in the high valleys of the Arag6n and Gallego rivers, Central. Spanish morphometric features (area, width, length, L/W relations, etc.). The over- deepening of the cirques. Key words: glacial cirques, cirque morphometry, cirque distribu-. GMT Cirque overdeepening and their relationship to morphometry - Cirques are glacially eroded features characteristic of mountain. Cirque overdeepening and their relationship to morphometry Black Sea and their connection with the overdeepening of river valleys of the Caucasian coast.
According to Ramos and Ghiglionethe orographic effect of the Andes generated a climatic contrast in southern Patagonia and influenced the styles of development of continental and alpine glaciations during the Last Glacial Maximum LGMas well as during prior cold periods. The study area is affected by a cold temperate climate, with significant E-W rainfall and temperature gradients that are associated with the presence of the Andes.
Cyclones involving the Westerly winds originate in the polar front, creating high amounts of rainfall and low temperatures that directly affect the local climate Garreaud, In other parts of the SBMR, the glacial cirques are developed into these intrusive units.
Geomorphology Previous studies discussed various aspects of the regional glacial geomorphology Marden, ; Fogwill and Kubik, ; Glasser et al. In contrast, we focused on the SBMR where glacial cirques provide evidence of mainly an alpine non-ice sheet style of glaciation Fig.
Using aerial photographs and satellite imagery, plus field observations, it is clear that glaciers still occupy some of the cirques in the SBMR and its surroundings. Glacial cirques have been subject to varied erosion processes Fig. Some cirques host remnants of ancient moraine-dammed lakes, and there is also evidence of eroded remnants of lateral moraines Fig.
No direct age control for the last occupancy is available for the glacier cirques or the SBMR. However, we can establish a tentative framework for the chronology of glacial variations based on nearby studies.
By implication, we infer that at least some cirques in the study area were re-occupied during the ACR. Subsequently, glaciers in the nearby Andes experienced repeated fluctuations throughout the Holocene e.
We consider that the SBMR cirques, at least those currently occupied by ice, experienced similar variability. However, it remains unclear whether cirques without glaciers were reoccupied during the Holocene.
Future studies will help to define the timing of cirque glacier activity throughout the study area and test the assumptions above. Right box shows the location of the study area. Elevations were obtained from the SRTM model. Cirque activity corresponds to 1. Mapped cirque floor elevations, which show the increase in elevation towards the east from m a.
At east margin of SPIF, to 1, m a. Photographs A and B see location on Google image show a hanging valley and glacial cirque, and photo C shows a glacial valley.
Current snow and glacier processes are particularly active along the high elevations on the eastern side of the SBMR i. Lateral moraine dotted gray line below the cirques shown in figure 2A location in figure 1B.
Striated white arrows block location shown in figure 1B. Methodology A total of glacial cirques were identified and mapped. Mapping was carried out using the following i topographic maps at a scale of 1: Three field campaigns were carried out during and to map cirques and observe, describe and interpret the general geomorphology of the SBMR. The identified cirques were digitized on satellite images and aerial photos. The boundaries of the cirques were defined considering the criteria used by Evans and CoxCossart et al.
The crest-lines were considered as the upper limits of the cirques. Lower limits were marked, in some cases, by the crests of frontal moraines.
However, for most of the cirques, the lower limit was considered as the transition between the gentle slope of the cirque floor and an abruptly sloping valley below.
Visible topographic features on the photographs, satellite images, and contour levels, were also considered in the definition of the cirque limits Fig. Otherwise, former glacier extensions are estimated based on the distribution of lateral and frontal moraines. Each cirque was digitized as a closed polygon with a unique ID number, upon which a table containing associated morphometric information was generated.
The calculation of variables such as length and width of the cirques was performed in ArcView 3. Following Delmas et al. The variables considered are: Surface Sfc corresponds to cirque area estimated in GIS, cirque length L was calculated as that of the mean axis line which passes through the midpoint of the cirque's lowest boundary usually a moraine or a bedrock rimand divides the surface of the cirque into two equal halves.
The width W is the longest perpendicular line to the L axis Federici and Spagnolo, A m a. Table 2 presents a summary of the morphometric characteristics of the cirques in the study area. The circularity C was obtained by dividing the perimeter of the cirque by the perimeter of a circumference of a circle that has the same area as the cirque Aniya and Welch, Also, erosion dominated by cirque-type glaciers should be greatest when Equilibrium Line Altitude ELAs are at or near the cirque floor height, which may occur during the initial stages of glaciation and during glacier retreat.
Each parameter was analyzed in terms of its minimum, maximum and average values. Correlations between dependent surface, perimeter, length, width, cirque depth, minimum elevation, mean slope and circularity and independent maximum elevation, aspect morphometric variables were detected using Pearson correlation coefficients. This technique was originally designed for paleogeographic reconstructions and uranium exploration and is based on the combination of different parameters characteristics of cirques in this case into single, combined values, which are eventually plotted on a composite map.
The methodology has the advantage that for a specific data point, any number of different parameters pertaining to that location can be combined into a single, non-dimensional value, as long as the inherent meaning of the different parameters are understood.
For the composite map, because the original parameters are expressed in different dimensions degrees, km2, and a dimensionless ratiothey cannot be combined unless they are first standardized into non-dimensional values with the same range. This is carried out as follows: The numerical value Gi of a specific parameter at each cirque is then converted by first calculating its transition value T, given by Gi-Gland then by multiplying T with the conversion factor C of the parameter.
Therefore, the first case would have a new dimensionless slope value of 0x2. The other two parameters will also have a range of standardized, dimensionless values between 0 and Therefore, for the cirque with the highest floor slope, the inverted slope value I would be Adding in this case the three standardized values discussed above for each cirque thus produces a dimensionless combined value Dc which reflects the type of cirque or transitions into other cirque types in an unbiased, objective manner.
However, they should still have a lower combined value than type 2 cirques, thus classifying as type 1cirques, if the combined value is less than Transitional cirque types, with values between The composite values of all the cirque stations are finally plotted on a map and interpolated, in this case using the Inverse Weighted Distance IWD technique in ArcMap. This shows the spatial distribution of the cirque types. Definition of key morphometric attributes of a hypothetical cirque.
Left side shows the length L and azimuth for aspect as a black line and width W as a white line. The right panel shows the characteristics of range H and the topographic profile i-ii. Cirques with current glacier activity Aspects ranges are wide, from E to S. Cirques with no glacier or perennial snow activity These have a similar aspect as group ii but with more of an E bias.
Cirque showing evidence of glacier activity, at the easternmost section of the SBMR location in figure 1B. Cirque showing perennial ice and snow activity location in figure 1B. This photo was taken in early summer Cirque shape Table 2 presents a summary of the morphometric characteristics of the cirques.
Surface area values vary between 0. The frequency distribution of L, W and H are shown in figure 8. Extreme values for Sfc, L and W are distributed geographically in two particular areas Fig. The spatial distribution of the cirque surface areas and shape parameters appear to be a function of the type of cirque activity, which we elaborate in the discussion.
The positive and statistically significant correlations between the shape parameters L and Sfc 0. This inference is further reflected by the average value of circularity Table 21. Frequency distribution of L, W and H.
Cirque overdeepening and their relationship to morphometry
Cirque mean surface km2 decreases with an increasing in depth H. Otherwise, when the average slope becomes gentle i. As shown in figure 1, large cirques are concentrated along the eastern margin of the SPIF and gradually get smaller towards the interior of the continent around the SBMR.
As shown in table 3, Emax has a positive correlation with the elevation range, mean slope and parameters that define the shape of cirques Sfc, L, W. The correlation between Emin and cirque shape parameters is negative, which is particularly noticeable in the higher sections of the SBMR. Slope values vary between 5. Particularly at greater H, the cross-section and incision of cirques are reduced as the glacier occupied a smaller area thereof, which limited erosion; therefore, these cirques have lower surface areas.
Otherwise, gentle slope cirques i. Aspect values in radians vary in a range between 0. Eighty-nine cirques, which represent Table 4 shows a summary of cirque attributes by aspect. Larger cirques show aspects that range from SE to W, and are located in a wide elevation range, from B to E altitudinal classesm a.John Lawrence Seminar Series: Jeff Dangl, University of North Carolina
Smaller cirques show aspects that range from SSE to N, and are located in higher sectors, mainly in altitudinal classes D and E 1, m a.
Larger cirques show a gradual size increase moving from W to S aspects, and smaller cirques from N to SW aspects. In both cases circularity decreases moving towards S-facing aspects, as here cirques tend to be longer than wider. Rose diagram showing frequency distribution of aspect. Cirques with a SW aspect tend to be located in the intermediate zone and altitudinal range E 1, m a. Otherwise cirques with a S to SE aspect show a homogeneous distribution within the study area and an altitudinal range between C to D 1, and 1, m a.
Moreover cirques with W to SW aspects tend to show larger surface areas.
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This is consistent with landform preservation, including cirques and alpine-glaciated topography, in the arid rain shadow of the Andes Rabassa et al. Morphological cirque types Based on cluster analysis, two morphological cirque types can be distinguished Fig. They differ mainly in their surface and shape parameters Sfc, L, W. Both types show a similar overall aspect distribution with a S to E bias and a similar mean slope. Morphological Type 1 shows a cirque floor average elevation of 1, m, and these usually show current glacial activity.
The analysis shows two major morphological Types where the differences are based on the cirque activity i. Morphological Type 2 tends to show the largest cirques compared with the morphological Type 1.
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Composite map Composite values of the ten parameters considered, vary between The high values are related to cirques that show current glacier or perennial snow activity Figs. These cirques are spatially concentrated in the highest sections altitudinal cirque classes D and E, 1, m a. Particularly for this case, cirque activity would be linked to topographical effects rain shadow and local weather conditions temperature descentwhich favor snowfall.
Low values are associated with cirques that show no evidence of present-day glacier or perennial snow activity e. These cirques have aspects that range from NW to NE and show a broad spatial distribution at lower elevations altitudinal cirque classes B and C. The current cirque activity is mainly controlled by elevation and aspect. On the other hand, the cirque aspect can favor the persistence of ice or snow coverage, especially if oriented E to SE, due to the effect of lower solar radiation during the warmest part of the day and lower direct sunlight in the afternoon; this effect has already been described in EvansEvans and CoxGarcia-Ruiz et al.
Composite map of cirque attributes in the study area. Cirques were categorized using 10 parameters extracted from the morphometric analysis Table 2. Black dotted lines show outlines of maximum extension glacier stages see text. This contributed to their higher areal development in relation to those located in the SBMR, which were isolated from the ice sheet outlet lobes. The lowest values blue spatially match with evidence of areas covered or adjacent to former outlet glaciers of the SPIF, and cirques with no evidence of current glacier or snow activity.
Discussion The glacial cirques development in the SBMR result from the combination of several climatic and non-climatic variables Delmas et al. The relative importance of these variables is difficult to rank. Previous studies have concluded that, based on glacial morphometry, it is difficult to establish clear relationships between independent environmental factors and dependent size and shape of the cirques variables due to the structural and lithologic complexity of mountain environments.
For example, Aniya and Welch inferred that the morphological differences between cirques may reflect stages in the cirque development rather than differences in environmental factors. In comparative terms, most of the glacial cirques of SBMR shares similar characteristics less than 2 km for L and W, and less than 1 km for Hin particular with the glacial cirques located at Mt. Vasilitsa, in northwest Greece, Hughes et al. The overall morphology and special distribution of the cirques in these relative well-studied European locations have been interpreted as predominantly controlled by lithology and geological structure, while for cirques development factors such as aspect and elevation are considered as the preponderant.
However, cirque elevation effects are diverse. Whereas for the Cantabrian Range an increase in elevation determines an increase in cirque size.
The elevation effect for cirques should be linked to controlling factors such as lithology and the geological structure of these massifs, as well as their spatial distribution in relation to moisture sources, Specially in the SBMR where the Westerly winds and associated climate systems and rainfall gradients play an important role in the cirques development, as we explain below. In general, the spatial distribution of different cirques types in SBMR would be the result of the combined actions of the following: Actually, west of the study area, around the accumulation zone of the SPIF, the annual precipitation reaches up to 10, mm Cassasa et al.
We assume this reflects the rainfall gradient at SBMR, which favors the development of small cirques eroded by alpine-type glaciers cirques Type 1prevails at eastern higher areas of SBMR mainly due to the low temperatures associated with the E-W thermal gradient, moreover Type 1 reflects an alpine glaciated landscape that is isolated from coverage by the Pleistocene Ice Sheet, a particular morphoclimatic domain mainly in this eastern Andes foothills which also provides a useful natural laboratory for the study of the past environmental changes.
Another important factor in the development of the SBMR morphoclimatic configuration corresponds to the difference in the cirques floors elevations cirques Type 1, is about m above the cirques Type 2. Ice segregation erodes the rock vertical rock face and causes it to disintegrate, which may result in an avalanche bringing down more snow and rock to add to the growing glacier.
The enlarging of this open ended concavity creates a larger leeward deposition zone, furthering the process of glaciation. The Lower Curtis Glacier in North Cascades National Park is a well-developed cirque glacier ; if the glacier continues to retreat and melt away, a lake may form in the basin Eventually, the hollow may become a large bowl shape in the side of the mountain, with the headwall being weathered by ice segregation, and as well as being eroded by plucking.
The basin will become deeper as it continues to be eroded by ice segregation and abrasion. A bergschrund forms when the movement of the glacier separates the moving ice from the stationary ice forming a crevasse.
The temperature within the bergschrund changes very little, however, studies have shown that ice segregation frost shattering may happen with only small changes in temperature. Water that flows into the bergschrund can be cooled to freezing temperatures by the surrounding ice allowing freeze-thaw mechanisms to occur.
When three or more cirques erode toward one another, a pyramidal peak is created. The Matterhorn in the European Alps is an example of such a peak. Where cirques form one behind the other, a cirque stairway results as at the Zastler Loch in the Black Forest.