Morphometric-morphologic analysis of drainage systems in Taftan volcanic cone by means of geographic information systems Shahram Bahrami Ph.D. student in geomorphology, Department of Physical Geography, College of Geography, University of Tehran, Iran. shahram_bahrami2003@yahoo.com  Mojtaba Yamani Assosiate Professor of geomorphology Department of Physical Geography College of Geography, University of Tehran, Iran. myamani@ut.ac.ir  Seyed Kazem Alavi Panah, Assosiate Professor ,Department of Cartography College of Geography, University of Tehran, Iran. salavipa@ut.ac.ir Abstract Semi-active volcanic cone of Taftan is located in southeast of Iran, northwest of Khash. While there is water vapor and sulfur dioxide exhaustion from its peak, the presence of deep valleys and irregular shape of drainage system demonstrates long term erosion and complicated development of drainage system. For the evaluation of effects of lithology, slope degree and slope aspect on drainage density, after digitizing of topographic and geologic maps, we prepared digital elevation model(DEM), slope degree and slope aspect, drainage system and lithologic maps of Taftan cone. Then drainage densities in all formations, slope aspects and slope degrees were extracted. Data analysis reveals that, except alluvial fans and terraces, drainage densities in old formations and steeper slopes are more than in recent formations and lower slopes. Drainage densities in eastern and southeastern aspects, which are corresponding to older formations, are more than other aspects. Investigation of drainage patterns in the study area shows that drainage patterns in younger lavas are parallel while in older lavas are dendritic. Nevertheless, there are irregularities in patterns.The comparison of old and new alluvial fans in IRS satellite images illustrates considerable differences in drainage patterns so that drainage patterns in new and old fans are divergent and convergent downward respectively. This is due to agradational process in new fans and degradational process in old ones. While morphometry and morphology of drainage systems are affected considerably by strength and geomorphologic development, it is difficult to accept entirely the drainage system chronological development hypothesis in Taftan cone, because there are inherent complexity of volcanic cones especially in rocks complexity, complicated primary conditions of lavas and pyroclastic rocks, variability of the content of formations and inaccessibility of macro-scale topographic maps. Introduction Volcanoes are constructional landforms that either are being built or were built and might were eroded by erosion. Magma composition and percentage of lava, tephra and volcanic ash often determine morphology, slope angle and erosion capability of any volcanic cone. Alternation of activity and quiescence, time elapsed since volcanic activity ceased, differences in lava and pyroclastics proportion, slope angle and climate affect on drainage density and pattern of volcanic cone. Drainage density as total stream length per unit area, represents the degree of fluvial dissection (Lin, 2004).Semi-active situation of Taftan stratovolcano (its water vapor and sulfur dioxide exhaustion), on one hand, and alteration of original shape due to erosion (such as deep valleys and irregular shape of drainage system), on the other hand, lead us to analyze drainage system and its relation to lithology, slope angle and aspect. Study area The study area is a rectangular that include volcanic cone of Taftan in southeast of Iran, 42km northwest of Khash (Fig.1(A). study area is located between 61° 00 and 61° 15 ´ E longitude and 28° 30´ and 28° 40´ N latitude, has an area of 398 km2 and altitudes of 1700 to 3943 m (Fig.1( B). The presence of Taftan stratovolcano is intimately related to the subduction of Neotethys oceanic lithosphere under the central Iranian continental plate. The main volcanic activity of Taftan belongs to Neogene compressional phase but the volcanic intensity has continued from upper Pliocene through recent time (Ghazban, 2004). The most distinct geological characteristic in Taftan cone is the presence of thick quaternary volcanic rocks. According to lithologic map (Fig. 2(A), there are following geological formations in study area: Qt2: young terraces, gravel plain (Dasht), and young fans (Holocene) Qt1: older composite fans and high terraces (upper Pleistocene) Qa3: youngest andesite of Kuhe Taftan (upper Pleistocene) Qa2: older andesitic flows and extrusive dome with minor detrital boulder breccias (Pleistocene) Qag: fanglomerate and lapilli tuf with andesitic and dacitic components (Pleistocene) Q1u: interbedded andesitic tuff, siltstone, mudstone (Pleistocene) Qa1: older andesitic flows with minor basaltic andesite and olivine basalt (lower Pleistocene) From petrological point of view, old and modern lavas of Taftan are similar, and they comprise a lot of pyroclastics. Taftan cone has two peaks including northwestern and southeastern peak. Northwestern peak is old and intensively eroded and has dissected margins and, in some places, andesitic flows are preserved. Southeastern peak, to some extent, has a conic shape and is covered by thick and new andesite flows. Everyday, there are some cubic meters of sulfur dioxide and water vapor exhaustion in southeastern peak (Fig. 2 (B).Also there are numerous white effusions with whistling sound in southern steep slopes. These effusions create white cloud that can be seen from 100 km distance. In general, erosion is higher in old lavas (such as deep valleys in west and south of cone (fig. 2(C)) than in modern lavas. Data base and method In this study, with the aim of evaluation of effects of slope angle, slope aspect and lithology on drainage density and pattern , at first we digitized 1/50000 scale topographic map and 1/250000 scale geologic map of study area using ILWIS software in a rectangular shape that comprised all of Taftan cone. Then, digital elevation model (DEM), slope angle, slope aspect, drainage system and lithologic maps of study area were prepared. At third step, after rasterizing and crossing of above mentioned maps, the total length of drainages in different classes of formations, slope degrees and slope aspects were extracted and accordingly, mean drainage density in each class computed. Finally, for studying of drainage pattern, drainage systems extracted from topographic map and from IRS satellite image, were analyzed. Results and discussion Deformation of any volcanic cone is a function of lithology, time elapsed since volcanic activity ceased and climatic parameters. Also, slope angle and aspect affect on erosion or dissection of cone. Therefore, we examine the drainage density, as an erosion index, in different classes of slope degrees and aspects and geological formations. Figures 2(A), 3(A), 3(B), and 3(C) show geologic, drainage system, slope aspect and slope degree maps of study area, respectively.  Fig. 1. the location map (A) and hypsometry of study area (B) .  Fig. 2. geologic map (A) water vapor and sulfur dioxide exhaustion from peak (B) and deep valley in southern aspect of Taftan cone(C) On the basis of rasterizing and crossing of above mentioned maps, mean drainage densities in different classes of geology, slope angle and aspect were computed (table 1). According to table 1, Qag and qal formations have higher mean drainage densities than other formations. Qal (modern andesite) has minimum drainage density. Table 1, also, shows that mean drainage densities tend to decrease from flat to steeper slopes.  Mean drainage densities are higher in southern, eastern and southeastern aspects than other aspects. Overlaying the geologic and aspect maps demonstrated that northern aspects are mainly coincidence with younger formations while southern aspects are underlied by older formations (qt1, q1u, qa2). Evaluating the drainage systems shows that drainage patterns changes in different geological formations. The overall channel pattern in Taftan cone is fairly radial. Patterns in alluvial fans and siltstone- mudstone (qt1, qt2 and q1u) are parallel and in andesitic lavaes (qa1, qa2 and qa3) are dendritic (fig.4( A)). In general, from new to old andesites, the dendritic pattern becomes more developed. Fig. 4(A) shows some selective drainage patterns in different geological formations. Evaluation of fig. 4(A) represents that, while there is orientation of parallel to dendritic pattern from new to old formations, irregularities can also be observed in patterns. For example, q1u (older formation) has fairly parallel and qa2 (younger formation) has more developed parallel pattern. Precise studying of IRS satellite image of Taftan cone revealed that channel pattern in old fans is convergent downward and in modern fans is divergent downward (fig.4 (C)that demonstrate the presence of flooding process(fookes,1986). Figure 4 (B) shows chronological development of an experimental drainage system in response to simulated rainfall. Although experimental studies suffer some drawbacks in comparison to natural systems, they are useful in providing conceptual models (Ritter, 1995).according to chronological development of drainage system (Schumm et al., 1987), passing of time and decreasing of slope degree cause drainage system to develop from parallel to dendritic patterns. Nevertheless, in spite of overall accuracy of Parker experiments (Schumm et al., 1987), there are irregularities of channel patterns in geological formations (fig. 4(A)). For example, qiu(relative old formation) has parallel pattern. These irregularities come from inherent complexity of volcanic cones especially in rocks complexity, complicated primary conditions of lava and pyroclastic rocks, variability of formation content and inaccessibility of macro-scale topographic maps.  Fig. 3. drainage systems(A) slope aspects (B) and slope degrees of study area.  Fig. 4. drainage patterns in geological formations (A)chronological development of an experimental drainage system in response to simulated rainfall(Parker, 1977, cited in Ritter et al., 1995) (B) and channel patterns in old and new fans in south of Taftan in IRS satellite image(C). Conclusion Analysis of data relating to drainage density illustrates that, except in alluvial fans, old formations and smaller slope angles have higher drainage densities than younger formations and larger slope angles. Drainage densities in southern, eastern and southeastern aspects (coincident with older geological formations) are higher than northern aspects (coincident with younger geological formations). Evaluation of channel pattern in relation with lithology represents that, with overall tendency of parallel to dendritic pattern from new to old formations, there are some irregularities in drainage patterns. It can be, to some extent, accepted the viewpoint of some researchers like Parker about drainage density and pattern relation with time passing and slope angle in study area but there are complexities in drainage characteristics especially in channel pattern. These complexities, partly, are arisen from inherent complexities of volcanic cones and from complicated primary conditions of lava and pyroclastic rocks. It seems that reconstruction of primary shape of Taftan cone and estimation of its erosion volume by means of digital elevation models (DEM) and satellite images will assist to better explanation of drainage development in future researches. Acknowledgement We thank A. Ghorabi and P. Jam for their support with the preparing some maps and constructive review of the manuscript. References
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