SIXTH INTERNATIONAL CONFERENCE ON ... › sites › default › files › A9 Sierra...SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY FIELD TRIP TO SIERRA NEVADA MASSIF GLACIAL GEOMORPHOLOGY

SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY

SIERRA NEVADA MASSIF GLACIAL GEOMORPHOLOGY AND PRESENT COLD

PROCESSES

Antonio Gómez Ortiz, Lothar Schulte, Ferran Salvador Franch, David Palacios Estremera, Carlos Sanz de Galdeano, José J. Sanjosé Blasco, Luis M. Tanarro García and

Alan Atkinson

FIELD TRIP GUIDE - A9

SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY

FIELD TRIP TO SIERRA NEVADA MASSIF GLACIAL GEOMORPHOLOGY AND PRESENT COLD PROCESSES Antonio Gómez Ortiz, Lothar Schulte, Ferran Salvador Franch, David Palacios Estremera, Carlos Sanz de Galdeano, José J. Sanjosé Blasco, Luis M. Tanarro García and Alan Atkinson Departament de Geografía Física i Análisi Geográfica Regional. Facultat de Geografía i História. Universitat Central de Barcelona. Baldiri Reixac, s/n. 08028 Barcelona. e-mail: [email protected] Introduction The A9 field trip of the Sixth International Conference on Geomorphology highlights the special morphobioclimatical nature of Sierra Nevada Massif situated in the southeast of Spain. This massif is characterized by the existence of high peaks (over 3.400 m), its low latitude (the southeastern zone of Europe) and its thermopluviometric pattern (long arid periods and intensive cold). These features make Sierra Nevada and especially its summits be an interesting scientific area to reconstruct the recent paleoenvironmental Quaternary history of the Mediterranean areas and to understand the present morphodynamics in cold and arid high areas of temperate latitudes. The guide is divided into two parts. The first one pays attention to the biophysics features and particularly to the glacial and periglacial geomorphology of summits. The second part contains the details of the viewpoints and the most outstanding geomorphological aspects of the field trip. To sum up, the field trip emphasizes the glacial and periglacial landforms and the cold processes that characterize the mountaintops from the study of one of the most representative areas of the range, the “Picacho Veleta unit”. The final results come from several research projects developed since 1998. FIRST PART: BIOPHYSICAL ENVIRONMENT OF SIERRA NEVADA SUMMITS 1. Geographical context Sierra Nevada belongs to the Betic Range that is situated in the southeast of the Iberian Peninsula. The Range is a southwest-northeast orogen that extends along 520 km from the vicinity of the Strait of Gibraltar (Atlantic Ocean/Mediterranean Sea) to the La Nao Cape (Mediterranean Sea). Sierra Nevada is a West-East trending massif that stands out from the depressions and valleys that surround it (Granada, Baza-Guadix, Guadalfeo, Andarax, etc.). The massif includes the highest peaks of the Iberian Peninsula (Mulhacén, 3482 m; Veleta, 3396 m; Alcazaba, 3366 m; etc.). It is at latitude 37º North and 2º45´/3º30´ West in the Andalucía region in Almería and Granada provinces along 90 km. Its maximum width reaches 40 km and it is 30 km away from the sea. The morphology of Sierra Nevada is characterized by the predominance of gentle landforms apart from the western summits affected by Pleistocene glaciations (picture 1 and figure 1). A dense and incised network of creeks and watercourses compartmentalizes the relief. The ones orientated to the South discharge into the Mediterranean Sea (Guadalfeo creek, Adra creek, Andarax creek, etc.) while those orientated to the North discharge into the Guadalquivir River (Genil creek, Fardes creek, etc.). They show an ephemeral regime due to the arid/semiarid climate of the western part of the Mediterranean area. Long dry periods are interrupted by autumn high intensity

Sierra Nevada massif glacial geomorphology

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storms that make the watercourses overflow, sometimes catastrophically. However, the altitude of the massif triggers a change in the climate, especially in the summits where the cold and the snow become important (at 2150 m between October and May the mean temperature is lower than 5º C and the 47,8 per cent of the mean annual rainfall is as snow). As a result, the thicket and the sclerophile forest of the lower altitudes disappear in favour of a xerophile grass of festucas at higher altitudes.

GRANADA

Baza

Guadix

S

37º N

ALMERIA

AdraMotril

Fiñana

3º W

al Río Guadiana

Cabo de Gata

R. Genil

R. Almanzora

N0 10 20 30 Km

40º

0º

Figure 1. Geographical situation of Sierra Nevada and the glaciated area.

Picture 1. Aerial photographs of Sierra Nevada western summits

A. Gómez Ortiz et al.

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Sierra Nevada (known as“Sun Hill/Snow Hill” in Muslim texts of the IX century) has been affected by anthropic activities since the old times. Currently, many villages are spread all over the area ascending up to 1550 m high. Their economy is based on farming and tourism trade. The summits that are home to high value scientific ecosystems and interesting landscapes have been declared Biosphere Reserve (UNESCO, 1986), Natural Park (Regional Government, 1989) and National Park (Spanish Government, 1999) (picture 2).

Picture 2. Alcazaba-Veleta area. Highest peaks of Sierra Nevada 2. Geological context Sierra Nevada is situated in the Inner Betic Zone that is formed by three large tectonically overlapped complexes. From bottom to top, they are the “Nevado-Filábride”, the “Alpujárride” and the “Maláguide”. The latter does not crop out in Sierra Nevada. In contrast, the Alpujarride Complex is found at low altitude areas surrounding the massif. The highest summits of this complex such as “Alayos”, “Trebenque”, the “Tesoro”, the “Dornajo” and the “Calar de Cantar” occur at the western part of the massif and they may reach 2000 m high. Although the Alpujarride Complex is superposed on the Nevado-Filábride Complex, it is not as taller as the Nevado-Filábride Complex. Both complexes are formed by metamorphic rocks. Finally, the Nevado-Filábride Complex is divided into two thrust nappes: at the bottom the Veleta nappe and at the top the Mulhacén nappe (figure 2). In this area, the Veleta nappe is formed by a single unit called “Las Yeguas unit” (Puga, 1971; Díaz de Federico et al., 1980). The field trip pays attention to Veleta nappe outcrops. This nappe is made up of over 3000 m thick dark carbonaceous schists (picture 3) (Díaz de Federico et al., 1980) that alternate with locally thick quartzite beds. We may also find lenses of amphibolites and scarce serpentinites and epidotites. These rocks are believed to constitute a Palaeozoic basement affected by Alpine metamorphism.


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Mirador Bajo2528

Picón de Jeres3094

Horcajo de Trevélez

3182

Cerro de Trevélez2878

Muerto2891Alcazaba

3366

Puntal de Vacares3129

Puntal dela Caldera3226

MojónAlto

3109

Papeles

2435

Charcón

H. Duque

Guéjar·Sierra

V·PT

T

T

T

TT

T

T

CañadillasV·P

Tesoro

2215

C SF

S

Pradollano

S

CC

C

S Caballo3013

Tajo de los Machos3081

3157

Tajo de Lagunillos

3398

Veleta

C

La Estrella

CAlegas2720

Trevélez

Mulhacén3482

C

C

C

CC

C

C

C

S

S

0 1 2 km

N

Neogene

Alpujárride (T:Trevenque unit,V·P:Víboras·Padules unit)

Mulhacén mantle(S:Sabinas unit, C: Caldera unit,S·F: San Francisco unit)

Veleta mantle

Overlaping

Fault

Antiformal of Sierra Nevada

Figure 2. Geological context (modified and simplified from Díaz de Federico et al., 1980). To the top, Triassic quartzite beds alternate with feldspathic micaschists that may reach 200 m thick. From the beginning of the Tertiary to the end of the Oligocene, different stages of compression and extension led to the thrusting of the Inner Betic Zone complexes. Subsequently, from the lower Miocene to the Middle Miocene an important extensional period (Galindo-Zaldívar et al., 1989; García-Dueñas et al., 1992) caused a readjustment of the tectonic complexes and their units. The upper units were displaced to the WSW. The deformation was first ductile affecting mainly the Nevado-Filábride complex. Afterwards it changed to brittle affecting specially the Alpujárride Complex. Most of the previously overlapped units were sheared and occasionally thrusted. In spite of the intense tectonic activity no high relieves were formed. The highest ones (1830 m) are found in the western zone of the massif. Those summits consist of Upper Miocene marine sediments (Sanz de Galdeano & López-Garrido, 1999) that suggest the main uplift stage occurred after the Upper Miocene. The lack of eroded sediments from the Nevado-Filábride Complex in


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the stratigraphical record before the end of the Upper Miocene corroborates the previous hypothesis.

Picture 3. Carbonaceous micaschists of the Veleta Unit affected by neotectonics and gelifraction processes. The main uplift period that caused the large antiform structure of Sierra Nevada began in the Upper Miocene as a result of a NNW-SSE to N-S compression regime (Sanz de Galdeano, 1998; Sanz de Galdeano & López Garrido, 1999; Sanz de Galdeano & Alfaro, 2004), which is still active today. The uplift enhanced the incision of an important drainage system. The axis of this antiform structure has an E-W orientation and it bends to the south in its western zone. 3. Geomorphological Context Cold quaternary processes concentrate above 2000 m. The snow line was 2400, 2800-2900 and 1965 in the Würm, Tardiglaciar and Messerli glacial phases respectively. The studies have demonstrated that summits landforms grade into periglacial landforms from the mountaintops downwards. 3.1. Summits landforms. Pleistocene glaciation Pleistocene glaciation was first studied at the end of the XIX century, although its existence was confirmed at the beginning of the XX century (Obermaier, 1916; Dresch, 1937). However, its geomorphological and paleoenvironmental meaning was not established until halfway through the last century (Paschinger, 1957; Messerli, 1965; Lhenaff, 1977). Nowadays, Pleistocene glaciation is well known, although the number of glaciations and their ages have not been recognized yet (Gómez Ortiz & Salvador Franch, 1998). The quaternary glaciers of Sierra Nevada Massif were similar to those that exist in dry mountains such as the High Atlas and Andes rather than in wet mountains such as the Alps or the Pyrenees. Its latitude contributed to minimize the effect of the Atlantic atmospheric perturbances and to favour the thermal Mediterranean influence. As a result the snow line was set at a much higher


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altitude than in other areas of the Iberian Peninsula (2300-2400 m in the North face; 2400-2500 m in the South face; Messerli, 1965). Other local factors that helped to characterize its glaciarism were its volume, altitude, preexisting relief, morphostructure, lithology and slope orientation. These local and regional factors made the glaciers exist in mountaintops and headwaters of creeks mainly in the western zone where the summits such as the “Cerro de Trevélez” (2877 m)-“Cerro del Caballo” (3013 m) reached the highest altitudes (figure 3). In Sierra Nevada the area occupied by ice and snow masses is characterized by the following geomorphological features: a) Influence of tectonic activity and lithostratigraphy on the creation of erosive landforms in cirques and valleys. b) Summits defined by either ridges and horns (hörner) or altiplanation terraces. c) Isolated and well limited accumulation areas. Transfluence passes were scarce. d) Steep and U-shaped glacial troughs. e) Moraine deposits of different ages along creeks. f) A large number of rock glaciers in the highest points of the cirques.

Río PoqueiraRío Puntal Río Trevélez

Río Alhorí

Río Maitena

Río Genil

Río Monachil

Río Dílar

Río Dúrcal

Trevelez

Río

Lró

anja

n

?

??

Río

Gua

rnón

0 5 km

N

1 2 3 4

Corral del Veleta

Figure 3. Glaciarism of Sierra Nevada. Identification of the Corral del Veleta cirque. 1. Summits; 2. Fluvial network; 3. Glacial cirques; 4. Glaciers.


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We may distinguish two types of glacial systems regarding their morphological meaning (table 1): Table 1. Classification of Glacial systems __________________________________________________________________________________________________ Type Glacial system/form Variety Examples __________________________________________________________________________________________________ convergent glacier tongues Genil, Poqueira, Trevélez ……………………………………………………………………………………………………………. Cirque and valley not well differentiated Monachil, Guarnón, San Juan, Valley isolated glacier tongue Lanjarón …………………………………………………………………….. Cirque and valley well differentiated Dílar, Vadillo, Maitena ………………………………………………………………………………………………………………………………… Moraines outside the cirque Siete Lagunas, Dúrcal, Alhorí, Lagunillos, Río Chico …………………………………………………………………………………………………………………… Cirque Lateral-moraines Cornavaca, Puerto de Trevélez, Hoya de la Mora ………………………………………………………………………………………………………………… Moraines exclusively inside the cirque Ventisquero del Gallo, Chorrillo, Peñón Negro, Nigüelas. __________________________________________________________________________________________________ Cirques Sierra Nevada cirques were formed in the headwaters of watercourses where the slope was suitable for the accumulation of snow. Those formed in the South face were favoured by an extra supply of snow due to the predominance of west winds (picture 4). During the Tardiglaciar and the Litlle Ice Age, the ice only remained in Cirques. The isolation of cirques, the compartmentalization of glacial systems and the accommodation to a preexisting relief are demonstrated by the lack of confluence points and transfluence passes.

Picture 4. Río Seco cirque (South slope of Sierra Nevada)


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On the other hand, local tectonic structures influenced the erosion activity of glaciers and the erosional landforms. The formation of thresholds, striated, polished and grooved rock surfaces and overdeepened basins was controlled by NNW-SSE and NE-SW trending joints and faults present on bedrock. Some examples are the Veleta (Green Water landscape), Cuenco del Goterón and Juntillas cirques. The most outstanding depositional landforms are rock glaciers that tend to fill cirques above 2,800 m. They might be formed during the Early Würmiense (Tardiglaciar) in different phases (Messerli, 1965; Lhenaff, 1977; Sánchez Gómez et al. 1990). The best examples occur at the headwater of Dílar creek (Cascajar del Cartujo) and in the Valdeinfierno unit (Genil headwater) (figure 4).

Tajos d Goterón

el

Barran

co de

Valdein

fierno

Alcazaba

3371

Puntal delGoterón

3072

Polished, raked substratum

N

0 50 100 m

Localised fracture network

Old rock glaciers

More recent rock glaciers

Gravity gelifract cone

River systems and irrigation grassland

Glacial cirque headway and bedrock in process of gelifraction

Rock ledge

Figure 4. Morphology of the Goterón cirque. Valleys Glaciers mainly tended to flow along the preexisting fluvial valleys leading to glacier tongues. However some of them grew along the mountain slopes becoming cirque glaciers (Cornavaca, Siete Laguna, Alhorí). Glacier tongues were little and didn’t reach long distances. Nevertheless, glaciers orientated to the North and especially to the Northwest had longer tongues. For example, the Dílar glacier tongue could reach more than 10 km long. The low latitude of the massif, the fact that accumulation areas were small and the scarce transfluence caused the early thawing. As a result the U-shaped glacial geometry of the valleys has disappeared quickly due to erosional processes.


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Water in soft matter

Rock ledge

Soligelifluction flows and lobes

Moraine matter flows

Middle Río Seco moraines

Slope debris soligelifluidal slides

Crescent mark

Moraine matter indistinguishable from Majada

Hoya del Capitán moraines(Veleta-Río Seco-Milhacén confluence)

Majada moraines

Middle Mulhacén moraines

Middle Sabinar moraines

Ravine, antique snowy

Developed cirques with uneven edges

Bedrock subjedt to gelifraction

Polished and raked bedrock

Other ledge

Eroded slope ledge

Narrow valley

Slope rupture

River system

Ice mass

Erosive high plateau

Alto delChorrillo

Río Poquerira

Hoya delCapitán

Río Mulhacén

La Majada

Peñones Negros

Río Seco

Río Veleta

Barranco del Sabinar

0 0,9 km

N

Figure 5. Morphology of the Hoya del Capitán cirque (Poqueira valley).


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Only those moraines deposited in low-gradient valleys (glaciers formed on cirque thresholds and on open thalweg valleys) such as the ones present in Poqueira, San Juan, Cornavaca and Lanjaron glacial systems, preserve their original morphology. In those places, several different stages of moraines formation can be identified. Each stage may be related to a glacial period (figure 5). All the moraines identified in the massif had similar features. They are sharp-crested ridges made up of a poorly sorted mixture of debris embedded in fine matrix. In the absence of good absolute dating, they have been dated depending on their morphosedimentary features and their distribution in the valleys. Outer, inner and intermediate moraines are supposed to be Prewürm-Würm, Würm-tardiglaciar and würm respectively. The Poqueira glacial system is a good example. Here, stepped moraine ridges occur between the altitudes of 1750 m and 3150 m 3.2. Altiplanation terraces and slopes landforms. Periglaciarism Sierra Nevada slopes were affected by periglacial processes during and after glacial morphodynamics leading to grèzes litées on hill slopes even at low altitude (1100 m in the Laroles Valley). The periglaciarism of the massif was first pointed out by Dresch (1937), although Ulrich-Brosche (1978) and Soutadé & Baudière (1971) were the ones who showed its morphological meaning and effects on the present dynamics of ecosystems. The most outstanding feature of Sierra Nevada periglaciarism is the existence of altiplanation terraces. They are attributed to old remains of periglacial erosional surfaces. The edaphological studies conducted in altiplanation terraces (Sánchez Gómez, 1989) suggest that they behaved as cryoplanation surfaces and they didn’t host fjeld type ice caps. On the other hand, periglacial processes led to patterned grounds, stone pavements, gelifluction lobes and stone accumulations depending on the topographic gradient (table 2 and picture 5). In addition, during the Tardiglaciar, a great volume of debris was accumulated leading to the previously mentioned rock glaciers. The snow deflacted from the altiplanation terraces was accumulated in the cirques orientated to the south. This fact controlled the dynamics of the existent rock glaciers such as the ones situated in Poqueira and Lanjaron valleys.

Picture 5. Patterned ground in the Cerro de los Machos pass


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Table 2. Topmost altiplanation terraces and landforms _____________________________________________________________________________________________ Altiplanation terrace 1 2 3 4 5 6 7 8 _____________________________________________________________________________________________ Caballo M 15,8 0,8 3013 2940 WNW 14,1 ABE ............................................................................................................................................……………………………. Tajos Altos M 38,7 1,5 3003 2940 NW 5,5 ABE 0,5 2920 ............................................................................................................................................…………………………… Lanjarón-Elorrieta M 33,7 1,0 3100 3080 SW 19,1 ADE 0,2 2890 ............................................................................................................................................…………………………… Cañar M 47,7 1,7 3081 3020 NW 6,1 ABCE 0,3 2980 ............................................................................................................................................…………………………… Cerro de los Machos M 9,4 0,4 3320 3240 N 10,1 ABD 0,4 3160 ............................................................................................................................................…………………………… Allanada del M 32,3 0,7 3440 3400 S 8,5 ABD del Mulhacén 0,3 3380 ............................................................................................................................................…………………………… El Cuervo M 16,1 1,5 3152 3100 S 3,4 ABC 0,2 3100 ............................................................................................................................................…………………………… La Atalaya M 26,2 1,5 3158 3100 S 3,8 ABE 0,1 3100 ............................................................................................................................................…………………………… Jeres-Cerro Pelado- M 96,4 3,5 3182 3100 S 2,3 ABCE -Horcajo 1,2 3100 .........................................................................................................................................…………………………….. Las Albardas M 236,9 4,5 2919 2900 S 1,2 ABCE 1,5 2860 ___________________________________________________________________________________________ 1. Substratum (M. micaschists); 2. Area (ha); 3. Maximun length and width (km); 4. Highest and lowest points (m); 5. Average altitude; 6. Predominant orientation; 7. Average slope (%); 8.Significant landsforms (A. Cryoplanation terraces; B. Tors; C. Mer de roches; D. Patterned grounds; E. Gelifluction lobes). ___________________________________________________________________________________________ 4. Little Ice Age During the Little Ice Age, (XV-XIX centuries), while the Alpine and Pyrenean glaciers expanded, the high peaks of Sierra Nevada Massif hosted small glaciers. This fact allows stating that this historical cooling period reached the Mediterranean latitude. Since halfway through the 18th century (Ponz, 1797; Boissier, 1839; Hellmann, 1881; Quelle, 1908; Solé Sabarís, 1942) travellers and scientists already informed about the existence of probably the most important ice mass of the massif formed during the Little Ice Age. It was situated in the “Corral del Veleta” cirque at the headwaters of the Guarnón Creek and it left moraine ridges of different ages of deglaciation (Gómez Ortiz et al. 1996; Schulte, 2002b) (figure 6). The geophysical (seismic and electrical profiles) and borehole data point out the present existence of a frozen mass at 1.90 m underground covered of debris in the eastern side of the cirque where the glacier lasted up to halfway through the 20th century (Gómez Ortiz et al. 1999). The frozen body may be associated with an alpine permafrost situated between the back wall and the Tardiglaciar lip moraines of the cirque. Periodical thermal checks suggest that it may be undergoing a degradation process.


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Figure 6. Topography of Sierra Nevada Massif (Bide, 1893). 5. Morphodynamics of summits The climatic conditions of Sierra Nevada summits are defined by the aridity and the thermal and nival regime (long periods of gelifraction processes on soils). Those conditions allow the development of the important cold processes that characterize arid mountains. The combined effect of cold, ice, snow and wind is very effective above 2.700 m in the South face. Table 3. Present cryonival processes and landforms

Site Preferential situation Main processes Landform and biogeographical affinity

Weathering Transport Cirque Transition bottom-

wall (talus) Gelifraction Avalanche, rock falls

landslides Protalus ramparts, talus cones, block streams and lobes

Glacial Valleys Rocky walls Gelifraction Rock falls Block slopes and cones “Borreguiles” Cirque heads and

headwaters of creeks Physical/chemical Cryoturbation and

solifluction Lobes vegetated by Carex, Ranunculus and Sphagnum

Altiplanation terraces >3000m

Gelifraction, deflation and cryoturbation either may preserve preexisting landforms (patterned grounds and mers de roches) or form new ones (patterned ground and stone lobes). Psicroxerphite grass (Festuca indigesta, F. Pseudoeskia)

Slopes Upper part >2900-3400m

Gelifraction, deflation, cryoturbation and meltwater runoff either preserve preexisting landforms (debris-mantled slopes) or form new ones (stone lobes). Psicroxerophite grass (Festuca indigesta, F. pseudoeskia) and low growing junipers (Genista baetica)

Middle part 2500-2900m

Gleifraction, wetting-drying, deflation

Cryoturbation, solifluction, meltwater runoff, creep

Stone lobes, gelifruction benches, patterned

Cold processes are variable. Gelifraction is favoured by the low strength of the substratum (fractured micaschists that led to block accumulations at the toe of vertical walls). Solifluction and cryoturbation are especially important in loose detrital deposits. Their action results in either a slow migration of superficial debris along the slopes or the formation of debris terraces,


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depending on the topographic gradient. The scarce vegetation cover is always a passive agent as it is unable to inhibit those movements. The resulted landforms, cryonival processes and altitude are connected as shows Table 3. SECOND PART: ITINERARY, VIEW POINTS AND ANALYSIS The field trip is designed to visit the Veleta unit situated in the highest area of the western massif. The way up is easy. A restricted-access bypass reaches the mountaintop ridges. The explanation of the glacial and periglacial processes and landforms is carried out from significant viewpoints. The field trip consists of four stops (figure 7):

Figure 7. Itinerary and viewpoints.

1. Carihuela pass; 2. Picacho del Veleta Peak 3. Corral del Veleta cirque 4. Cerro de los Machos pass


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1. Carihuela pass. Carihuela Pass (3199 m) is one of the main communication routes between the North and the South faces of Sierra Nevada. Many years ago, people from the Alpujarra villages were obliged to cross it to get to Granada city. Thanks to its situation it acts as a junction point of three glacial valleys: Veleta, Lagunillos-El Nevero and Dílar. In addition, Carihuela Pass shows excellent views of the Mulhacén-Loma del Tanto-Chorrillo Ridge and the Veleta cirque and valley head. 1.1. Cirque and headwater of the Veleta valley. The Veleta cirque, also called Aguas Verdes cirque is orientated towards the SSE and it is formed by several cirques. It is limited by the “Tesoro Cut (Loma Púa)-Raspones de Río Seco” and Veleta and Machos Peaks of more than 3300 m high. During the Last Glacial Maximum transfluent ice flowed through Río Seco, Valdeinfierno and Dílar valleys and some peaks acted as horns (Veleta, Tajos de la Virgen, Púlpito).

1 2 3 4 5 6 7 8 9 10

Corral del Veleta

Corral deValdeinfierno

Circo deRío Seco

Loma Púa

Veleta3.398 m

Río Veleta0 300 900 m

N

Figure 8. Morphology of the Veleta cirque. 1. Horn; 2. Sep-walled cirque; 3. Ledge, furrow; 4. Joints and faults families; 5. U-shaped valley; 6. Glacial basin hewn through a tectonic fault; 7. Bedrock subject to gelifraction and remains of glacial abrasional surfaces; 8. Hanging valley and transfluence pass; 9. Ice cap (Corral del Veleta cirque); 10. Late-melting glacier (Vasares del Veleta).


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The snow deflated from the Altiplanation terraces of the Picacho del Veleta peak fed the Veleta Cirque. Even at the present times snow-patches orientated towards the South survive in the surroundings of the Cilindro shelter in summer. The Veleta, Río Seco and Mulhacén glaciers formed the Poqueira glacial system that reached 6.4 km in length and led to a sequence of stepped moraines. Their tongues reached the altitude of 1704 m. The zone of ablation occurred at an altitude of 1980 m in the “Hoya del Capitán” basin. In the Veleta Cirque, erosional landforms such as overdeepened glacial basins (Aguas Verdes, Púlpito and Cabras ponds) and incised channels (Chorreras Negras) were controlled by the lithology and structure of the substratum (figure 8). This is made up of low shear strength micaschists affected by a NW-SE and NE-SW trending alpine faults. Rockfall deposits accumulate at the toe of vertical walls. The Veleta glacial trough is a straight, wide and U-shaped valley of four km long (picture 6) that starts soon after the cirque threshold is surpassed. In its right margin we find several knickpoints and small hollows cut into bedrock blocked by rock glaciers. In its left margin the hanging Púlpito cirque and moraines are at 125 m above the main channel. Hanging lateral moraines occur in both valley sides.

Picture 6. Veleta glacier head and Aguas Verdes pond 2. Picacho del Veleta peak The Veleta Unit forms part of the antiform structure axis that generates the highest peaks ridge of Sierra Nevada. This unit is made up of dark carbonaceous micaschists and light quartzites. On the other hand, it hosts the Picacho del Veleta peak (3,398 m), which became a horn during the Pleistocene, and several cirques that surround it such as the Veleta cirque orientated towards the Southeast and the Corral del Veleta cirque towards the North).


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The Picacho del Veleta peak connects with the Machos Peak through a gentle pass to the east and with the headwaters of Dílar and Monachil creeks to the west. Its top shows one of the most beautiful panoramic view of the massif (picture 7).

Picture 7. Picacho del Veleta peak 2.1. Thermal record from the Picacho del Veleta peak borehole Periodic temperature measurements of the permafrost allow distinguishing annual and centuries-old changes in the energy balance. The european PACE project (Permafrost and Climate in Europe) established the first permafrost systematic monitoring network in the european mountains. The stations were placed from Janssonhaugen (Svalbard) to Sierra Nevada (Spain) (Janssonhaugen (Svalbard) – Tarfalaryggen (Sweden) – Juvvasshoe (Norway) – Schilthorn and Stockhorn (Switzerland) – Stelvio Pass (Italy) – Sierra Nevada (Spain)) (Harris et al., 2003; figure 9). In Sierra Nevada drillholes were conducted in the Picacho del Veleta peak (3,397 m high) up to 114,5 m deep at 3,380 m high by rotary-percussion drilling technique. In a first stage 30 and in a second stage 10 thermal sensors were placed inside a protection PVC pipe (figure 10). First phase (2001 and 2002) According to the rules of the PACE project, during 2001 and 2002 the first thermal cable included 30 Yellow Spring Instruments YSI 44006 type NTC thermistors (negative temperature coefficient thermistor). The electrical resistivity of the sensors was 2.95 x 104Ω at 0ºC. They were calibrated in VAW (Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie) that belongs to ETH. Their absolute accuracy was ±0.05ºC. The thermal profiles of figure 11 show the record of the only two temperature measurements obtained (September 8th 2001 and August 27th 2002 at 12:00 am GMT). The absence of permafrost in the Picacho del Veleta Peak is suggested by: a) The temperature values of the 2002 survey were above 2.15 ºC and b) the existence of water at 80 m deep.


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0 250 500 km Svalbard, Noruega (UO-NO)

Tarfala, Suecia (SU-SE)

Cardiff, Gran Bretaña (UWC-UK)(Geotechnical Centrifuge Centre)

Zermatt and Shilthorn,Suiza (JLUG-DE)

Sierra Nevada, España (UCM/UB-ES)

Corvatsch, Suiza(ETH-CH y UZ-CH)

Zugspitze, Alemania (JLUG-DE)

Valtellina, Italia (TUDSR-IT)

Jotunheimen, Noruega(UO-NO y TEDAT-UK)

0-

500-

4000-

3500-

3000-

1000-

1500-

2000-

2500-

30 40 50 60 70 80

Latitude, degrees North

Alti

tude

, met

ers

1. Janssonhaugen Svalbard 2. Tarfalaryggen3. Juvvasshoe4. Stockhorn

5. Stelvio Pass6. Schilthorn7. Murtèl-Corvatsch8. Veleta Peak, Sierra Nevada

1

2

3

4

56

7

8

Figure 9. Geographical situation and altitude of permafrost monitoring stations in European mountains (PACE). The temperature of both thermal profiles are almost similar from 1.20 m deep downwards. There is just 0.15º C difference that is less than the deviation of the instrumentation. At 7 m deep the temperature sets at around 2.45 ºC. Below this depth the variation of temperature was minimum. The zero annual amplitude (ZAA) was set at 20 m deep. Under this depth there weren’t significant temperature variations.


18

3.095,53.098,08

3.053,5

3.065,5 3.073,0

3.072,53.095,5

3.137,0

3.398

3.352,5 3.319,5

3.298,5

3.323,5

3.324,5

3.299,0

3.329,0

3.135,0

3.145,03.127,0 3.014,0

1

3.250

3.300

3.200

3.150

3.100

3.0503.000

2.950

2.900

3.350

3.300

3.2 50

3.2 00

3.15 0

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3.050

3.00 0

Veleta Peak

Cerro de los Machos

0100 100 m

4.102.04.102.0

4.101.0 4.101.0

468.0

468.0

467.0

467.0

116m-BoreholeSierra Nevada, Veleta Peak, Spain

Altitude: 3380 m; Depth: 114,5 m1

Figure 10. Location of the Picacho del Veleta peak borehole. The Picacho del Veleta peak thermal record shows that there isn’t an increase of temperature due to geothermal gradient. Actually, between 20 m and 100 m deep the thermal gradient is 0.004ºC m-1 and it may be even negative. This fact has been also observed in other european mountains included in the PACE project (Isaksen et al., 2001; Harris et al., 2003). The influence of thermal flows from the Corral de Veleta (North) and Aguas Verdes (Southeast) cirques walls might explain this phenomenon. The mean annual ground surface temperature (MAGST) has been estimated around 2.6ºC. The 20 m deep bh1 thermal profile carried out in Flüela Pass in the Swiss Alps at an altitude of 2470 shows a similar behaviour. The absence of permafrost possibly inhibits the development of a geothermal gradient (Luntschg et al., 2004). Second phase (2002 a 2004) A new cable equipped with ten autonomous thermal sensors of the Type UTL-1 (supplied by Geotest AG firm, Switzerland) substituted the previous NTC thermistors due to technical problems in august 2002. The number of sensors decreased and the temperature deviation increased (from ±0.05ºC to ±0.27ºC) in comparison with the previous thermal cable. Sensors recorded temperature every two hours.


19

The thermal record allows reconstructing the maximum and minimum values from September 2002 to August 2004 (figure 12). The maximum temperature variation occurred just 20 cm below the surface and the zero annual amplitude (ZAA) was at 20 m deep as in the first phase thermal measurements. The extreme minimum temperature was -1.2 ºC at 20 cm deep. From 80 cm deep downwards, temperature was no longuer negative. Therefore, only the upper 80 cm of the ground is undergoing temperature changes above and below 0ºC. The mean annual ground surface temperature (MAGST) was around 2.3 ºC, three tenths less than in the first phase survey. This difference may be attributed to the less sensitivity of the UTL-1 sensors. The average annual atmospheric temperature measured by the UTL-1 sensors at the Picacho del Veleta peak (3395 m) was 0.83 ºC from September 21st 2003 to September 20th 2004 (figure 13).

Schu

lte, 2

003

Upper profile

Geothermal profile of the 115m-borehole of the Veleta Peak (3.380 m) Total depth (115 m)

Figure 11. Geothermal log of the Picacho del Veleta peak (8-9-2001 y 27-8-2002). The average ground temperature data recorded in Picacho del Veleta peak borehole (figure 14) every four months from September 1st 2003 to August 26th 2004 suggests: a) there isn’t permafrost as it was already observed in the first phase survey, b) The ZAA is located at 20 m deep, c) The lowest temperature values happened during spring in the first 5 m of the ground and during summer between 5 m and 10 m deep, d) Solar radiation at the Corral cirque side walls may explain the lack of a geothermal gradient as it was stated for the 2001-2002 period, and f) the unknown thermal effect of the flowing water found at the base of the borehole.


20

MAGST

ZAA

Ground temperature [ ]°C

0

10

20

30

40

50

60

00 2 4 6 8 10 12 14 16-2-4 3380 m a.s.l.

Maximum and minimum temperatures 01.09.2002 - 28.08.2003 Maximum and minimum temperatures 01.09.2003 - 26.08.2004

-21.5°C 28.06°CAnnual air temperature amplitude 2003-04*

Annual ground temperature amplitudes the Veleta Peak borehole (3380 m a.s.l.)

within

Figure 12. Annual ground temperature amplitude within the Veleta peak borehole (3380 m) (2202-2204)

Winter SummerSpingAutumn

Temp

eratu

re (º

C)

Figure 13. Air temperature in the Veleta peak (2003-2004 field survey).


21

Conclusions The positive temperature values recorded by the thermistors, the mean annual ground surface temperature (MAGST) of around 2.3 ºC and the occurrence of water at different depths suggest the lack of alpine permafrost. As a result, the postnival expansive thermal wave quickly counteracts the negative values accumulated at the surface. Ground temperature values are 2.5 ºC higher than the ones recorded by the PACE borehole in Schilthorn (46 ºN, Switzerland) at 2,900 m high (Harris et al., 2003). Below the ZAA the positive thermal gradient remains constant probably due to its proximity to the side walls of the Cirque. This fact has been also observed in the Stockhorn and Schilthorn boreholes despite the present existence of permafrost (Harris et al. 2003). The lack of permafrost in Sierra Nevada does not allow the calculation of the geothermal gradient and the possible influence of the present climatic change (Lachenbruch & Marshall, 1986). The surface-wave radiation reaches 20 m deep. The monitored boreholes of Tarfalaryggen (Sweden) and Juvvasshoe (Norway) show the same value (Isaksen et al., 2001). The seasonal lag of the radiation wave is recorded at a depth of 7 m.

Mean seasonal ground temperatures within the Veleta Peak borehole (2003-2004)

Temperature [°C]

3380 m

a.s.l.

3395 m

Figure 14. Mean seasonal ground temperature within the Veleta peak borehole (3380 m) (2003-2004). 2.2. Permafrost distribution model in the Veleta unit The first model of permafrost distribution in a Mediterranean area was developed in the Veleta Unit (Tanarro et al., 2001). The model established a statical relationship among BTS data (Bottom


22

Temperature of the Winter Snow Cover), altitude, solar radiation and snow cover. According to Gruber & Hoelzle (2001), this relationship decisively influences on the occurrence of permafrost. 121 BTS temperature measurements were carried out at the base of the snow cover when it was more than 80 cm thick and two months stay. Haeberli (1973) points out that permafrost exists when the BTS temperature obtained under these conditions is below -3ºC BTS temperature values below -3ºC were recorded in the north face of the Picacho del Veleta peak and the northern wall of the Corral del Veleta cirque. The variables that might control these values were the altitude, the potential solar radiation and the snow cover during summer. The altitude was established from a 1:10.000 scale digital elevation model (DEM). The potential solar radiation values were obtained using the SRAD model. This model developed by Moore et al. (1993) allows determining the solar radiation distribution from July to October when the snow cover is minimum and radiation is maximum. The minimum radiation values occurred at the northern wall of the Corral del Veleta cirque. A raster type model of the snow cover in summer time was obtained by the aerial photographs of four years (1957, 1985, 1989 and 1999) in July. The different zones were classified depending on the existence or the lack of snow. A multiple regression analysis of the four variables was applied to develop a permafrost distribution model using ArcInfo GIS software. The function used was (Gruber y Hoelzle, 2001): BTS = 11.789 - 0.0054 * Altitude + 0.131 * Radiation + 0.095 * snow thickness in summer Three different methods validated the model (figure 15). Firstly, the occurrence of a cryokarst (collapse structures) informed that permafrost was undergoing degradation processes. Secondly, the temperature under the snow cover was constant (-5ºC). Finally, seismic refraction and geoelectrical resistivity surveys showed characteristic physical properties of the ice mass. For example, different areas presented resistivity values of 562.220 Ohm/m According to the model, permafrost occurs at the north face of the Picacho del Veleta peak and the back wall of the Corral del Veleta cirque. The model informs that solar radiation is the variable that best fit the permafrost distribution. Therefore it seems that permafrost existence in the Picacho del Veleta peak is controlled by the topography that favours the occurrence of areas where solar radiation is minimum and snow patches remain. Future BTS data of a larger number of points and the establishment of a ground thermal control network at different depths will improve the current model. 3. Corral del Veleta Cirque The Corral del Veleta is the oldest cirque of the Guarnón glacial system. It is located at the toe of the North face of the Picacho del Veleta peak in the headwater of the Guarnón creek. It is a 600 m long oval hollow whose back wall is 300 m high. During the Little Ice Age it hosted a small glacier that has already disappeared. However, several ice masses remain trapped in the eastern side of the cirque. Those ice bodies are probably undergoing degradation processes.


23

Figure 15. Possible Permafrost in the Veleta unit.


24

3.1. Holocene glacial fluctuations Introduction During the XX century important studies about the evolution of glaciers have led to a better understanding of the Holocene climatic changes (Maisch, 1981; Röthlisberger, 1986; Zumbühl & Holzhauser, 1999). The morphological, sedimentological and dendrochronological record of different european mountain ranges such as the Alps and the Scandinavian Range informs about the youngest and oldest Holocene glacial fluctuations with chronological precision (e.g. Wanner et al., 2000; Karlén & Kuylenstierna, 1996). On the other hand, a large number of studies developed since the end of the XIX century in South Europe evidence the glacial variability that existed in high mountain ranges. Messerli (1967) pointed out the altitudinal and latitudinal distribution of glaciers during the Holocene and Pleistocene glaciarism in the Mediterranean areas. However, the correlation and reconstruction of the climatic changes in the Mediterranean regions isn’t easy due to the shortage of well-differentiated Holocene moraines, the insufficient geochronological data, The insufficient stratigraphic record and the absence of written texts referring to the extension of glaciers. In the Iberian Peninsula, the understanding of glacial fluctuations started in the nineties of the XX century. For example, lichenometric dating and written texts of the Maladeta Cirque (central Pyrenees Range) suggest that the Pyrenees Range underwent four ice expansion periods during the Little Ice Age in 1600-1620 AC, 1820-1830 AC, 1915-1925 AC and 1985-1995 AC (Chueca Cía & Julián Andrés, 1996). The rest of the mountain ranges of the Iberian Peninsula had a poor record of moraine deposits that difficults the reconstruction of the recent climatic changes. Apart from the Pyrenees, Sierra Nevada massif shows the greatest information. For the last 130 years Sierra Nevada has become an interesting area to study end and lateral moraines ridges due to the volume and geographical situation of the massif (Obermeier, 1916; Paschinger, 1957; Messerli, 1965, Gómez et al., 1996; Schulte, 2002a). The Corral del Veleta cirque located in the Guarnón valley and the Hoya del Mulhacén cirque situated in the Valldecasillas valley are two of the most outstanding areas of the massif. Upper Pleistocene moraines of the Guarnón valley The Guarnón valley hosts a moraine deposited during the maximum ice advance of the last glaciation at an altitude of 1790 m and three post-LGM (Last Glacial Maximum) moraines at 2010 m, 2250 m and 2360 m above sea level (Gómez et al., 2002). The youngest Tardiglacial moraine (LPM or Late Pleistoce Moraine) may be correlated with the outer end and lateral moraine ridges of the Corral del Veleta cirque situated at an altitude of 2.980 m (figure 15). The LPM consists of heterogeneous, up to metric-scale blocks that are perpendicularly orientated to the glacier flow direction. Its absolute age is unknown due to the absence of radiometric dating. However, its spatial configuration and topography indicate a tardiglacial age, probably Younger Dryas. The tardiglacial equilibrium line altitude (ELA) was lower than the Holocene ELA (figure 16). This fact suggests that the glacier exceeded the cirque threshold flowing along the Guarnón valley during the Tardiglacial. The reconstruction of the Tardiglacial glaciation and ELA was carried out by Schulte et al. (2002 a) by the dating of lacustrine sediments found in the overdeepened basin of the Hoya del Mulhacén cirque.


25

4.102.04.102.0

4.101.5 4.101.5

468.0467.5

468.0467.5Late Glacialmoraines

Stone-bankedsolifluction lobes

Rocky outcrops

Peat

Principal escarpment

Structural steps

Break of slope bycovered structural steps

Channels

Debris flows

Fluvial network

Lakes

Perennial ice slab

LEGEND/LEGENDA

Holocene moraines

L.I.A. Moraines, Hm4a

L.I.A. Moraines, Hm4b

(Morrenas Holocenas)

(Morrenas de la PEH)

(Morrenas de la PEH)

(Rampa morrénica)

(Nichos glacio-nivales)

(Sustrato pulido y estriado)

(Placas de hielo perennes)

(Pared rocosa)

(Desprendimientos)

(Lengua de bloques activo porcrio- reptación)

(Formas activas por crio-reptación)

(Formas inactivas por crio-reptación)

(Colada de derrubios)

(Lóbulos de gelifluxión)

(Grietas de deslizamiento) (Flujo de detritos)

(Turberas “Borreguiles”)

(Escarpes principales)

(Canales)

(Material removido degénesis diversa)

(Lagos)

(Red fluvial)

(Ruptura de pendiente porescalones estructurales cubiertos)

(Escalones estructurales )

(Círculo de piedras)

(Banda de piedras)

(Campo de bloques )

(Depresión por fusión de hielo muerto)

(Lóbulos de solifluxión con frente de piedras)

(Crestas pronivales)

(Crestas indiferenciadas)

(Taludes de gravedad)

(Afloramientos rocosos)

(Morrenas Tardiglaciares)

Gelifluxión lobes

Slide scarsMudflow

Holocene moraines

Pronival ramparts

Active ice-creepfeaturesInactive ice-creepfeatures

Active ice-creeptongue

Rock avalanches

Removed material from diverse genesis

Scree slope of rock fall

Smoothened rock surface

Blocks field

Sorted circle

Sorted stripes

Glacio-nival hollows

Dead-ice depression

Rockwall

Diffused ridges

3.250

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3.298,5

3.323,5

3.324,5

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Pico del Veleta Cerro de Los Machos

0100 100

Figure 16. Morphology of the Corral del Veleta cirque.

Holocene moraines of the Corral de Veleta cirque

They occur closer to the back wall of the cirque than the Late Pleistocene moraines between the altitudes of 2996 m and 3060 m. We have mapped five end and lateral moraine ridges of different ages.

Every ridge shows its own size and geomorphological and sedimentological features (figure 16). The HM1 (first Holocene moraine formation period) is associated with the maximum extension of the glacier flowing over the cirque threshold. Its front that ended at an altitude of 2996 m has become blurred due to erosional processes. Moraine deposits are thin due to the steep slopes of the cirque.

The HM2 may be related to different ice advance periods. The moraine deposits formed during this phase are made up of a great amount of fine-grained sediments (<2 mm) that constitute the 60 per cent of the total. Blocks (36%) and pebbles (29 %) become the most representative sizes of the coarser fraction (>2 mm; 40% of the total) (measurement point number 7, cross-section CV-98-T2, figure 2) (figure 17).

The amount of fine-grained sediments allows distinguishing HM2 moraines from HM3, HM4a and HM4b ones. Gelifraction processes acting during a long time may explain the great amount of fine and medium size clasts. In this context, it seems that the present atmospheric temperature values may corroborate the former hypothesis. The thermal sensors situated in the Picacho del Veleta peak at an altitude of 3395 m and 300 m above the HM2 moraines, have recorded 134 days (36,6 %) in which the temperature was around 0º C between September 21st of 2003 and September 20th of 2004.


26

The HM3 period led to a 30 cm thick subglacial and ablation till that comprises subangular, coarse and middle size clasts (52 %) and little fine size matrix.

30500 50 100 150 200

m

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3130

NS

7

1

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2

45

6

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<5º

<5º

<5º5º

Corral del Veleta, Sierra Nevada (Spain)Section CV-98-T2

LIA morainesHm4b HM4a

LIA flutes

Rock fall talusPerennial ice slab

Clast size

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> 50 cm

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Spot 1Elevation: 3.117 m

Clast orientation respect to flow direction

Flujo 0º / 15 º0 º /

15 º

15º / 30 º15º /

30 º

30º / 45º30º /

45º

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60º / 90º60º / 90º

0 10 1020 20 40 40 % 60 60 %30 30 50 50

Clast dip angle

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30 50

Clast size

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3 0 º

30º / 45 º30º /

45º

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60º / 90º60º / 90º

0 10 1020 20 40 40 % 60 60 %30 30 50 50

Clast dip angle

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40 60 70 %

30 50


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30

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100cm%

> 50 cm

25-50 cm

10-25 cm

4.5-10 cm

2-4.5 cm

Clast dip angle

0º / 15 º

15º / 30 º30º / 45º45º / 60º

60 º / 90º

0 10 20 40 60 70 %30 50 0 1020

40 60 70 %

30 50



15 º

15º / 30 º15º /

30 º

30º / 45º3 0º / 4

5º

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0 10 1020 20 40 40 % 60 60 %30 30 50 50

Clast size

0

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100cm%

> 50 cm

25-50 cm

10-25 cm

4.5-10 cm

2-4.5 cm

Clast dip angle

0º / 15 º

15º / 30 º30º / 45º45º / 60º

60º / 9 0 º

0 10 20 40 60 70 %30 50 0 1020

40 60 70 %

30 50


Flujo 0 º / 15 º0º /

15 º

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30 º

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45º

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0 10 1020 20 40 40 % 60 60 %30 30 50 50

pot 4Elevation: 3.086 m

Clast size

0

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100cm%

> 50 cm

25-50 cm

10-25 cm

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2-4.5 cm




15 º

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3 0 º

30º / 45º30º /

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0 10 1020 20 40 40 % 60 60 %30 30 50 50

Clast dip angle

0º / 15 º

15º / 30 º30º / 45º45º / 60º

60º / 9 0 º

0 10 20 40 60 70 %30 50 0 1020

40 60 70 %

30 50

Clast size

0

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50

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90

100cm%

> 50 cm

25-50 cm

10-25 cm

4.5-10 cm

2-4.5 cm



Flujo 0º / 1 5 º0º /

15 º

15º / 30 º15º /

30 º

30º / 4 5º30º /

45º

45º / 60º45º / 60º

60º / 90º60º / 90º

0 10 1020 20 40 40 % 60 60 %30 30 50 50

Clast dip angle

0º / 15 º

15º / 30 º30º / 45º45º / 60º

60º / 90 º

0 10 20 40 60 70 %30 50 0 1020

40 60 70 %

30 50

Clast size

0

10

20

30

40

50

60

70

80

90

100cm%

> 50 cm

25-50 cm

10-25 cm

4.5-10 cm

2-4.5 cm




15 º

15º / 30 º1 5º /

30 º

30 º / 45º30º /

45º

45º / 60º45º / 60º

60º / 90º60º / 90º

0 10 1020 20 40 40 % 60 60 %30 30 50 50

Clast dip angle

0º / 15 º

15º / 30 º30º / 45º45º / 60º

60º / 90 º

0 10 20 40 60 70 %30 50 0 1020

40 60 70 %

30 50

Holocene moraine Hm2

L.M

. Tan

arro

, 199

8

Figure 17. Morphosedimentological cross-section.

Moraines ridges of HM4a and HM4b are sinuous, their thickness shows variations and they overlay older Holocene moraine deposits. They are made up of angular blocks that constitute 65 % of the total. We didn’t find fine-grained sediments (measurement points numbers 5 and 6, cross-section CV-98-T2, figure 9) (figure 17). Subglacial till shows streamlined features such as glacial flutes (measurement points numbers 5 and 6, cross-section CV-98-T2) (figure 17).

Chronology of the Holocene glaciation in the Corral del Veleta cirque

The absolute ages of HM1, HM2 y HM3 are unknown but it seems that they happened during the Holocene. We can ensure that HM1 and HM2 periods are older than the Little Ice Age. The Holocene age of HM3 is doubtful. We believe that HM3 may occur at the beginning of the Little Ice Age or during medieval times as its deposits are little weathered (Röthlisberger, 1986; Zumbühl & Holzhauser, 1999).

The HM4b period led to an inner moraine ridge that represents the youngest deposit of the Corral del Veleta cirque. The HM4b is associated with the last glacier advance that happened in 1876 AC (Hellmann, 1881). This ice expansion event may be correlated with the glaciers advance of the Alps in 1850 AC (Röthlisberger, 1986; Zumbühl & Holzhauser, 1999) and the middle Pyrenees between 1820 AC and 1830 AC (Chueca Cía & Julián Andrés, 1996).


27

The morphoestratigraphical distribution of the HM4a moraine ridges informs that HM4a deposits had to be oldest than the HM4b ones. The 210Pb dating of the lacustrine sediments of a pond situated between Hm4a and Hm4b moraine ridges (Schulte, 2002a, 2002c) indicates that the HM4a period may be related to an ice expansion during the Little Ice Age. This period may be correlated with the early glaciers advance in the Alps (1570–1650 AD; Röthlisberger, 1986: 308; Wanner et al., 2000) and the middle Pyrenees (1600-1620; Chueca Cía & Julián Andrés, 1996).

The studied glacio-lacustrine sediments form a 14 cm thick rhythmite sequence of silts and sands that is overlying gravel and block size subglacial till of the HM4a period. Therefore, the lacustrine sequence was sedimented after HM4a. The rhythmite sequence is formed by different sedimentary units. The age of a sample taken at 7 cm deep was 1908 AC. If we suppose that the sedimentation rate has been constant, we may hypothesize that its deposition started 200 to 250 years ago halfway of the XVIII century (Schulte, 2002b). Therefore, the HM4a period might be associated with an advance of glaciers at the beginning of the Little Ice Age and it might be correlated with the ice expansion recorded in the Alps in the 1570-1650 AC (Röthlisberger, 1986: 308; Wanner et al., 2000).

The Little Ice Age deglaciation was fast. In 1899 Rein’s measurements of the extension of the Corral del Veleta cirque glacier demonstrated that the ice mass had reduced half its area in just 13 years. It seems that the glacier disappear throughout the first half of the XX century (Gómez et al., 1999). According to Rodríguez et al. (1996), the thermal and pluviometric record of La Cartuja station located in Granada city at an altitude of 774 m (1902-1994) and University Youth Hostel station situated in Sierra Nevada massif at an altitude of 2.550 m (1960-1994) points out a significant change in the rainfall regime. This change consists in a decrease in winter rainfalls and an increase in summer rainfalls. Correlation problems It is difficult to correlate the glacial fluctuations observed in the Corral del Veleta cirque with the ones recorded in the Alps due to the lack of chronological data. These data allow highlighting the following thoughts: a) The distribution of moraines at different altitudes indicates that the snow line has migrated to the mountaintops. Using the Höfer method (1879) we have estimated a 628 m upward migration of the snow line in Sierra Nevada after the Last Glacial Maximum. In contrast, Maisch (1981) considered that in the Alps the snow line underwent a 1200 m migration during the same period of time; b) Pleniglacial and tardiglacial periods represent important morphological stages in the South of the Iberian Peninsula; c) In the Corral del Veleta cirque, we have detected two periods of glaciers advance during the Little Ice Age. They are known as HM4a (the oldest stage) and HM4b that is considered to be the youngest period and to be form during the second half of the XX century. Both periods (HM4a and HM4b) may be correlated with ice expansion events of the Alps and Pyrenees. Schulte (2002a) tried to establish a relationship between the glacier advances in the Guarnón valley and the fluvial terraces of the Aguas River in the Vera Depression situated close to the Mediterranean coast 150 km away from Sierra Nevada massif. The correlations are still difficult and doubtful despite the number of moraine ridges and terrace levels are the same and the fact that Pleistocene sedimentary units are different from the Holocene ones. The discrepancy of absolute ages comes from the dating methods used. AMS radiocarbon dating is difficult because sediments of semiarid environments contain a little amount of organic matter


28

and pollen grains. Cosmogenic nuclides accumulation and thermoluminescence dating methods may be used in the coming future in spite of their probable age deviation. In addition, future research should focus on determining the absolute chronology of the Sierra Nevada glacial fluctuations so as to know the effects of climatic changes in the Mediterranean mountain ranges and surroundings. 3.2. Geophysics and permafrost During the 1995 summer there wasn’t snow cover in the Corral del Veleta cirque. This fact allowed us to distinguish the different landforms. Talus cones, gelifruction lobes and rock glaciers occurred between the western wall of the cirque and the LPM (Late Pleistocene Moraine). Table 4. Electrical and seismic sounding. Features. __________________________________________________________________________________________________ Zone A Zone B Zone C Zone D ……………………………………………………………………………………………………………………. Geographical Western zone Middle zone Eastern zone Eastern zone situation …………………………………………………………………………………………………………………………… Average altitude 3062 m 3086 m 3105 m 3123 m ………………………………………………………………………………………………………………………………… Logs length 92,5 m 92,5 m 42,5 m 192,5 ………………………………………………………………………………………………………………………………… Average depth 14 m 14 m 10 m 25 m ………………………………………………………………………………………………………………………………… Landform debris fan moraine ridge rock glacier rock glacier ………………………………………………………………………………………………………………………………… Maximum minimum 153 minimum 451 minimum 3431 minimum 150 Resistivity (Ohm/m) maximum 4862 maximum 5702 maximum 58914 maximum 562220 ………………………………………………………………………………………………………………………………… Seismic velocity (m/s )maximum 4200 maximum 3800 maximum 3600 maximum3600 __________________________________________________________________________________________________ The existence of ice masses (permafrost) covered by debris was suggested by the interpretation of vertical electrical sounding carried out during the 1995 summer. Later electrical and seismic sounding of the PACE project (Permafrost and climate in Europa) developed by Terradat-LTD & ETH in 1995, highlighted the existence of high resistivity deep layers exclusively in the eastern zone of the Corral del Veleta (table 4). Table 5. Physical features of the core _________________________________________________________________________________________________ Reach Thickness Lithology Sedimentogical and petrographical features ……………………………………………………………………………………………………………………………….. A 120 cm Micaschist Non weathered heterometric blocks in a consolidated structure at the rock glacier front …………………………………………………………………………………………………………………………………. B 30 cm Sediment Micaschist, gravels and sand embedded in melting ice fragments …………………………………………………………………………………………………………………………………. C 40 cm Ice masses C1 (15 cm). Frozen mass with micaschist clasts. The ice is formed by amorphous crystals and it contains lots of air passages. ……………………………………………………………………………... C2 (25 cm). Dense and crystalline ice mass. __________________________________________________________________________________________________


29

A borehole was carried out in August 1999 at an altitude of 3105 m in the front of a rock glacier in the eastern zone of the cirque (zones C and D) 250 m away from the Lagunilla del Corral pond (Table 4). There, the highest resistivity values (58914 and 562220 Ohm/m) (figures 18 y 19) and the type of landforms (gelifruction lobes with transverse ridges and rock glaciers) suggested the possible existence of deep ice masses. The drill core was continuous and reached 190 cm deep (table 5 and Picture 8).

Picture 8. Borehole log of the permafrost top in the Corral del Veleta cirque The existence of frozen sediments from 1,20 m deep downwards, the high resistivity of zone D (562220 Ohm/m) and the location of ice bodies in the first 50 cm under the ground in other places of the debris-mantled slope of the Corral de la Veleta cirque, ensures the occurrence of alpine permafrost in the eastern zone of the cirque. The permafrost would be covered by debris and it would occupy 3200 m2. Its origin is associated with the deglaciation of the Corral del Veleta glacier after the Little Ice Age at the end of the XIX century. Halfway through the XX century the glacier had already become a single ice body situated in the eastern zone of the cirque. Debris material covered it becoming a motionless black glacier. This origin explains its distribution and the irregular depth of the permafrost top. In Sierra Nevada massif, we are only sure that permafrost exists in the Corral del Veleta cirque, although BTS results indicate that it may also occur in other places such as the Cerro de los Machos peak (3240 m). Permafrost survival lies in the insulating effect of the debris cover. In addition, the topography and orientation of the Corral del Veleta cirque made its eastern zone be an especially cold place. Solar radiation is minimum and snow cover remains most of the year. As a result, the underlying ice melts slowly although the thermal measurements we are carrying out inform that the solar radiation effect reaches the top of the permafrost (Gómez Ortiz et al., 2004)


30

3.3. Relationship between slope geological processes and snow cover Rockfalls from the walls of the Corral del Veleta cirque has led to a debris-mantled slope affected by gelifluction lobes, debris flows and rotational landslides. The existence of permafrost may explain the formation of these slope movements (Gómez Ortiz et al., 2001). Every winter the debris-mantled slope is covered by several metres of snow that usually remains all the year. The insulating effect of the snow cover may protect the substratum from temperature changes (Gómez Ortiz et al. 2003). Besides, ice fixes the blocks and prevents slope movements (Thorn, 1988). In contrast, when the snow cover abruptly melts during summer time, the shear strength of the slope decreases leading to large slope movements (Strömquist, 1985; Rapp & Nyberg, 1988). Every year since 1995 we have monitored the residence time of the snow cover and the geomorphological processes in order to understand the relationship between snow cover dynamics and slope movements (table 6 and figure 18). The monitoring has been carried out by oblique aerial photographs in September when snow cover is minimum. Slope movements and snow cover have been mapped and the photographs have been georeferenced using a 5 m resolution digital elevation model (DEM).

Source: TerraDat & ETH (1998)

-2.0-4.0-6.0-8.0-10.0-12.0-14.0-16.0

Elevation

West

Very High Resistivity Zone

East

0.0

10.0

20.0

30.0

40.0

3431 5150 7730 11603 17418 26146 39247 58914

Interation 5 RMS error = 4.5

Resistivity in Ohm.mUnit Electrode Spacing = 2.5 m.

High Velocity (3600 m/s)from seismic Line D

Vertical exaggeration in model section display= 1.0First electrode is located = 0.0 m.Last electrode is located = 40.0 m.

Figure 18. Topography and Electrical D-log in the Corral del Veleta cirque. Evolution of the snow cover (1995/2004) In 1995 the climate was dry and only some snow patches remained. In contrast, during 1996 and 1997 the bottom of the Corral del Veleta cirque was covered of snow all the time. In 1998 the snow occupied 65 per cent of the slope. In 1999 the climate was very dry and the snow disappeared at all. In 2000 the slope was almost uncovered. In 2001 as in 1998 the snow covered 60 per cent of the slope. In 2002 as in 2000 rainfalls were scarce and only some snow patches occurred at the head of the eastern talus cones. In 2003 the situation was better than in 2002 and the snow remained in the contact between the slope and the cental and eastern cirque walls and in different points of the western zone. In 2004 the extension of the snow cover was greater than in previous years and it occupied the bottom of several basins (table 6 and figure 20).


31

30.0

Elevation (m)

20.0

10.0

0.0

-10.0

-20.0

40.0

80.0

150 486 1575 5102 53557 173525 56222016530

Low Resistivity Moraine Material

High Velocity (3600m/s)from seismic Line C

Ice-creep tngue

Very High Resistivity ZoneRock fall talus

South West North East

Unit electrode spacing= 5.0 m

Resistivity in Ohm.m

Vertical exaggeration in model section display= 1.0First electrode is located = 0.0 m.Last electrode is located = 200.0 m.

120

160

0.0

Source: TerraDat & ETH (1998)

Iteration 5 RMS error = 11.2

Figure 19. Topography and Electrical C-log in the Corral del Veleta cirque. We can distinguish three areas depending on the snow residence time. The first one is located between the debris-mantled slope and the eastern and central walls of the cirque. There snow can be found all the year. The second one is situated in creeks and between the debris-mantled slope and the western wall of the cirque. In these places the snow only remains in very wet years. Finally, the third one covers the central zone of the debris-mantled slope and the western side of the cirque. Here, the snow tends to disappear every summer.

3.053,5

3.065,5 3.073,0

3.072,5

3.095,5

3.137,0

3.398

3.352,5 3.319,5

3.298,5

3.323,5

3.324,5

3.299,0

3.250

3.300

3.200

3.150

3.100

3.0503.000

2.950

2.900

3.350

3.300

3.250

4.102.04.102.0

4.101.5 4.101.5

468.0467.5

468.0467.5

Pico del Veleta Cerro de Los Machos

Rocky outcropsRockwall Scarps in bedrock Lake

Very low summer snow-cover remain Low summer snow-cover remain Medium summer snow-cover remain High summer snow-cover remain Maximum summer snow-cover remain

Elaborated by Luis Tanarro Figure 20. Evolution of the snow cover in the Corral del Veleta area in summer (from 1998 to 2004).


32

Table 6. Snow cover in the debris-mantled slope of the Corral del Veleta cirque during September ________________________________________________________________________________ Time of residence Percentage of covered area Years ……………………………………………………………………………………………………………………… Maximum time >65 % 1996, 1997 Intermediate time 25/65 % 1998, 2001, 2004 Minimum time <25 % to 0 % 1995, 199, 2000, 2002, 2003 ________________________________________________________________________________

Geomorphological dynamics of the debris-mantled slope (1995/2004) A talus cone situated in the eastern side of the cirque is undergoing the most important slope movements. In 1995 several debris flows formed (60 cm long and 50 cm wide). The largest blocks moved towards the surface and they were stacked at their toes while fine-grained sediments occurred in the core of the deposit. In the accumulation zone, they showed low-gradient surfaces with transverse ridges. Their toes were irregular and they presented vertical throws of 1 m high. During the same year, the slope was affected by an important landslide. The depleted mass was more than 6m long, 4 m wide and 2.5 m high. Three years later, in 1998, when the talus cone was uncovered of snow again these landforms had already disappeared. During this year, the snow melting led to new debris flows that displayed similar features to the ones formed in 1995. In 1999 two large rotational landslides affected the eastern and central zone of the talus cone. The one formed in the central part evolved to a debris flow. Apart from the landslide, many gelifluction lobes broke the slope topography. Slope movements affected 40 per cent of the eastern and central sectors and 15 per cent of the western sector of the talus cone. In September 2000, the preexisting movements became blurred due to the formation of three debris flows whose failure surfaces were above the 1999 landslide scarps. These flows were from 10 m to 25 m long and 0.5 m to 1.5 m wide. In 2001, many superficial debris flows broke the topography of the debris-mantled slope. They were between 5 and 10 m long. In addition, at the end of the spring a huge rockfall covered 50 per cent of the talus central zone. The snow that was trapped by the fallen rocks remained all the year. In 2002, the melting of the trapped snow led to thermokarstic collapse structures. Besides, very plastic debris flows formed at the head of the debris-mantled slope. They were between 20 m and 50 m long and from 8 m to 15 m wide. In 2003, a few debris flows occurred and the central zone affected by the 2001 rockfall became blurred. The changes observed in 2004 were few and they were associated with meltwater runoff. Conclusions After 10 years of study it seems that the increase in the number of slope movements is related to the absence of snow cover during summer time although there are differences depending on the zone of the debris-mantled slope (picture 9). The existence of frozen layers within the substratum and the amount of fine-grained sediments are also important factors for the development of slope movements. The debris-mantled slope that connects the bottom and the vertical walls of the cirque can be divided into three zones depending on the activity of the slope processes. The western zone is not affected by slope movements except for the bed of the creeks where snow tend to remain. Slope movements, especially debris flows and landslides, often take place in the central zone. Processes in the eastern zone aren’t as active as the ones that happen in the central zone. The movements tend to be slow and they are associated with frost heaving, thermokarstic collapses and solifluction flows.


33

Picture 9. Debris-mantled slope in the Corral del Veleta cirque The obtained data indicate that the northern wall of the Corral del Veleta cirque is undergoing a transition from an old glacial period to a present deglaciated paraglacial period. In the former, cirque walls were affected by unloading and gelifraction processes that supplied a big amount of debris. The material was evacuated by the glacier forming moraine ridges. During the latter, a debris-mantled slope was generated. At the beginning of the XX century, during this transition period we can distinguish a post-glacial stage in which the Corral de la Veleta glacier mass balance became negative. As a result, the glacier was unable to transport the supplied materials that came from the cirque walls leading to an incipient debris-mantled slope. The thickness of the debris was minimum in comparison with the thickness of the ice mass. Solar radiation at the toe of the cirque walls was minimum. Therefore, the ice melted at a slower rate than at the top of the cirque walls. The slope was affected by slow slope movements and processes such as frost heaving, thermokarstic collapses and solifluction flows. In a later period, the debris-mantled slope already appeared but in an early phase of development. Although the debris layer was thicker, there were important frozen beds very close to the surface. As a result, their permanence or melting depended on the snow cover variation. Slope movements were fast, catastrophic and they had a very irregular distribution. In the last period the frozen layers disappeared and the slope reached its mature phase. The slope became a real accumulation area and its morphogenetic activity was controlled by the debris supply and the snow cover. 3.5. Dynamics of the Lagunilla del Corral rock glacier It is an active rock glacier situated in the western zone of the Corral del Veleta cirque (Gómez Ortiz et al., 1999, 2004). It descends from the debris slope at an altitude of 3174 m to the altitude of 3100 m in the vicinity of the Lagunilla del Corral pond. It is a 120 m long, L-shaped rock glacier with an average width of 30 m and an average topographical gradient of 23º. The boulders


34

and stones that constitute its surface may reach 10 m thick. It occupies an area of 3700-4000 m2. Its central and frontal parts are characterized by the existence of arcuate ridges and lobes. Its origin, present morphology and dynamics are associated with the continuous supply of rock debris from the eastern wall of the Corral del Veleta cirque, the intervention of cold processes in the debris and the successive and different slope movements that affect it. We are especially interested in its evolution, movement, geometric configuration, the temperature of its layers and the temperature of the permafrost top (picture 10).

Picture 10. Corral del Veleta cirque and Corral del Veleta rock glacier. Table 7. Straight rods average displacement (cm/year) ___________________________________________________________________________________________________ Rod number rock glacier zone 1995-1999* 2001-2004** __________________________________________________________

H V H V ………………………………………………………………………………………………………………………………… B1 Front -- -- 5,9 -19,3 B2 -- -- 8,8 -24,8 B3 -- -- 12,8 -22,6 B4 -- -- 4,8 -20,9 …………………………………………………………………………………………………………………………………… B5 Back front -- -- 4,5 -17,7 B6 -- -- 4,1 -16,5 B7 -- -- 6,3 -19,0 B8 12,4 -8,6 3,7 -14,6 B9 -- -- 6,6 -19,4 ………………………………………………………………………………………………………………………………….. B10 Main body 1,9 -9,8 1,7 -19,3 B11 6,1 -15,5 4,2 -21,4 B12 -- -- 2,7 -16,9 B13 -- -- 4,0 -18,7 B14 -- -- 6,5 -13,2 B15 -- -- 4,4 -12,8 B22 -- -- 2,0 -44,7 B23 14,5 -27,1 24,9 -39,6 B24 5,7 -44,8 7,1 -25,7 B25 -- -- 12,9 -18,2 ____________________________________________________________________________________________________ * Topographical measurement; ** Measurements using topographical, geodetic and photogrammetric techniques. H. Horizontal displacement (advance), V. vertical displacement (subsidence) ____________________________________________________________________________________________________


35

We have used Sanjosé Blasco (2003) and Sanjosé Blasco et al. (2004) methodology and techniques (Geodetic, topographical and photogrammetric monitoring) in the central and frontal parts of the rock glacier in order to determine its movement (figure 21). The installation of straight rods in its perimeter, transverse lines and points allow us to measure vertical (subsidence) and horizontal (advance) topographical variations once a year since 1995 (table 7). The average subsidence of the glacier indicates that during 2002-2003 and 2003-2004 the volume-loss was 1500 m3 and 336 m3 respectively. In 1999, a drillhole were carried out in the rock glacier front at an altitude of 3.107 m reaching the permafrost top. Thermal sensors of the datalogger tiny-talk type (-35º/+70ºC temperature range, and in past years UTL-1) were installed inside at depths of 0m, -0.15 m, -0.40 m, -0.9 m and 1.9 m. We used Ramos’ (1998) methodology for data capture (table 8 and figure 22).

3130

3120311031003090

SCV

23

7 8

13

22

24

6

lagoon

25

23

11

10

12

1514

1

5

4

9

Perimeter

Transect

Resistivity

Reference bar

Borehole and thermal control

Topographical base

30 m0 10 20

4.1

01

,54

.10

1,6

467,7 Figure 21. Topography and measurement points of the Corral del Veleta rock glacier.

The research of the Corral del Veleta rock glacier demonstrates the relationship between the thermal evolution of the ground (with or without snow cover) and the instability of the rock debris. The dynamics of the rock glacier is deduced by the vertical and horizontal displacement of the rock debris layer and the volume balance. The different mechanical response of the debris layer to canges in the superficial and internal temperature may explain its dynamics.


36

0 5 10 20 30-5-10-20800

0

-15

-40-50

-90

-190

-30 40

0 5 10 20 30-5-10-20-30 40

800

0

-15

-40

-90

-190

TMR (2001 - 2002)

GR (2001 - 2002)GR (1999 - 2000)

TMR (1999 - 2000)

-50

400 400

PV (2003 - 2004)

GR (2003 - 2004)

Extreme min. Mean

Temperature (ºC)Extreme Max.

Ther

mal

sam

ple

leve

l (cm

)

Figure 22. Sub-surface temperatures of the Corral del Veleta rock glacier and Verro de los Machos pass and air temperatures of the Veleta summit. 4. Cerro de los Machos pass Cerro de los Machos pass (3299 m) connects the Picacho del Veleta peak (3398 m) and the Cerro de los Machos peak (3327 m). The pass is the remain of an old quaternary cryoplanation terrace cut into bedrock, limited by cirques and characterized by the existence of patterned grounds. Nowadays, the pass has become a place where cold processes are effective and significant due to repetitive snow melting (picture 11).

Picture 11. Cerro de los Machos pass and Picacho del Veleta peak

Tabl

e 8.

Ext

rem

e an

d av

erag

e te

mpe

ratu

ra v

alue

s for

air

and

grou

nd su

rfac

e, in

the

Cor

ral d

el V

elet

o ci

rque

, Col

lado

de

los M

acho

s Pea

k an

d m

eteo

rolo

gica

l ref

eren

ce

site

s

1999

-200

0 (N

ov-a

ugus

t) 20

01-2

002

(Nov

-aug

ust)

2003

-200

4 (N

ov-a

ugus

t)

Site

em

T xT

eM

T eA

em

T xT

eM

T eA

em

T xT

eM

T eA

TMR

(air)

3.

095

m a

sl

-20.

7 0.

4 15

.9

36.6

-1

6.5

0.2

15.7

32

.2

PV(a

ir)

3.39

5 m

asl

-2

1.5

0.4

28.6

50

.1

(-2)

-22.

2 -1

.3

36.5

58

.7

-22.

3 -1

.1

35.1

57

.4

--

--

--

--

--

(-

50)

-7

.5

-0.8

15

.0

22.5

-7

.3

-1.7

10

.7

18.0

--

--

--

--

--

CV

-rg

(-2)

3.

107

m a

sl

-19.

6 2.

2 23

.0

42.6

--

--

--

--

--

--

--

--

--

(-15

)

-4.0

-1

.1

20.6

24

.6

-5.0

-1

.3

28.9

33

.9

CV

-rg

(-15

) 3.

107

m a

sl

-2.8

-0

.6

12.9

15

.7

(-40

)

-4.0

-1

.1

14.2

18

.2

-4.9

-1

.8

15.7

20

.6

--

--

--

--

--

(-

90)

-4

.0

-1.4

8.

4 12

.4

--

--

--

--

(-90

)

-2.5

-0

.8

8.2

10.7

(-

190)

-3.1

-1

.5

-0.6

2.

5 -4

.3

-3.2

-2

.1

2.2

--

--

--

--

--

C

M-p

g (-

10)

3.29

7 m

asl

-1

3.4

1.4

28.1

41

.5

--

--

--

--

CM

-pg

(-5)

3.

297

m a

sl

-12.

9 0.

6 29

.3

42.2

(-

50)

-7

.2

1.0

14.5

21

.7

--

--

--

--

(-50

)

-7.4

0.

4 12

.1

19.5

TM

R: m

icro

met

eoro

logi

cal r

efer

ence

stat

ion

(PA

CE

Proj

ect)

(309

5 m

) PV

: Vel

eta

Peak

C

V-r

g: C

orra

l del

Vel

eta

rock

gla

cier

bor

ehol

e C

M-p

g: C

olla

do d

e lo

s Mac

hos p

atte

rned

gro

und

(--)

: no

data

emT:

ext

rem

e m

inim

um te

mpe

ratu

re

xT: m

ean

tem

pera

ture

eM

T: :e

xtre

me

max

imum

tem

pera

ture

eA

: ext

rem

e th

erm

al a

mpl

itude


37


38

4.1. Cryoplanation and patterned grounds It seems that Sierra Nevada summits didn’t host fjeld type glaciers (Sánchez Gómez, 1990). The reason is that snow deflaction inhibited the development of ice sheets. This is evidenced by the absence of striated, polished and grooved rock surfaces. Under these conditions periglacial processes (gelifraction, geliturbation and gelifluction) were important forming cryoplanation or altiplanation terraces. The altiplanation terraces of Sierra Nevada are highly eroded, individualized and isolated due to the later retreatment of glacial cirque walls. The Machos cryoplanation terrace that is limited by Valdeinfierno and Corral del Veleta cirques is a good example. In addition, this surface host the most interesting patterned ground of Sierra Nevada and the best ones of the Iberian Peninsula. The patterned ground occupies 0.5 ha and it includes different forms depending on the topographical gradient. Sorted circles and stone roses occur when the slope angle is less than 3º. If it is between 3º and 6º the circles deform and become sorted stripes. All of them resulted from frost heaving (Tricart & Cailleux, 1967). The stones that constitute them are metric and decimetric size (picture 12). The surroundings of patterned grounds show terraces, vegetation stripes, stone lobes and block fields at slope angles between 6º and 9º. When the slope increases the previous forms grade into block lobes. When the slope angle exceeds 20º the resulted forms are closely associated to gravitational processes as it occurs in the boundaries between the terrace and Valdeinfierno and Veleta cirques. In contrast to other cryoplanation terraces of Sierra Nevada, lichen and psicroxerophite grass have settled on Macho terrace patterned grounds. Therefore, they are inherited forms that were generated in cirques and headwaters during the last phases of the Pleistocene glaciarism.

Picture 12. Detailed picture of Machos cryoplanation terrace patterned ground field 4.2. Geophysics and thermometry Thanks to Pace Project, a vertical electrical sounding survey was carried out in the Cerro de los Machos peak in 1999 in order to determine the existence of permafrost. The electric logs points


39

out the probable existence of discontinuous permafrost between 1 m and 4 m deep. The maximum values of resistivity were 100.000 Ohm/m (Terradat & ETH, 1998-99) (figura 23). Datalogger type thermal sensors (NTC100 thermistor Tiny-Talk model and TMC-1T thermistor UTL-1 model) were installed at -5 cm, -10 cm and -50 cm deep. We only have the continuous and daily data of the 1999-2000 and 2003-2004 field surveys from November to August (table 8). We observed in the table that the thermal profiles show a constant and positive mean temperature of 0.4ºC. This value may be in agreement with the existence of permafrost 1 m under the surface. Other important data are the large number of days in which the temperature was negative or it ranged around 0ºC during the 2003-2004 survey (table 9 and figure 24).

5000030000150007000

0.50.5

2.5

5.0

0.0 6.0 16.0 24.0 32.0 40.0Depth Teration 8 RMS error -4.0%M.

Inverse Model Resistivity Section

Figure 23. Resistivity tomography in pass Cerro de los Machos pass. We can see from the table 9 that the air temperature was 102 days bellow 0º C and 115 days ranging around 0º C. For the same period, ground temperature shows negative values 195 days at -50 cm deep and 162 days at -10 cm deep. In addition, freeze and thaw cycles occurred 2 days at -50 cm deep and 47 days at -10 cm deep. These temperature data demonstrate that the ground freezes while there is a snow cover and it melts during the postnival period. On the other hand, the fluctuation of temperature values around 0º C (47 days at -10 cm deep and 2 days at -50 cm deep) suggests that the snow cover does not exert an insulating effect probably due to the deflaction of the snow cover. Table 9. Freezing days and freeze/thaw cycles (Period 2003-2004, Nov-August)

__________________________________________________________________ Sampling site D+ D+/- D- __________________________________________________________________ PV (air) 3095 m 88 115 102 ……………………………………………………………………………………… CV-rg (-15 cm) 23 1 279 CV-rg (-50 cm) 23 1 279 ……………………………………………………………………………………… CM-pg (-10 cm) 89 47 162 CM-pg (-50 cm) 102 2 195 __________________________________________________________________

PV: air temperature in the Veleta peak (3396 m) 4 m over the ground; CV-rg: bore-hole in the Corral del Veleta rock glacier (3107 m); CM-pg: Paterned ground in Cerro de los Machos pass (3297 m).


40

Extreme min. Mean

Temperature (ºC)

Ther

mal

sam

ple

leve

l (cm

)

TMR (1999 - 2000) PV (2003 - 2004)

CM (1999 - 2000) CM (2003 - 2004)

Extreme Max.0 5 10 20 30-5-10-20

800

0-10

-50

-30 40800

0

-50

400400

0 5 10 20 30-5-10-20-30 40

-10

Figure 24. Sub-surface temperatures of the Cerro de los Machos pass and air temperatures of the Veleta summit. All these data do not allow ensuring the existence of permafrost in the Cerro de los Machos peak. Therefore, we will need to control the temperature at deeper layers in the coming future. Acknowledgements We are sincerely grateful to E.U. ENV4-Ct97-0492 and Spanish Government BSO0745 research projects and to David Serrano Giné for his graphical support. Servei de Paisatge from Universitat de Barcelona is thanked for it special collaboration, as well as Parque Nacional de Sierra Nevada for it stakeholder.


41

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