Glassy volcanic pitchstones

of Volcan Popocatepetl, Mexico

pitchstone [410 kb] pitchstone [533 kb]

Fig. 1: Two samples collected from Popocatepetl on two separate visits in the 1980s. Left: 090, displaying subconchoidal fracture. Right: the larger, shiny 297.
Popo and the somewhat lower but more craggy peak of Ixtaccihuatl are typically approached from Mexico City, via the market town of Amecameca. A road leads east from Amecameca, up to a splendid mountaineering hut, in the col known as the Paso de Cortez, between the two volcanoes (see Secor, 1981). Sample 297 was collected on 07 November 1983, "effectively in situ" from a cliff on the west side of the road leading northeast from the mountaineering hut, curling under a knoll on the ascent to a radio tower. 090 was collected loose, on the west side of the crater rim of "Popo", a few metres north of the summit of the volcano, on 05 October 1981.

"Rock of the Month # 237, posted for March 2021" ---

Popocatepetl lies close to the great urban region of Mexico City (Mexico, D.F.) and its sprawling suburbs. The name is from the Aztec language, Nahuatl, and means "smoking mountain". At 5,452 m (17,887') it is the second-highest volcano in Mexico, after Citlaltepetl (El Pico de Orizaba). It is the second-highest of a series of late Cenozoic volcanoes and innumerable smaller structures like cinder cones, one of the principal, largely-andesitic stratovolcanoes of the Trans-Mexican Volcanic Belt. The natural hazards it presents cannot be ignored, since the regional population is 20 million or more, even though radiocarbon dates of charred trees place the last great eruption at about 820 A.D. (Williams and Leen, 1999). After that, the last historic eruption was in 1720. Waitz (1921) visited the crater, and presented a fine photograph of a steam eruption on 12 October 1920. Fumarolic activity is to be expected, and may be detected visually, or by a scent of sulphur on the air. The samples illustrated here were collected on visits in October 1981 and November 1983.

In 1993, after almost 70 quiescent years, Popo unleashed volcanic gases and earth tremors, and at Christmas 1994 small explosions happened in the crater, after which, sporadic ash clouds have been emitted (Williams and Leen, 1999). Monitoring increased once the fumarolic activity increased in early 1993 (Siebe et al., 1994). In recent time, Popocatepetl has been active since January 2005. The activity is generally mild, with emissions of steam and ash. The NASA Earth Observatory web pages, using imagery from Landsat and other satellites, chronicle the recent activity, including ash emission on 06 January 2021. The image below, from the NASA program, shows a plume of steam, gas and fine ash drifting ENE in the upper-atmosphere winds, out over the Gulf of Mexico on 25 January 2016.

Popo by NASA-2016  [223 kb]

Fig. 2: An example of the downwind fallout from T "El Popo". While much activity is in the form of vapours from fumaroles, this eruption generated ash which caused suspension of flights at an airport in Puebla, while ash was cleared from the runway. See the Earth Observatory link above for more details.

Radiocarbon dates, mostly on charcoal and carbonized wood samples from pyroclastic flows and the fast-moving clouds of gas and ash known as nuées ardentes, around Popo extend back to to the late Pleistocene, though the origins of the regional east-west Trans-Mexican Volcanic Belt extend back several million years, far beyond the limits of the radiocarbon method. Samples from Popo and other mountains were used to investigate production of the 14C isotope by cosmic ray interactions in the upper atmosphere (samples: Wilson, 1987; analysis: Jull et al., 1989).

Volcanologists have studied Popo extensively, and there are ample reports of earth tremors associated with volcanism (Arciniega-Ceballos et al., 2000); the mechanics of magma ascent (Annen and Zellmer, 2008); emission of volcanic gases (Goff et al., 2008; Oppenheimer et al., 2003); and the hazards posed by Popo, now and in the past (Medina, 1980; McGuire et al., 2000; Siebe et al., 2004).

The two samples

Returning to our samples, a covered thin section was prepared from 090. It is a black, glassy volcanic rock with conchoidal fracture and subvitreous lustre. Such rocks are commonly termed obsidian, though this implies a siliceous compositon. Samples which are relatively less vitreous to almost waxy in lustre have been termed pitchstone. Igneous rock nomenclature was reviewed in the handy reference work by Le Maitre et al. (1989, pp.99,106). Here we find that:

  • a) Obsidian is "a common term for a volcanic glass, usually with a water content <1%, often dark in colour, massive and with a conchoidal fracture", while
  • b) Pitchstone is referred to as "a volcanic glass with a lustre resembling pitch and usually containing a few phenocrysts and a water content between 4% and 10%. This is unlike obsidian that usually has a water content <1%".
Pitchstone is a term used quite frequently in the classic literature on western Scotland. While the Turnstone collection has just one obsidian (1155, Taxco area, Mexico) numerous pitchstone localities are represented, in such famed areas of the Tertiary Volcanic District as Arran, Mull, Rhum and Skye. The instances are often subvolcanic hypabyssal intrusions, apparently fast-cooled melts just below the paleosurface. The term appears elsewhere in the world, such as along the Deccan Traps and Western Ghats of India from Gujarat south to Karnataka, and on Disko Island in western Greenland.

The thin section of 090 contains an estimated 67 volume percent glass. There are three types of coarser crystal (phenocrysts) within the glass. By far the most abundant (28%) is plagioclase feldspar, as crystals mostly <1.5 mm long, but up to almost 3 mm. Albite and simple twin laws are evident, and a minority of grains show signs of compositional zonation. The optical estimate for feldspar composition is An49 (calcic andesine). This feldspar chemistry would be consistent with an andesitic pitchstone, but not with a more siliceous (rhyolitic or rhyodacitic) obsidian. The feldspar crystals are crowded with tiny inclusions of brownish glass, the largest just 70x50 microns in size. Also present are sub-mm microphenocrysts of orthopyroxene (hypersthene) and clinopyroxene (augite). The section contains several small (up to 800x500 microns) granular inclusions dominated by feldspar, with opaque grains which may be iron sulphides, up to 250 microns in size.

Chemical definitions of rocks (such as water content) are of limited use, absent assay data. A quick scan of the literature offers some other clues, in differentiating obsidian from pitchstone. Obsidian and pitchstone may both be of silica-rich (rhyolitic) bulk composition: obsidian tends to be massive and glassy, with a conchopidal fracture, while pitchstone contains more phenocrysts, has a relatively dull llustre and a flatter fracture. Pitchstone is often somewhat devitrified, and of variable composition (not solely felsic, as in rhyolite). Pitchstones occur as dykes and sills and chilled margins, as in the complex minor intrusions of the Hebridean examples quoted above. The subvitreous lustre, absence of textbook conchoidal fracture, and abundant phenocrysts (as per examples from Arran and Eigg) suggest pitchstone rather than obsidian. The intermediate feldspar composition is taken to be consistent with an andesitic rather than a rhyolitic composition. Some rhyolitic pitchstones exhibit circular, perlitic fractures, not evident in thin section for 090.

There has been extensive work on the petrology of the lavas (predominantly andesites), tephra (ash-fall deposits, composed of glassy shards and mineral grains), pumice and lahars (muddy flows typically formed on volcanoes where there is a ready source of water, such as a snow field or glacier). Boudal and Robin (1988) note that the modern volcano Popocatepetl, including the upper parts of the mountain, is some 30,000-50,000 years old, composed of diverse products, but that the modern summit is a relatively youthful edifice, built up in the past 7,000 years. Their review includes whole-rock analyses and mineral chemistry of the lavas, and considers the chemical evolution of the volcanic pile. Late Holocene tephras offer confirmation of episodic explosive volcanism (Panfil et al., 1999). The intricate structures of the plagioclase can be illustrated beautifully using a Nomarski interferometer (Pearce and Clark, 1989). The petrography of tephra lapilli from six explosive volcanic eruptions, April 1996 to February 1998, has been studied (Straub and Martin-Del Pozzo, 2001). A strontium and neodymium isotope study was made of dacitic to andesitic lavas and domes, the nearby Chichinautzin cinder cones, and granitic to metapelitic xenoliths in pumices (Schaaf et al., 1997). The paleomagnetism of 16 lava flows from the volcano. mainly andesites, plus one trachyandesite, has been documented (Conte et al., 2004). Lastly, an interesting study was made of base and precious metals in pumice samples from the flanks of the volcano. The mineral assemblage is similar to that in high-sulphidation epithermal gold deposits. Samples from the Paso de Cortes contain not just pyrite but also sphalerite, chalcopyrite and sulphosalts, plus some rare minerals, including tellurides such as (?) calaverite and kostovite. These minerals crystallized from magmatic gases trapped in vesicles (Larocque et al., 1997).


Annen,C and Zellmer,GF (editors) (2008) Dynamics of Crustal Magma Transfer, Storage and Differentiation. Geol.Soc. Spec.Publ. 304, 288pp.

Arciniega-Ceballos,A, Valdes-Gonzalez,C and Dawson,P (2000) Temporal and spectral characteristics of seismicity observed at Popocatepetl volcano, central Mexico. J.Volc.Geotherm.Res. 102, 207-216.

Boudal,C and Robin,C (1988) Relations entre dynamismes éruptifs et réalimentations magmatiques d'origine profonde au Popocatépetl. Can.J.Earth Sci. 25, 955-971 (in Fr.).

Conte,G, Urrutia-Fucugauchi,J, Goguitchaichvili,A, Soler-Arechalde,AM, Morton-Bermea,O and Incoronato,A (2004) Paleomagnetic study of lavas from the Popocatepetl volcanic region, central Mexico. Int.Geol.Rev. 46, 210-225.

Goff,F et al. (1998) Geochemical surveillance of magmatic volatiles at Popocatepetl volcano, Mexico. Bull.Geol.Soc.Amer. 110, 695-710.

Jull,AJT, Donahue,DJ, Linick,TW and Wilson,GC (1989) Spallogenic 14C in high-altitude rocks and in Antarctic meteorites. Radiocarbon 31, 719- 724.

Larocque,ACL, Stimac,JA and Siebe,C (1997) An epithermal-like vapor-phase assemblage in pumice from Volcan Popocatepetl, Mexico. GSA Abs.w.Progs. 29 no.6, 360, Salt Lake City.

Le Maitre,RW, Bateman,P, Dudek,A, Keller,J, Lameyre,J, Le Bas,MJ, Sabine,PA, Schmid,R, Sorensen,H, Streckeisen,A, Woolley,AR and Zanettin,B (1989) A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell Scientific Publications Ltd, Oxford, 193pp.

McGuire,WJ, Griffiths,DR, Hancock,PL and Stewart,IS (editors) (2000) The Archaeology of Geological Catastrophes. Geol.Soc. Spec.Publ. 171, 417pp.

Medina,F (1980) Vulcanologia y evaluacion del riesgo volcanico en Mexico. Annales del Instituto de Geofisica 26, 55- 73 (in Sp.).

Oppenheimer,C, Pyle,DM and Barclay,J (editors) (2003) Volcanic Degassing. Geol.Soc. Spec.Publ. 213, 432pp.

Panfil,MS, Gardner,TW and Hirth,KG (1999) Late Holocene stratigraphy of the Tetimpa archaeological sites, northeast flank of Popocatepetl volcano, central Mexico. Bull.Geol.Soc.Amer. 111, 204-218.

Pearce,TH and Clark,AH (1989) Nomarksi interference contrast observations of textural details in volcanic rocks. Geology 17, 757-759.

Schaaf,P, Siebe,C, Macias,J-L and Stimac,J (1997) Popocatepetl lavas and domes, associated xenoliths and cindercones: an isotopic profile of the past 22000 years. GSA Abs.w.Progs. 29 no.6, 481, Salt Lake City.

Secor,RJ (1981) Mexico's Volcanoes: A Climbing Guide. The Mountaineers, Seattle, 120pp.

Siebe,C, Delgado,H, Fischer,TP, Williams,SN and Obenholzner,J (1994) Popocatepetl reactivated: first results of ongoing fumarolic gas monitoring and stratigraphic studies. GSA Abs.w.Progs. 26 no.7, 533pp., 114, Seattle.

Siebe,C, Rodriguez-Lara,V, Schaaf,P and Abrams,M (2004) Radiocarbon ages of Holocene Pelado, Guespalapa, and Chichinautzin scoria cones, south of Mexico City: implications for archaeology and future hazards. Bull.Volc. 66, 203-225.

Straub,SM and Martin-Del Pozzo,AL (2001) The significance of phenocryst diversity in tephra from recent eruptions at Popocatepetl volcano (central Mexico). Contrib.Mineral.Petrol. 140, 487-510.

Waitz,P (1921) Popocatepetl again in activity. Amer.J.Sci. 201, 81-87.

Williams,AR and Leen,S (1999) Popocatepetl, Mexico's smoking mountain. National Geographic 195 no.1, 116-137, January.

Wilson,GC (1987) Untitled (on samples from high altitudes on volcanoes in Mexico, and carbonaceous sediments from the Sudbury area of Ontario). Technical report, 14pp.

Graham Wilson, posted 04,07,09,28 February 2021

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