr21. In the subequatorial northern Andes (38S–158S), orographic interception of the trade winds delivers .2 m·yr21 of rainfall to the Amazon side
of the range and ,0.2 m·yr21 to the Pacific
side, and westerly winds produce the opposite
relationship in the temperate latitudes south of
338S. The central part of the range (158S–
338S) is in the subtropical belt of deserts,
where there is little precipitation on either side
of the range, or on the high plateau of the
Altiplano. These major climate boundaries in
the Andes are not dependent upon orographic
effects, but are robust features of the general
circulation in the Southern Hemisphere, and
therefore may be considered a priori conditions under which the mountain range
developed.
TOPOGRAPHIC ANALYSIS
We focus on four aspects of the large-scale
geomorphology of the Andes: (1) a generalized index of erosion intensity based on regional slope and fluvial discharge, (2) crossrange asymmetry, (3) regional hypsometry
q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400.
Geology; July 2001; v. 29; no. 7; p. 579–582; 3 figures.
579
Figure 1. A: Maximum (dark line) and mean (gray area) elevation in 18 latitude bins. Red circles are elevations of modern perennial snowline
and blue circles are lowest elevation of Pleistocene glacier extent, both from Schwertfelder (1976). B: Topography and convergence
velocity. Vectors headed in open circles denote long-term velocity of Nazca and Antarctic plates relative to South American plate (DeMets
et al., 1994); those headed in closed circles denote global positioning system (GPS) velocities at coastal sites, relative to stable South
America (Norabuena et al., 1998; Kendrick et al., 1999). C: Mean annual precipitation, overlain on shaded-relief map of western South
America. D: False-color image of South America showing areas with steep slope in yellow, high precipitation in blue. Red pixels have
calculated IE above 90th percentile relative to all pixels in image. E: Cross-range asymmetry, defined to be fraction of range volume above
sea level that drains to west: values greater than 0.5 (lighter shade of gray) indicate that bulk of range is west of divide.
(the elevation distribution of the topography),
and (4) the relationship between the maximum
elevation and the perennial snowline. We used
topography from the global 30 s GTOPO30
digital elevation model; topography, slope,
and flow direction from the 1 km HYDRO1K
DEM; and mean annual precipitation digitized
from Hoffmann (1975). For purposes of our
analysis, we defined the eastern boundary of
the Andes as the approximate limit of Tertiary
or older units mapped on continental-scale
geologic maps (UNESCO, 1978).
Erosion Index
Rates of fluvial and hillslope erosion are
governed by processes characterized by different erosion laws, but the net large-scale
erosional potential of a landscape increases
with precipitation, drainage area, and slope.
Thus, we evaluated large-scale patterns in erosion potential by using a simple parametric
measure of erosional intensity (IE) based on
the product of local slope (S) and discharge
580
determined by summing the annual precipitation (P) over the matrix of upslope grid cells
each of drainage area A:
IE 5
O P A 4S.
3
i
i
(1)
We used this simple approach because (1) it
is not clear which process formulation is most
appropriate for modeling landscape-scale erosion rates across 1 km grid cells in which net
erosion reflects an aggregation of finer scale
effects from multiple, interacting processes;
(2) vegetation and land use, which cannot be
predicted from digital elevation models, complicate simple relationships between precipitation and erosion rate; (3) erosion models at
this scale inherently require calibration because slopes calculated from coarse-resolution
grids are gentler than actual gradients (Zhang
and Montgomery, 1994); and (4) data on differences in erosivity due to soil type and parent lithology generally are not available at the
scale of interest. In the Andes, the pattern of
IE values shows that the zone of maximum
predicted erosion is on the eastern side of the
range in the northern Andes and on the western side in the southern Andes. Only small,
localized areas of high IE are predicted in the
central Andes (Fig. 1D).
Cross-Range Asymmetry
We defined a cross-range asymmetry index
as the ratio of the volume of the topography
above sea level on the west side of the divide
to that of the entire range within a given latitude band (Fig. 1E). Between 28S and 428S
most of the range is to the east of the drainage
divide, whereas south of 428S most of the
range is west of the drainage divide. North of
28S, the inclusion of the areas draining to the
Caribbean Sea with areas draining to the Pacific Ocean places most of the range on the
west side of the drainage divide. Cross-range
asymmetry tracks latitudinal variations in
moisture delivery due to prevailing wind
directions.
GEOLOGY, July 2001
1957), here the aggregate pattern is geographically consistent with variations in erosional
processes. In the northern Andes, concave-up
hypsometric curves, which are characteristic of
fluvially dissected landscapes, reflect the dominance of fluvial erosion in a wet tropical climate. In the southern part of the range, glaciers
have selectively eroded high elevations, creating a shoulder in the hypsometric curves. In
the central Andes, the hypsometric curves are
nearly linear, with a convexity imposed by the
relatively flat hypsometry at elevation of the
Altiplano. This form describes a weakly incised
tectonic wedge and mechanically limited plateau, implying that fluvial incision is ineffective
relative to tectonic uplift. This strong association of hypsometry with climatically driven
variations in geomorphologic processes demonstrates that both the nature of the dominant
erosional mechanism and its rate relative to tectonic uplift are fundamental to the overall topographic expression of the Andes.
Figure 2. Normalized hypsometric curves for 38 latitude bins of Andes; curve color
corresponds to location in northern (red), central (yellow), and southern (blue) Andes.
Hypsometry
Hypsometric curves, which show the proportions of a landscape at different normalized
elevations, have strikingly different, but regionally consistent, shapes in the northern, central,
and southern Andes (Fig. 2). These latitudinal
variations suggest that fluvial, tectonic, and glacial processes, respectively, dominate the morphology of the range in these different zones.
Although individually these hypsometric
curves could reflect different developmental
stages in a classical interpretation (Strahler,
Figure 3. A: Volume of Andes above sea level calculated from 18 latitude bins. B: Excess erosion
rate, relative to largest 18 bin, is required to explain volume difference under uniform tectonic
convergence. We calculated required latitudinal variation in erosion rates under constant tectonic convergence by calculating missing mass above sea level in each 18 latitude bin as VXSa/
At, where VXS is excess volume in given bin compared to largest bin (148–158S), A is bin area,
t is time (taken to be 25 m.y.), and a 5 rc/(rm 2 rc), where rc 5 2.7 g·cm23 and rm 5 3.3 g·cm23.
Note that because of selection of strictly east-trending bins for analysis, region between 138S
and 178S, where range trends northwest rather than north, has anomalously large volume in
each bin. C: Mean annual precipitation. D: Mean erosion intensity index value.
GEOLOGY, July 2001
Maximum Elevation
The tendency for the elevation of the perennial snowline to track mountaintops is well
known (Mill, 1892), but the causal basis for
this relationship and the relative efficiency of
glacial erosion remain more controversial. In
the Andes, the maximum elevation and the
snowline are greater than 5 km north of 308S,
and both decrease toward the pole thereafter,
such that only a small fraction of the topography remains above the snowline at any latitude (Fig. 1A). The distinct shoulder to the
hypsometry of the southern Andes also descends with the perennial snowline. The correspondence betweeen total relief and snowline elevation supports the hypothesis that
higher rates of erosion in glacial and periglacial environments effectively limit the relief of
mountain ranges (Brozovic et al., 1997). This
implies that high topography cannot persist at
high latitudes and that the high Andes terminate at 358S in part because they intersect the
perennial snowline at this latitude.
DISCUSSION
The observation that topographic changes
along the Andes correspond with large-scale
variations in climate suggests that zonal climate
patterns affect the orogen-scale morphology of
the Andes. This conclusion has implications
both for general understanding of landscape
evolution and for specific large-scale tectonic
interpretations for the Andes. For example, Isacks (1988) neglected the effect of mass removal by erosion when inferring latitudinal
variations in convergence from a crustal mass
balance. However, the latitudinal variations in
mean IE also track variations in present excess
crustal volume in the Andes (Fig. 3). An ex-
581
treme interpretation of this correlation, taking
the opposite assumptions to the analysis of Isacks (1988), would hold net convergence constant from 458S to 58N and explain the current
width of, and crustal volume in, the Andes as
the result of latitudinal variations in erosion
rate. In this case, the observed distribution of
crustal volume requires latitudinal variations in
average erosion rate during the lifetime of the
Andes of ,2 mm·yr21 (excluding the glaciated
southern Andes). These required variations are
within the range of reasonable erosion rates and
broadly correlate with the independently determined IE values. Hence, it is reasonable to suggest that climatically influenced gradients in
erosion rates contribute to the latitudinal variation in range width and crustal volume.
Other local structural variations may be the
result of variable erosion. Range-wide changes
in geology are broadly consistent with this idea:
the crystalline rocks of the northern and southern Andes reflect deeper exhumation, and the
preserved sedimentary and volcanic cover of
the central Andes indicates that exhumation
there has been minimal. For example, the Eastern Cordillera and Subandean zone of Bolivia
have undergone 2–6 km of exhumation since
10 Ma north of 198S (Benjamin et al., 1987),
but ,1 km since that time to the south (Masek
et al., 1994; Gregory-Wodzicki, 2000). This
difference is immediately apparent in the truncation of the prominent fold-and-thrust belt by
the apparent erosional ‘‘bite’’ in the area with
high rainfall to the north.
We are not arguing that tectonic variations
are unimportant in the evolution of the Andes.
In fact, the major changes in the topography
and mass distribution in the Andes also correlate with tectonic parameters such as the orientation and dip of the subducting slab (Jorden
et al., 1983; Gephart, 1994) and major geologic
provinces (Gansser, 1973). For example, the
high volume segment of the central Andes between 108S and 308S corresponds to the steeply
dipping segment of the Nazca slab, particularly
when one recognizes that our analysis likely
overestimates the mass between 138S and 178S
(Fig. 3). This situation complicates the interpretation of crustal mass distribution as the result of one or the other of these two seemingly
covariant forcings. We view tectonics and erosion as a coupled system, with potential for
feedback between climate-driven erosion and
tectonic forcing on shallow crustal processes
(Willett, 1999), or even deep, mantle processes.
Could the large accumulation of mass in the
Altiplano, which seems to have been permitted
by slow surface erosion, also affect the dynamics of the subduction zone?
CONCLUSIONS
Our results support the idea that global climate patterns influence orogen morphology.
582
Specifically, we see three archetypes of climatic
control on large-scale landscape form: (1) normal fluvial erosion in the northern Andes where
high precipitation rates maintain a narrow
mountain range; (2) tectonic dominance of
landscape form in the central Andes, where
there is little erosion except in big river valleys,
leading to crustal thickening by tectonic wedge
propagation, the formation of a mechanically
limited plateau, and linear hypsometry; and (3)
glacial land sculpting that preferentially erodes
the highest ground in the southern Andes, resulting in an excess of elevation at the glacial
limit and a systematic decline in maximum elevation toward the pole. The coincidence of
low inferred erosion rates (on the basis of calculated IE values) in the desert latitudes and the
greatest width of the Andes suggests that lack
of erosion plays an important role in mass accumulation in the mountain belt. If the development of the Altiplano reflects the mechanical
limit to crustal thickening (Pope and Willett,
1998), then its existence implies that tectonic
thickening has outpaced erosional mass removal; its position in the global desert belt suggests
that this dominance of tectonic shortening was
possible, at least in part, because of the arid
climate of this latitudinal band. We conclude
that the large-scale distribution of crustal mass
in a mountain belt is controlled by not only
tectonic shortening, but also by the type and
intensity of erosional processes.
ACKNOWLEDGMENTS
Supported by a Hertz Foundation Graduate Fellowship (to Balco) and in part by National Science
Foundation grant EAR-9903157. We thank Peter
Zeitler and Bryan Isacks for their constructive critiques of the manuscript.
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Manuscript received November 7, 2000
Revised manuscript received March 7, 2001
Manuscript accepted March 15, 2001
Printed in USA
GEOLOGY, July 2001
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