Download Holocene yardangs in volcanic terrains in the southern Andes

Earth Surface Processes and Landforms
Earth Surf. Process. Landforms 26, 657–666 (2001)
HOLOCENE YARDANGS IN VOLCANIC TERRAINS IN THE
SOUTHERN ANDES, ARGENTINA
MOSHE INBAR1* AND CORINA RISSO2
Department of Geography, University of Haifa, 31905 Israel
2
Departamento de Geologı́a, Universidad de Buenos Aires, Buenos Aires, Argentina
1
Received 1 June 2000; Revised 24 October 2000; Accepted 8 December 2000
ABSTRACT
Yardangs of different sizes were developed in the Payun Matru Volcanic Field, a semiarid area east of the southern Andes
mountains. Yardangs from volcanic terrains have not been described previously from Earth, although studies from Mars
interpreted linear ridges found by Mariner and Viking images as yardangs.
The Payun Matru Volcanic Field is an extensive plateau at 2000 m a.s.l. covered by basaltic lava and ignimbrite flows.
Strong westerly winds affect the extensive plateau. Micro- and mesoyardangs are formed on the ignimbrite rock blanket, and
macroforms or megayardangs, several kilometres in length, are developed in the basaltic lava flows as long parallel troughs.
They all have a distinctive 320° azimuth, which is the prevailing wind direction. No yardang features are noted in the more
recent lava flows, younger than 1000 years, indicating that their formation needed a longer time or they developed in earlier
periods with stronger winds.
The yardang development is explained by the strong unidirectional winds, the poor vegetational cover due to the aridity of
the region, the available quartz sand and volcanic ash particles as abrasive agents, and the volcanic lithology texture and flow
structure. Copyright # 2001 John Wiley & Sons, Ltd.
KEY WORDS: yardangs; wind erosion; volcanic
geomorphology; Holocene; Payun Matru; southern Andes
INTRODUCTION
Wind erosion surfaces and forms are known in many arid and semiarid regions in the world (Breed et al.,
1989). However, yardangs are not well reported and studied. Yardangs are defined as positive topographic
forms a metre to a kilometre long, elongate and streamlined, resulting from erosion of lithified substrates by
the wind (McCauley et al., 1977). They were originally named from a local word in Chinese Turkistan
(Hedin, 1903). Their typical shape is of an inverted boat, and they are streamlined according to the prevalent
wind direction, with length to width ratios of 3:1 or 4:1 (Grolier et al., 1980). Their best development is noted
on large plateaus, where strong winds are unidirectional (Laity, 1994).
Yardangs from volcanic terrains have not been previously described on Earth; however, there are studies
from Mars, where linear ridges found by Mariner 9 and Viking images of volcanic terrain were eventually
interpreted as yardangs (Ward, 1979). They were also recorded in the geological record, in the Permian of
Utah (Tewes and Loope, 1992).
The southern Andes and Patagonia of South America are characterized by strong westerly winds, but few
research efforts have been made on the aeolian morphology and processes in this region. The only reported
yardangs from South America are from the Pacific coastal arid area of Peru (McCauley et al., 1977a).
During a volcanic geomorphological study carried out by the authors in 1998 and 1999, linear features
were recognized as yardangs in airphotos and checked in the field. Previous geological works in the area
identified them as structural geomorphological features (González Dı́az, 1972; Llambias, 1966).
* Correspondence to: M. Inbar, Department of Geography, University of Haifa, Mount Carmel, Haifa 31905, Israel. E-mail:
[email protected]
Contract/grant sponsor: Universidad de Buenos Aires; contract/grant number: UBA-Ex138
Contract/grant sponsor: University of Haifa
Copyright # 2001 John Wiley & Sons, Ltd.
658
M. INBAR AND C. RISSO
Figure 1. The Payun Matru Volcanic Field
The aim of this paper is to report on yardangs in the Payun Matru Volcanic Field, a semiarid area east of the
southern Andes mountains, and to analyse the geomorphic factors for their formation, and their relationship to
the upper Pleistocene and Holocene lava flow events in the area.
THE PAYUN MATRU VOLCANIC FIELD
The Payun Matru Volcanic Field (PMVF) is located between latitudes 36 and 37°S and longitudes 70 and
68°W in the Mendoza province of Argentina, about 100–200 km east of the main Andes Cordillerra. It is an
extensive plateau, covering an area of 5200 km2 at an altitude of 2000 m, and contains about 300 volcanic
vents, most of them to the west of the main andesitic Payun Matru shield volcano (Figure 1). The Payun
Matru plateau is enclosed to the west by the Andes chains that rise up to more than 3000 m; on the eastern side
it is bordered by low hills and the pampas lowlands. Older lava flows are of the pahoehoe type, while recent
ones are typical aa blocky flows, with elongated crests and sharp lobal edges. The older cones and lava flows
are of Plio-Pleistocene age, but the majority of the cones and lava flows are of a young Upper Pleistocene to
Holocene age. No volcanic activity has been witnessed in the last 200 years, but local oral traditions refer to
eruptions in the area, and it is unlikely that these traditions last more than hundreds of years.
The basalt rocks are of the alkaline olivine type, with 46–52 per cent SiO2 and 35–6 per cent alkalines
(Na2O ‡ K2O). Rock texture is porphyritic with granular matrix and olivine and plagioclase phenocrysts.
Matrix minerals are olivine, plagioclase and pyroxene microlithics. The trace element composition indicates
a common genetic source, and the basalts have a geochemical composition characteristic for backarc and
orogenic environments (Bermúdez and Delpino, 1989).
The climate is semiarid, with annual precipitation around 200 mm. Summers are hot and winters cold with
average temperatures of 20 °C for January and 2 °C for July. Strong westerly winds affect the area, and the
predominant wind directions are northwest and west (Servicio Meteorológico Nacional, 1991). Soils are loess
type and shallow, mostly rocky; vegetation is xerophytic and mostly composed of shrubs and herbaceous
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HOLOCENE YARDANGS IN VOLCANIC TERRAINS
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Figure 2. The ignimbrite flow blanket on the southwestern flank of the Payun Matru volcano, partially covered by a sub-recent lava
flow
taxa. The possibility that the yardangs were formed by glacial processes is rejected because there are no
glacial features in the area. The actual snowline is above 3000 m, and probably was above most of the area
during the glacial time, excluding the higher parts of the larger volcanoes.
Sandstone sedimentary formations, about 20 km to the west of the PMVF, are the source of sand blown to
the volcanic area; small deposits of sand are found on the volcanic cone escarpments and in the yardang areas.
THE IGNIMBRITE YARDANGS
On the western slopes of the composite Payun Matru volcano, a pyroclastic flow forming an ignimbrite
Copyright # 2001 John Wiley & Sons, Ltd.
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M. INBAR AND C. RISSO
Figure 3. U-shaped yardangs carved in the ignimbrite rock. Length is 40–50 m, width 25 m and depth 03 m
blanket of early Holocene age covers several square kilometres (González Dı́az, 1972). On the lower part of
the escarpment a sub-recent basaltic lava flow partially overlies the ignimbrite blanket (Figure 2). The
ignimbrite rock is consolidated, with distinctly elongated crystals, and the formation is known as Tobas del
Portezuelo (Llambias, 1966) or El Portezuelo (González Dı́az, 1979). The rock is crystalline, welded with a
dark pink colour and it contains many volcanic clasts from different sources. It has a porphyritic texture with
30–40 per cent phenocrysts, mainly plagioclase and in lesser proportion clinopyroxene and olivine minerals.
The plagioclase phenocrysts are broken, angular and filled by the matrix material. The matrix is mainly
hyallophyllitic and consists of feldspar tables and interstitial glass, with apatite, zircon and magnetite as
minor minerals.
The ignimbrite blanket is homogeneous with an annular form around the old Payun volcano dome. The best
exposures are to the northeast and southeast of the volcano, being covered in other areas by younger lava
flows (González Dı́az, 1972). Two kinds of yardangs are found in this lithology: micro and meso forms,
according to the division proposed by Cooke et al. (1993). The microyardangs are long, parallel, U-shaped
channels, 50 m long, 25 m wide and 03 m deep, carved into the ignimbrite blanket (Figure 3). The largest is
100 m long and 8 m wide. The channel floor is smooth with ripple-shaped ridges parallel to the main yardang
trough direction. The erosive straight channels are in a 320° direction, similar to the prevailing winds, and are
partially covered by blown quartz sand grains derived from the exposed sandstone formations to the west of
the area. Figure 3 illustrates the cross-section and shape of the ignimbrite trough, showing an accumulation of
quartz sand on the lee side; the lack of vegetation may indicate that strong winds winnowing fine particles
impede the growth of plants and root development.
Other wind-shaped features in the same ignimbrite rock are the demoiselles, mesoyardangs about 15 m
high and 3–4 m long, well streamlined with the bow part to the wind direction, very similar to the ones carved
in the Tarawan Chalk of the Farafra Depression in Egypt (Breed et al., 1989, p. 286). They are distributed on a
relatively small area of a few square kilometres near the flank of the Payun Liso volcano (Figure 4a and b).
The main axis depends on wind direction and not on structural rock joints, which in this case do not affect
their shape, as shown in Figure 5. Width/length ratio is approximately 025, when total drag is at a minimum
(Ward and Greeley, 1984).
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HOLOCENE YARDANGS IN VOLCANIC TERRAINS
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(a)
(b)
Figure 4. Carved demoiselles yardangs in the ignimbrite blanket. (a) A 25 m high yardang; in the background is the Payun Liso
volcano. The structural joint is perpendicular to the main yardang axis. (b) Sand-blasted flutes and grooves in the ignimbrite rock,
oriented to the prevailing wind direction, 320°, indicated by the compass
Copyright # 2001 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 26, 657–666 (2001)
662
M. INBAR AND C. RISSO
Figure 5. Structural joint on a yardang. Yardang has width/length ratio of 025. Main axis direction depends on the wind direction and
not on the joint
THE LAVA FLOW YARDANGS
Long megayardangs, several kilometres in length, are discernible in airphotos and in the Landsat image
(Figure 6). These are unusually large features, similar to the large yardangs in Iran, China and Chad (Dresch,
1968; McCauley et al., 1977; Halimov and Fezer, 1989).
Field observations reveal lower areas, about 100 m wide between higher lava ridges. The lower parts are
covered by a thin blanket of blown quartz sand. The depressions are interrupted by lava crests, indicating
abrasion activity on the lava flow ‘crevasses’, widening them and leaving higher sharp crests, up to 10 m high
(Figure 7). Several features reach up to 10 km in length. They are linear and parallel on a distinctive 320°
azimuth, which is the prevailing wind direction. They are developed in the basaltic lava flows of E1 Mollar
formation of Holocene age (González Dı́az, 1972).
In the Mojave Desert, Blackwelder (1934) used the term ‘yardang troughs’ for similar geomorphic
features. Small ravines in the area are said to be the triggers that start the formation of troughs, by drifting
sand and grinding the bottom and particularly the sides of the yardang. Ridges are left in a ragged condition.
These major features may be developed by wind abrasion starting in microtopographic irregularities of the
lava flows, and although no smooth paved surfaces form, as in the ignimbrite rock area, the patches of quartz
sand found on the lava depressions are evidence of sand abrasion. The initial channelling in the basalt flows
accelerates local winds, increasing the abrasion processes, followed by deflation, erosion and the widening of
the troughs. Flow surfaces act as aeolian traps, increasing wind velocities and erosional processes, as in the
Cima volcanic field, Mojave Desert, California (Wells et al., 1985).
Pyroclastic ashfall added fine abrasive material, although the main erosive agent was the sedimentary
quartz grains.
Other large aeolian features are fine ash tails along cinder cones on the same lava field, following the main
wind direction.
DISCUSSION
No similar features are noted in lava flows that are younger than 1000 years, indicating that the yardangs
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HOLOCENE YARDANGS IN VOLCANIC TERRAINS
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Figure 6. Parallel yardangs, several kilometres long, west of the Payun Matru volcano, developed in a Holocene basaltic lava flow
developed in earlier periods with stronger winds or needed a longer time for their formation, but the lack of
vegetation cover may indicate that aeolian morphological processes are still active. Yardangs can form
quickly (Cooke et al., 1993), and the Payun Matru yardangs probably first developed in the last glacial period.
The yardangs are formed by unidirectional winds and are probably indicative of the resultant effective wind
(Ward, 1979). The present effective winds are on a 320° azimuth (Servicio Meteorológico Nacional, 1991),
indicating no major shifting in the wind direction since the time of the yardang formation. In northern Chile
(27–33°S), Veit (1996) inferred that westerly winds were stronger in several Holocene periods, based on
geomorphological and pedological studies. However, no major climatic change in the Holocene in the
Patagonian Andes was found in palynological studies (Markgraf, 1983), which strengthens the assumption
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Figure 7. A yardang trough, west of the Payun Matru volcano, about 80 m wide with an 8–10 m lateral crest
that a similar vegetational cover prevailed during Holocene times. The major physiographic changes in the
Payun plateau in Holocene times were the volcanic eruptions of cinder cones and lava flows. Yardang troughs
are observed in two areas: east and west of the main Payun Matru volcano. The western field is younger,
probably Holocene in age, while the eastern field is of Upper Pleistocene age. Yardang development is more
evident on the eastern side, and there are no major physiographic differences between the two lava flows
except their age, which may explain the more developed erosion features in the old eastern field, although it is
possible to assume that both systems formed through a continuous process during the same periods of time.
In the lava field yardangs, the troughs are shallow and wide, indicating that the major erosion was in
widening, with only a few metres in downcutting. The development sequence of the yardangs seems to start
with downcutting, widening and ridge streamlining.
In the Payun Matru area, abrasive quartz sand seems to have been an effective agent in eroding the basaltic
rock, and strong winds deflated the eroded particles. The ignimbrite and basalt are hard rocks and they must
have undergone an abrasive phase in order to be deflated. Grooves and flutes in the hard ignimbrite rock
(Figure 4b), and the undercutting at the bows and flanks of the sphinx-like features, are evidence of the
abrasion processes. Quartz sand particles, found today in the yardang features, were the abrasive agents and
sculptors of the volcanic hard rocks. On basaltic rocks in the Amboy lava field in California, pits and grooves
formed by a eolian erosion were found in pahoehoe basalt. The surface roughness of the lava increased the
wind threshold velocity (Greeley and Iversen, 1986), and a similar process probably affected the development
of the Payun Matru megayardangs.
From laboratory and field studies elsewhere, it has been found that streamlining occurs by deflation and
headward erosion (Ward, 1979). Similar wide troughs hundreds of metres wide have been found in Chad
(Mainguet, 1972), as well as on Mars on ignimbrite or highly porous lava (Ward, 1979).
Other hypotheses for the formation of the Payun Matru yardangs would be as fluvial or glacial features
along structural lines (González Dı́az, 1972). No present or palaeofluvial, palaeoglacial sediments or
morphologies are recognized in the area, and there is no evidence of water erosion activity in their formation.
The development of yardangs in hard, consolidated basaltic rocks is not common in volcanic terrains on
Earth and further studies are required to find the physical processes involved in their formation. Most studied
yardangs are developed on fine-grained sediments (Greeley and Iversen, 1985). However, yardangs were
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HOLOCENE YARDANGS IN VOLCANIC TERRAINS
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discovered in hard rocks like dense crystalline limestone and granite in Egypt’s Western Desert (Grolier et
al., 1980).
SUMMARY
1. In the Payun Matru Volcanic Field, distinctive features of yardangs carved in hard ignimbrite rock, and
large yardang troughs in basaltic lava flows are found, streamlined with the prevalent northwesterly winds.
2. The yardangs were formed at episodes of strong winds, which were on the same 320° azimuth as the
present wind system, probably indicating an extended period of aridity and strong winds in late
Pleistocene to early Holocene times. They were not found in the young, sub-recent lava flows, and there is
no evidence of present development of yardangs.
3. The rock texture of the ignimbrite flow allowed the formation of distinctive small features, where on the
hummocky lava field large discontinuous yardang troughs were formed.
4. Yardang development in the PMVC is due to the following factors:
a. Wind strength, frequency and direction: strong unidirectional winds develop in the extensive plateau.
b. Aridity: there is a meagre vegetation cover, and on new volcanic flows thousands of years may elapse
until vegetation is developed. The region has prolonged dry periods.
c. Abrasive sediments: available quartz sand and volcanic ash particles are the abrasive agents of the
volcanic rocks.
d. Lithology: the ignimbrite and basalt are hard rocks, but the irregularities of the lava flow enhanced
wind activity, and the coarse ignimbrite mineralogical texture favoured the abrasive processes.
ACKNOWLEDGEMENTS
The research was funded by grants provided by the Universidad de Buenos Aires Grant UBA-Ex138 and
University of Haifa. The authors would like to thank Professor Haim Tsoar and Professor Nicholas Lancaster
for critical reading of the manuscript and valuable comments. Professor Andrew Warren and an anonymous
reviewer contributed to the clarity of the paper.
REFERENCES
Bermúdez A, Delpino D. 1989. La Provincia Volcánica Andino-Cuyana. Asociación Geológica Argentina XLIV, 35–55.
Blackwelder E. 1934. Yardangs. Bulletin of the Geological Society of America 45: 159–166.
Breed CS, McCauley JF, Whitney MI. 1989. Wind erosion forms. In Arid Zone Geomorphology, Thomas DSG (ed.). Belhaven Press:
London, and Halsted Press: Toronto; 284–307.
Cooke RU, Warren A, Goudie AS. 1993. Desert Geomorphology. UCL Press: London; 295–300.
Dresch J. 1968. Reconnaissance dans le Lut (Iran). Bulletin Association Geographique Français 362(3): 143–153.
González Dı́az E. 1972. Descripción Geológica de la Hoja 30c Volcán Payun Matru. Dirección Nacional Geológica, Boletı́n 130:
Buenos Aires.
González Dı́az E. 1979. Descripción Geológica de la Hoja 31d La Matancilla. Dirección Nacional Geológica, Boletı́n 173: Buenos
Aires.
Greeley R, Iversen JD. 1985. Wind as a Geological Process. Cambridge University Press; Cambridge.
Greeley R, Iversen JD. 1986. Aeolian processes and features at Amboy lava field, California. In Physics of Desertification, El-Baz F,
Hassan MHA (eds.). Martinus Nijhoff: Dodrecht; 290–317.
Grolier MJ, McCauley JF, Breed CS, Embabi NS. 1980. Yardangs of the Western Desert. In Journey to the Gilf Kebir and Uweinat,
Southwest Egypt, 1978, El-Baz F et al. (eds). Geographical Journal 135; 7–86.
Halimov M, Fezer F. 1989. Eight yardang types in Central Asia. Zeitschrift für Geomorphologie 33(2): 205–217.
Hedin S. 1903. Central Asia and Tibet. Charles Scribner: New York.
Laity JE. 1994. Landforms of aeolian erosion. In Geomorphology of Desert Environments, Abrahams AD, Parsons AJ (eds). Chapman
& Hall: London; 506–535.
Llambias E. 1966. Geologı́a y petrografı́a del Volcán Payun Matru. Acta Geológica Lilloana VIII: 265–315.
Mainguet M. 1972. Le Modélé des Grès. In Problèmes Generaux. Institute Geógraphique National: Paris.
Markgraf V. 1983. Late and postglacial vegetational and paleoclimatic changes in subantarctic, temperate, and arid environments in
Argentina. Palynology 7: 43–70.
Copyright # 2001 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 26, 657–666 (2001)
666
M. INBAR AND C. RISSO
McCauley JF, Grolier MJ, Breed CS. 1977a. Yardangs of Peru and other desert regions. Interagency Report, Astrogeology 81, US
Geological Survey.
McCauley JF, Breed CS, Grolier MJ. 1977b. Yardangs. In Geomorphology in Arid Regions. Eighth Annual Geomorphology
Symposium, Binghampton, NY, Doehring DO (ed.). Allen and Unwin; 233–269.
Servicio Meteorológico Nacional (Argentina). 1991. Estadı́sticas Meteorológicas: 1901–1950, 1971–1980, 1981–1990. Serie B-1, N1.
Tewes DW, Loope DB. 1992. Paleo-yardangs: Wind scoured desert landforms at the Permo-Triassic unconformity. Sedimentology 39:
251–261.
Veit H. 1996. Southern westerlies during the Holocene deduced from geomorphological and pedological studies in the Norte Chico,
Northern Chile, Paleogeography, Paleoclimatology, Paleoecology 123(1–4): 121–145.
Ward AW. 1979. Yardangs on Mars: Evidence of recent wind erosion. Journal of Geophysical Research 84: 8147–8166.
Ward AW, Greeley R. 1984. Evolution of the yardangs at Rogers Lake, California. Geological Society of America, Bulletin 95: 829–
837.
Wells SG, Dohrenwend JC, McFadden LD, Turrin BD, Mahrer KD. 1985. Late Cenozoic landscape evolution of lava surfaces of the
Cima Volcanic Field, Mojave Desert, California. Geological Society of America, Bulletin 96: 1518–1529.
Copyright # 2001 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 26, 657–666 (2001)