JournaL cifGLacioLogy, Vat. 44, No . 1 46, 1998 Glacial retreat and its geomorphologic effects on Mexico's active volcanoes, 1994-95 DAV ID PALAC IOS, JAVI ER DE MARCOS Department cif Pl rysicaL Geography, CompLutense Uni v ersity, E- 28040 Madrid, Spain ABSTRACT. Th e no rth ern slop es of Mexico's great, ac ti ve str atovo lc anoes, Popocatc- pe tl (5450 m a.s.!') a nd Pico de Ori zaba (5700 m a.s. !. ), are covered by glac iers. Thi s paper analyzes th e retreat of th e glaciers be tween February 1994 and October 1995, and it s geo- morphological conse qu ences. Th e res ult s discusse d are a co ntinuation of r esea rch co n- ducted on the volc anoes up to 1 99 4. Alth ough the glaciers re tr eated a gr ea t d ea l during the 19 80 s and ea rl y 1 99 0 s, the process acce lerated fr om 1 994 to 1 995. Th e h eavy eros ion th at had been obse rved on the deglaciated wa ll of the volcanoes durin g th ese decades incr ease d as the glac iers retreated. Se dim ents o ri gina ti ng from materi al loosened fr om th e ex posed lateral wa lls added to th e supr ag lacial laye r. Te mp era tur e rea din gs taken from boreholes made in the glaciers indi ca ted that a bl ation had bee n rcdu ced as a r es ult of th e in s ul ating effects of the debris mantl e. Thi s a ll owed wid es pread fo rm ation of depos - its th at buried ice in areas where the glac ier had already disa pp eared. Th ese r es ults co n- firmed th e conclusions of ea rli er studies th at d eg lac ia ti on of str atovo lc anoes has r es ulted in th e accelera ti on of slope processes a nd th e forma ti on of buri ed ice deposi ts. INTRODUCTION Pop oca tcpe tl (5450 m) is in the ce ntre of th e trans-NIex i ca n vo l ca nic belt (19°03' , 98°35' W), and Pico de Ori zaba 01' Citlaltcpe tl ( 570 0 m) is on the easte rn edge of the ran ge (19 ° N, 97 ° 16' W) (Fi g. I). Th ey a re th e t wo highes t vo lc anoes in Mexico, a nd are th e only ones that are ac ti ve and h ave glaciers on their slopes. Pico de Ori zaba h as not been vo l- ca ni ca ll y active since th e 17th centur y; Pop oca tepe tl is mor e activ e, and as h emi ss ions ha ve occ urr ed since 1994. Des pit e these diff erences, th e glaciers on bo th vo lca noes h ave retr ea ted at the same rat e. remains a pp ea r on both vo lc a noes as roc ky spur s that ac t as an orogra phi cal barrier aga inst winds and insola ti o n. Thi s protec ti on a ll ows snow to acc umul ate wh ere th e eastern side of the s purs and the north e rn slope of the cones mee t. . Po p oca tcpe tl and Pico de Ori zaba have similar mor- phology, including th e remains of old volca ni c struc tur es th at were largely des troyed wh en ca lderas formed on th e no rthwestcrn slopes of th e large upper vo lc a ni c co nes. Th e Th e la rgest glaciers on Pop oca tepe tl and Pi co de Oriza- ba are loca ted on their no rth f ace and have fo rm ed since th e las t gr eat eruptions betwee n the 15th and 17 th centuri es . Th ese no rth e rn glaciers evolved as a res ult of th e shelt ering eff ec t of r ocky spurs. Both glac iers advanced to their lowes t altitud es at th e heig ht of the Littl e Ic e Age, a t th e end of the 19 th cent ur y. Except for a si ight advance of th e ir termini in the ea rl y 1970s, the two gl ac iers have rapidly receded during the 20th ce ntur y. A numb er of studie di sc uss th e evolution of th ese g lac iers (Lorenzo, 1964; He in e, 1975; Whit e, 1981; Hein e, 1988, 1 994; Pal acios 1995, 1996, in press; Pa lacios a nd Vazquez-Selem, 1996). Fi g. 1. L ocat ion map. 63