Geomorphology 123 2010 142 153 Contents lists available at ScienceDirect Geomorphology j o u r n a l h o m e p a g e w w w e l s ev i e r c o m l o c a t e g e o m o r p h Geomorphic assessment of the debris avalanche deposit from the Jocotitl n volcano Central Mexico Sergio Salinas 1 Jorge L pez Blanco Depto de Geograf a F sica Instituto de Geograf a Universidad Nacional Aut noma de M xico Ciudad Universitaria C P 04510 M xico D F M xico a r t i c l e i n f o Article history Received 25 July 2009 Received in revised form 8 July 2010 Accepted 9 July 2010 Available online 15 July 2010 Keywords Jocotitl n volcano Geomorphometry Principal components Cluster analysis Hummocks Ridges a b s t r a c t Edi ce collapse of the Jocotitl n volcano produced a debris avalanche deposit whose morphology is characterized by conical hummocks and elongated ridges We consulted aerial photographs orthophotographs and conducted eld work to de ne eld relationships between mound morphology and stratigraphy Based on eld evidence and geomorphic and geologic interpretation we sub divide the deposit into three sectors north northeastern and eastern We determine the emplacement mechanisms of the different sectors based on their distinct morphologic and lithologic features In this context we generate a geomorphometric database comprising 17 variables for each mound and apply multivariate statistical methods principal component analysis and cluster analysis to de ne the relationships between them The principal components incorporated 73 of the total data variance and seven geomorphometric variables perimeter major axis area height distance to the source axis ratio and circularity index de ned two groups hummocks and ridges The circularity index and the axis ratio best characterize the elongated form of hummocks composing the deposits Contrasts in lithological characteristics such as material strength and mobility provide evidence for a transition from a sliding mass debris avalanche deposit to a debris ow like emplacement Differences in deposit morphology suggest two collapse mechanisms magmatic intrusion Bezymianny type sector collapse and a tectonic mass slide an earthquake could provide the trigger for both collapses The eastern lower ank of the volcano then collapsed gravitationally due to a movement along a fault tectonic activity possibly related to the Acambay Tixmadeje Fault System This produced the northeastern sector of the debris avalanche deposit dominated by large elongated ridges The spatial arrangement of both sectors the N and NE suggests that the two failure events occurred simultaneously Finally the eastern sector of the deposit was emplaced by posterior remobilization of material from the two other sectors through a debris ow 2010 Elsevier B V All rights reserved 1 Introduction 1 1 Volcanic debris avalanche deposits Major edi ce collapse is a common phenomenon in the growth history of many composite volcanoes It occurs mainly in andesitic to rhyolitic stratovolcanoes e g Mt St Helens in 1980 Lipman and Mullineaux 1981 Siebert 1984 Siebe and Sheridan 1990 The instability and weakening of a volcanic edi ce is the result of several factors which include repeated magmatic intrusions and emplacement of the volcano in a tectonically active region prone to large earthquakes Francis and Wells 1988 Voight and Elsworth 1997 Seismic activity not only directly affects the stability of the volcanic edi ce but may also deform its basement weakening the volcanic edi ce Borgia et al 2000 Corresponding author 52 5 6224119x20 fax 52 5502486 E mail address sss geo sica unam mx S Salinas 1 Present address Depto de Volcanolog a Instituto de Geof sica Universidad Nacional Aut noma de M xico Ciudad Universitaria C P 04510 M xico D F M xico 0169 555X see front matter 2010 Elsevier B V All rights reserved doi 10 1016 j geomorph 2010 07 006 A debris avalanche is a rapidly moving incoherent waterundersaturated mass made up of a mixture of blocks sand and silt which ows under the in uence of gravity There are three principal processes which can trigger the collapse of a volcano 1 a volcanic eruption magmatic intrusion 2 a phreatic explosion or alteration i e hydrothermal alteration and 3 seismic tectonic activity Siebert et al 1987 The direction of edi ce failure depends on structural lineaments which usually correspond with the direction of maximum distension in the tectonic regime Voight and Elsworth 1997 The resulting topography of the collapse sector is a debris avalanche deposit DAD that is morphologically characterized by mounds For this study we distinguish two types of mounds 1 hummocks or mounds with irregular to conical form with no elongation and 2 elongated mounds or ridges which depending on the main direction of the ow can be transverse or longitudinal to the ow direction Siebert 1984 Both types of mounds compose a hilly topography proximal to the volcano and decrease in size and number towards more distal parts Generally the deposit is bordered by prominent marginal levees and characterized by small closed basins forming between hummocks S Salinas J L pez Blanco Geomorphology 123 2010 142 153 Glicken 1991 Dufresne and Davies 2009 note that longitudinal ridges are often oriented parallel to ow directions and disposed radially with respect to the source at proximal distance due to the lateral spreading of the mass ow Therefore the ow direction of the debris avalanche is indicated by the trajectory of the long or longitudinal axis of the mounds Dufresne and Davies 2009 In distal parts transverse ridges can be produced by compression or deceleration of the sliding mass when it impacts a topographic barrier or when the avalanche enters the sea e g Augustine volcano Siebert et al 1995 The spatial distribution and geomorphology of the mounds depend mostly on the material composing them the trigger mechanism of the collapse the runout distance and the age of the deposit degree of erosion and land cover The lithologic facies describe the internal structure of the DAD which is determined by the changes that the material experiences during transportation Palmer et al 1991 Flow dynamics can alter the competence of the material with respect to its original characteristics Debris avalanche lithofacies are classi ed into three types 1 block lithofacies composed mainly of blocks N1 m in size with a minor volume b30 vol of sandy interclast matrix all components of the deposit comprising material b2 mm in diameter 2