This paper discusses the results of a lichenometrical and geomorphological study of one of the few remaining active snow hollows in the central region of the Iberian Peninsula. The study area, located on a glacial shoulder, is called Hoya del Cuchillar de las Navajas. A protalus rampart occurs at the base of the hollow. Our studies, conducted between 1992 and 1998, were designed to determine the geomorphological characteristics of Hoya, the mobility of the deposits, and the characteristics of the snow cover. These data formed the basis for a study of the lichen colonization on the blocks and on the wall surrounding the snow hollow. All of the lichen species found were analyzed according to their abundance, distribution and the extent of their surface cover. Measurements of the diameter of the thalli of the species Rhizocarpon geographicum were also obtained. Thalli of this species were found to require a mean snow-free growing season of at Ž. least 95 days 13.5 weeks per year. Maximum mean thallus diameters indicate that the protalus rampart was formed during the Little Ice Age and became inactive 130 years ago.
ABSTRACT. The no rth ern slopes of M ex ico's great, ac ti ve stratovolc a noes, Popocatcpetl (5450 m a.s.!') a nd Pico de Orizaba (5700 m a.s. !.), ar e covered by glaciers. Thi s paper a na lyzes the retreat of th e glaciers betwee n Februa ry 1994 a nd O ctober 1995, a nd its geomorphological consequences. The res ults di sc ussed a re a continu ation o f resea rch co nducted on th e volca noes up to 1994. Although the glaciers retreated a g reat deal during the 1980s and ea rly 1990s, the process accelerated from 1994 to 1995. Th e heavy eros ion tha t ha d been obse rved o n th e deglacia ted wa ll of th e volca noes during th ese decades increased as the glaciers retreated. Sediments origin ati ng fr om materi a l loose ned from the exp osed latera l walls added to the supraglacia l layer. Temperature readings ta ken from bo reholes made in th e glaciers indicated th at abl a tio n had been rcduced as a result of th e insul ating effects of th e debri s m a ntl e. This allowed widespread form a ti o n of deposits th a t buri ed ice in a reas where the glacier had already di sappeared. Th ese r es ults co nfirmed the conclusions of ea rli er studi es th a t deglaciati o n o f stratovolca noes has res ulted in the acceleration of slop e processes a nd the form ati on of buri ed ice deposi ts.
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