Geologic setting of the Peña de Bernal Natural Monument, Querétaro, México: An endogenous volcanic dome

Peña de Bernal, at 20°45′N, 99°57′W (Fig. 1), is a prominent peak that was declared a natural reserve area by the State Government of Querétaro in 2007 (Fig. 2). Since 30 September 2009, it has been included in UNESCO’s Intangible Cultural Heritage sites (UNESCO, 2009) in order to preserve the traditions of Otomí and Chichimeca indigenous peoples as well as the Peña de Bernal itself. Furthermore, Peña de Bernal is known as the main entrance to the Sierra Gorda mountains area, a range that was declared in 2001 as a Biosphere Reserve by UNESCO (2001). In a few publications and in some web sites, including the official municipal (county) web page of the Bernal area and that of the National Institute of History and Anthropology (INAH) of Mexico, Peña de Bernal is identified as one of the highest monoliths in the world, with monolith defined as a prominent and large monolithologic rock (Fig. 2). The exact height of Peña de Bernal has not been well constrained, with reported heights ranging between 288 and 360 m (e.g., Coenraads and Koivula, 2007; INAH, 2009; Municipio Ezequiel Montes, 2009). From the topographic data of the official Mexican agency for National Geographic Studies and Information (Instituto Nacional de Geografía Estadística e Informática [INEGI], 2002) and from altimeter measurements carried out by us at the base and at the top of Peña de Bernal, we have calculated a height of 433 m on the northern side (Fig. 3). Height is 405 m on the southern side of Peña de Bernal with respect to the plain where Bernal town lies (2049 meters above sea level [masl]). Either elevation chosen from these new values shows that the Peña de Bernal is apparently the highest monolith of the world.

Previous regional geologic mapping has considered Peña de Bernal as a Paleocene monzo-granodioritic pluton (Segerstrom, 1961) or a subvolcanic Miocene felsic body (Carrillo-Martínez, 1998). However, the mechanism of emplacement has remained unknown. Before this work, there were no scientific studies specifically on Peña de Bernal’s geologic nature and origin. The purpose of this study is therefore to provide detailed information on the geologic setting, including the stratigraphy, structure, geochronology, and geochemistry, to answer fundamental questions of when and how Peña de Bernal was formed and why it is topographically prominent with respect to its surroundings.

Peña de Bernal is located at the intersection of three major geologic provinces: the Miocene–Quaternary Mexican Volcanic Belt (Demant, 1978; Aguirre-Díaz et al., 1998; Siebe et al., 2006), the mid-Tertiary Sierra Madre Occidental ignimbrite-dominated sequence (McDowell and Clabaugh, 1979; Aguirre-Díaz et al., 2008), and the Mesozoic Sierra Madre Oriental fold-thrust belt (Suter, 1987; Bartolini et al., 1999), which in this area corresponds to the Sierra Gorda sector (Fig. 1). A simplified geologic map of the Peña de Bernal area is shown in Figure 4. The stratigraphy is briefly described below, from the oldest to the youngest unit (Fig. 5).

Las Trancas Formation was defined by Segerstrom (1961) as a Jurassic–Early Cretaceous (Kimmeridgian–Barremian) marine sedimentary sequence. It consists of dark-gray fissile shales, calcareous limonites, and dirty limestones with pyrite and minor amounts of graywackes and flint (Segerstrom, 1961, 1962; Chauve et al., 1985; Carrillo-Martínez, 1990). In the Bernal area, this unit occurs as a thinly bedded sequence of fissile shales and limestones with minor amounts of graywackes. Bedding is generally undulated, with thicknesses in the order of tens of meters. The shales and limestones of Las Trancas Formation were intensively deformed during the Cretaceous–Tertiary (K-T) Laramide orogeny (Chauve et al., 1985; Padilla-y-Sánchez, 1985), producing incompetent folding, eastward thrust transportation, and a low-grade regional metamorphism (Suter, 1987; Carrillo-Martínez and Suter, 1990; Carrillo-Martínez, 1998; Bartolini et al., 1999) that gave a fissile and slate character to the shales and limestones, but without reaching the point of the schist facies.

El Doctor Formation is a middle Cretaceous (Albian–Cenomanian) unit that was defined by Wilson et al. (1955) from a locality near the town of El Doctor, in the Sierra Gorda portion of the Sierra Madre Oriental fold-thrust belt, and ∼35 km to the NE of Bernal (Fig. 1). This formation is characterized by its origin as a reef and, thus, is composed mainly of platform calcareous small fossils and microfossils. In the mapped area, El Doctor Formation crops out to the NW of San Antonio de la Cal (Fig. 4), as an elongated outcrop between Las Trancas and Soyatal formations. The outcrop is ∼100 m thick and consists mostly of gray fossiliferous limestones in thick to medium thick banks.

Soyatal Formation is a Late Cretaceous (Turonian) marine sedimentary sequence composed mainly of limestones and flint lenses (White, 1948). Index fossils near Bernal and found by us include ammonites of Nowakites sp. at the locality of Dedhó, and Texamites sp. in the Tolimán canyon, confirming the Late Cretaceous epoch of this unit. Soyatal Formation is thinly to medium bedded, showing boudinage primary bedding. It is dark gray when fresh, and light gray to white when weathered. As in Las Trancas, this unit was folded and northeastwardly tectonically transported during the K-T Laramide orogeny (Chauve et al., 1985). Las Trancas Formation was thrusted on Soyatal Formation during this orogeny (Suter, 1987; Carrillo-Martínez, 1998), forming klippes at top of younger Soyatal. In the Bernal area, the top of Soyatal Formation is eroded, and thus, a complete thickness estimate was not possible, but a minimum of 300 m is considered our best approach.

Continental detrital deposits crop out to the NW of the mapped area and nearby the town of San Martín (Fig. 1). Within the mapped area these deposits occur as small patches in the SW lower flank of Peña de Bernal too small to be shown in Figure 4, and to the southwest of Cerro Azul, in the northeastern portion of the area. Thickness ranges from 0 to 20 m. Near Bernal this unit consists mainly of conglomerates with clast sizes of 2–30 cm in diameter, mainly from shales of Las Trancas Formation and black flint and limestones of Soyatal Formation. Clasts are rounded to semi-rounded and are supported by a red clay matrix, in parts weathered to a calcareous soil. The deposits at Cerro Azul are finer and consist of cross-bedded sandstones, green and red, with no base exposed, and covered by younger pyroclastic rocks. These clastic deposits rest unconformably either on Soyatal or Las Trancas formations. Tentatively, we position this unit in the lower Tertiary, after the K-T orogeny, in correlation with similar continental deposits of central Mexico, such as the Eocene red conglomerates of Guanajuato and Zacatecas (Edwards, 1955; Tristán-González et al., 2009), and the Cenicera formation of San Luis Potosí (Labarthe-Hernández et al., 1982).

Overlying the continental red bed deposits mentioned above, there is a sequence of epiclastic deposits consisting of green to yellow tuffaceous sands, in beds 20–100 cm thick, with cross bedding in places. Best exposures occur west of Cerro Azul, where the unit reaches up to 50 m in thickness (Fig. 4). The lower part of the sequence consists of beds of rounded to subangular clasts of quartz, plagioclase, and volcanic glass, supported by a clayey, chloritized matrix. The upper part is mainly a clay deposit, in beds of 2–20 cm. It is possible that these deposits, and particularly the upper clay beds, represent reworked basal ash deposits precursor to the emplacement of Cerro Azul ignimbrite, which conformably overlies this unit.

Cerro Azul ignimbrite is a widespread, densely welded, orange-brown unit, with up to 30 vol% of phenocrysts (2–3 mm) of quartz, sanidine, and biotite, supported in a matrix of devitrified glass shards. The thickness at Cerro Azul is up to 45 m, but more commonly it is around 30 m thick. The base of the ignimbrite includes a pink unwelded to poorly welded ash layer, up to 1 m thick, that lacks bedding, phenocrysts, and/or lithics. Conformably and continuously overlying this basal layer is the massive part of the welded ignimbrite. Above the ash layer, the ignimbrite contains sparse lithics of limestone and chert. K-Ar age dating performed on this unit yielded an age of 31.5 ± 0.7 Ma (Table 1). Therefore, this ignimbrite corresponds to mid-Tertiary volcanism related to the Sierra Madre Occidental.

Andesitic lavas overlie Cerro Azul ignimbrite (Fig. 5). These lavas have been informally named as Cerro Azul andesite because the best exposures are found at Cerro Azul and at hills to the west of it. The andesite lavas usually form plateaus and apparently were fed from fissures. Some andesitic dikes with similar aspect and texture were observed nearby cutting the Cerro Azul epiclastic deposits and the Cerro Azul ignimbrite. Whole-rock K-Ar dating of this unit resulted in 29.4 ± 0.8 Ma (Table 1). It is generally a propylitized gray to green rock or dark gray, when rare fresh andesite is found. In some sites, it has vesicles or amygdules of agate or chalcedony. Andesite is fine grained, with sparse phenocrysts with sizes of less than 1 mm of plagioclase, Fe-Ti oxides, and clinopyroxene (augite), which sometimes is bordered by a rim of amphibole.

El Matón ignimbrite is one of the largest in distribution and volume of the region, and is commonly observed between Tolimán and San Miguel (outside the mapped area; Fig. 1). It is a partly welded, pink, rhyolitic ignimbrite, with eutaxitic texture. This is a crystal-rich ignimbrite, with up to 40 vol% of phenocrysts of quartz, sanidine, and oxidized biotite. Most crystals are subhedral and broken, with sizes between 1 and 3 mm. A matrix of devitrified glass shards is preserved. The degree of welding increases upward, from poorly to densely welded. It overlies Cerro Azul andesite, making the top of the hill with this name (Fig. 5). It also overlies Soyatal Formation and the lower Tertiary clastic sediments. K-Ar dating yields an age of 29.4 ± 0.7 Ma (Table 1). The ignimbrite is stratigraphically younger than 29.4 ± 0.8 Ma Cerro Azul andesite, although both resulted with the same isotopic age.

San Martín andesite includes a series of lavas erupted from San Martín volcano that are between the towns of San Martín and Bernal (Fig. 1). A couple of small outcrops can be observed at the southwestern part of the mapped area (Fig. 4). San Martín andesite overlies Las Trancas and Soyatal formations, and El Matón ignimbrite outside the mapped area. The lavas are dark gray to light gray and generally form thick lobes rather than plateaus. Lavas are porphyritic, with up to 20 vol% of phenocrysts of plagioclase and hornblende and a groundmass made of glass and microlites of the same phases plus sparse olivine. The structure of the lavas includes flow banding and flow foliation. K-Ar dating of two different lava units of San Martín volcano yielded ages of 11.0 ± 0.3 Ma and 10.2 ± 0.2 Ma (Table 1), suggesting a long-lived polygenetic history of this volcano. San Martín volcano is similar in age and composition with other middle Miocene stratovolcanoes in the region, such as La Joya and El Zamorano, which are related to the Mexican Volcanic Belt (Aguirre-Díaz, 2008).

Peña de Bernal is a highly crystalline and very resistant rock, gray to light gray when fresh and brown when weathered. It is composed of up to 80 vol% crystals and 20 vol% matrix glass; phenocrysts make up to 30 vol% of the total. Mineralogy includes plagioclase + orthopyroxene + hornblende + biotite + sanidine + quartz + Fe-Ti oxides + apatite + zircon (Fig. 6). It has a porphyritic texture with phenocryst content varying within 15–30 vol%. Amongst phenocrysts, plagioclase and hornblende are the most common and largest up to <2.5 mm; biotite and pyroxene are smaller and relatively less abundant. Mafic phases are generally oxidized either completely or along their rims (Fig. 6). The matrix is made of a microcrystalline mineral assemblage with all the phases mentioned above, plus interstitial glass. Plagioclase phenocrysts are generally zoned with complex oscillatory zoning and fritted zones (Fig. 6). Sanidine is rare, but it has been observed as phenocrysts. Glass is pale brown and occurs between the crystals. Textures suggest a long-term, complex cooling history of the Peña de Bernal magma, with resorption of phases and disequilibrium and reequilibration conditions, particularly recorded in the plagioclase. The large mineral assemblage includes most phases of the Bowen’s series (except olivine), suggesting enough time for the nucleation and growth of all these phases with the liquid line of descent (Wilcox, 1979).

Two samples of Peña de Bernal were analyzed to determine its age. Samples were processed in separate laboratories and using different techniques in order to cross-check the results and to have a good accuracy of the timing of formation. The K-Ar age obtained at the University of Brest, France, yielded 8.7 ± 0.2 Ma (Table 1). The ⁴⁰Ar-³⁹Ar plateau age obtained at the Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE) research center in Ensenada, Mexico, yielded also 8.7 ± 0.2 Ma on plagioclase separate (Fig. 7 and Table 2). Therefore, 8.7 ± 0.2 Ma should be the age of Peña de Bernal dacite. The middle Miocene age of Peña de Bernal is consistent with the general age of other silicic domes in the central-northern sector of the Mexican Volcanic Belt (Aguirre-Díaz et al., 1998; Aguirre-Díaz, 2008).

Besides the main igneous mass that forms the peak of Peña de Bernal, there are two other plugs with lower elevations, and all three plugs crop out in a 4.8 km² area (Figs. 2 and 4). The main plug extends for ∼3.5 km from the highest peak that forms Peña de Bernal to the southwesternmost part at San Antonio de la Cal (Fig. 4). The other two plugs are La Tortuga, 0.85 × 0.45 km, at the western portion of the main outcrop, and the Northern plug, at the northeasternmost part of the outcrop (Fig. 4). The rock is similar in all plugs with some textural or dome foliation variations; textures vary mainly in crystal size with crystals bigger in the main plug and smaller in the other two. Apparently, plugs were emplaced simultaneously, as there are not sharp contacts between them but rather transitional contacts. Peña de Bernal peak shows dome foliation that varies from ∼40° inward dips at the margins to nearly vertical dips at the center of the dome (Fig. 8). This foliation follows the general elongated trend of the main plug to the NE. Peña de Bernal dacite is in intrusive contact with the country rock, which is primarily the Las Trancas Formation, although it is also in contact with the Soyatal limestone in some places, in particular along the southwestern margin of the outcrop area (Fig. 4). It is not a thermal metamorphic aureole caused by the dome intrusion, and contact metamorphism of the country rock is also not observed. The metamorphism of Las Trancas Formation was caused by a regional tectonic event (Suter, 1987) and not by the intrusion of Peña de Bernal dome. The contact between the Peña de Bernal dacite and the country rock is sharp (Fig. 9A). Most notable is the fact that it looks like a normal fault contact, with vertical slickensides marked in the country rock, in particular in Las Trancas Formation (Fig. 10). The Peña de Bernal dacite near the contact is fractured but not brecciated, forming fractures with a peculiar orthogonal arrangement (Fig. 10A). Also observed along the margin of Peña de Bernal dome are sheared marginal zones that form wavy trends in map view between Peña de Bernal and the country rock (Fig. 10B). These observations indicate a rigid (fragile) behavior of the Peña de Bernal dacite along its margins during intrusion, which is discussed below.

Two whole-rock analyses of the Peña de Bernal rock yielded a dacitic composition following the total-alkalis (TAS) classification of Le Bas et al. (1986) (Fig. 11A). Whole-rock analyses resulted in SiO₂ = 67 wt% and Na₂O + K₂O = 6 wt% (normalized volatile-free, Table 3), which are plotted in the corresponding TAS diagram, together with other volcanic units of the mapped area, such as the Oligocene rhyolitic ignimbrites and the Miocene San Martín andesite (Fig. 11A). For comparison, analyses for other units from the region are plotted too, such as the Miocene andesite of El Zamorano volcano and a Pliocene basaltic andesite from the Cenizas locality, both reported by Aguirre-Díaz and López-Martínez (2001).

Cerro Azul ignimbrite chemistry resulted in SiO₂ = 72.5%, Al₂O₃ = 11.7%, Na₂O = 1.3%, K₂O = 5.35%, with a high value for loss of ignition (LOI) of 4% (Table 3), probably because we analyzed partly devitrified pumice fragments, indicating a rhyolitic composition in the TAS diagram (Fig. 11A). Major-element chemistry of Cerro Azul andesite yielded SiO₂ = 58.0%, Al₂O₃ = 15%, TiO₂ = 0.82%, Fe₂O₃ = 4.6%, Na₂O = 2%, and K₂O = 1.37% (Table 3), corresponding to a basaltic andesite composition in the TAS classification. Chemical analyses of El Matón ignimbrite yielded SiO₂ ∼73%, Al₂O₃ = 11%–13%, Na₂O = 1.3%–2.7%, and K₂O ∼4.4%. It classifies as rhyolite in the TAS diagram (Fig. 11A). However, it should be noted that the analyzed pumice was devitrified. Chemical data of two analyzed samples from San Martín andesite show SiO₂ = 60.4%–64.9%, Al₂O₃ = 15.4%–17.0%, TiO₂ = 0.4%–1.0%, Fe₂O₃ = 2.8%–5.3%, Na₂O ∼3.6%, and K₂O = 1.3%–2.5% (Table 3). These values result in a TAS classification of andesite to dacite (Fig. 11A).

Peña de Bernal dacite as well as other volcanic units sampled from the Bernal area all are calc-alkaline, low-K silicic rocks, as shown in the SiO₂-K₂O plot with the classification of Peccerillo and Taylor (1976; Fig. 11B). According to these results, the suite of Oligocene, Miocene, and Pliocene rocks, and in particular the Peña de Bernal dacite, corresponds to a continental margin subduction zone setting (Wilson, 1989). This tectonic setting agrees well with the fact that the Peña de Bernal lies within the Mexican Volcanic Belt, which has been described as a volcanic province formed during a continental margin subduction zone tectonic regime (Aguirre-Díaz et al., 1998; Siebe et al., 2006).

Peña de Bernal dacite is relatively poorer in silica and alkali content with respect to other volcanic rocks in the area, such as the rhyolitic ignimbrites of El Matón and Cerro Azul. On the other hand, it is relatively more evolved than the neighboring andesites, such as those erupted from the San Martín volcano and the Cenizas andesite in the Amazcala caldera area (Fig. 12). This may be explained by the crystal-rich content of Bernal dacite (up to 80 vol%) with respect to these rhyolites, because crystals and in particular plagioclase tend to lower the silica and NaO₂ + K₂O contents in the whole-rock composition. The glass composition described below confirms this point.

Microprobe analyses of glass show a higher Na₂O + K₂O content than the whole-rock composition, yielding a trachytic composition of SiO₂ = 65 wt% and Na₂O + K₂O = 13 wt% (Table 4; Fig. 13). Mineral chemistry (Table 5) indicates a wide range in plagioclase composition from An₂₀ to An₆₀ (Fig. 14). This range is confirmed in textures observed in large plagioclases with oscillatory zoning and fritted cores (Fig. 6D). They generally have Ca-rich cores and Na-rich margins, suggesting a crystallization that followed the liquid line of descent of the Bowen’s series (Wilcox, 1979). Two microprobe analyses confirm that orthopyroxene compositions correspond to enstatite (Fig. 14). Energy dispersive spectroscopy (EDS) analyses (not shown in Table 5) confirm the presence of biotite and hornblende and of the accessory phases, apatite and zircon.

Oxygen fugacity and corresponding equilibration temperatures were calculated based upon microprobe analyses on Fe-Ti oxides and pyroxene (Table 5). The temperature and oxygen fugacity (T-fO₂) parameters were calculated using the quartz-ilmenite-fayalite system proposed by Lindsley and Frost (1992) and the computer program QUILF (Andersen et al., 1993). Plotting the Peña de Bernal dacite data in a T-fO₂ diagram together with several buffers, a range of values is observed from T °C = 711.5, log fO₂ = –15.53 to T °C = 377, log fO₂ = –27.70, with several intermediate values (Fig. 15 and Table 6). The results plot between the nickel-nickel-oxide (NNO) and the hematite-magnetite (HM) buffers, indicating relatively high fO₂ conditions, which are common in subduction zone settings.

The Peña de Bernal area includes rocks of three major geologic provinces of Mexico: the Mesozoic Sierra Madre Oriental fold-thrust belt, the mid-Tertiary Sierra Madre Occidental volcanic province, and the Neogene–Quaternary Mexican Volcanic Belt. Focusing on the volcanic rocks, these include 31.5–29.4 Ma ignimbrites, 29.4 Ma andesites, 11–10 Ma andesites, and the 8.7 Ma Peña de Bernal dacite, which represents the youngest volcanic event in the mapped area (Fig. 4 and Table 1). The Oligocene ignimbrites and andesites crop out to the north of Peña de Bernal. At the base of this sequence is a layered succession of reworked pyroclastic deposits that in turn overlies continental red or green sandstones. The lower layered sequence resembles mid-Tertiary red continental deposits described in other places of central Mexico, such as the upper part of the Conglomerado Rojo of Guanajuato Formation, apparently of Eocene age (Edwards, 1955; Echegoyén et al., 1970). Furthermore, the contact between Guanajuato’s red conglomerate with overlying green Loseros Formation (Echegoyén et al., 1970) is similar to the succession of continental red sandstones and Cerro Azul epiclastic deposits of the Bernal area. The ignimbrites include sanidine, quartz, and minor biotite, and they can be correlated with similar 30–31 Ma ignimbrites reported nearby and to the WNW of the Bernal area and next to, but unrelated to, the Amazcala caldera (Aguirre-Díaz and López-Martínez, 2001).

San Martín andesite was derived from the neighboring San Martín volcano 3 km to the west of Bernal (Fig. 4). It was dated at 11–10 Ma (Table 1) and is one more of several middle Miocene andesitic stratovolcanoes at the north-central border of the Mexican Volcanic Belt, including San Pedro, Palo Huérfano, Chichimequillas, La Joya, and El Zamorano (Valdez-Moreno et al., 1998; Verma and Carrasco-Núñez, 2003; Aguirre-Díaz, 2008).

El Zamorano volcano includes a spine-type lava dome in its crater that is similar to the Peña de Bernal. It is a light-gray, porphyritic rock, almost completely crystalline, and with phenocrysts of plagioclase, hornblende, and quartz. Dome foliation jointing varies from vertical at the central part of the dome to subhorizontal and inward dipping at the borders, similar to the foliation observed in Peña de Bernal. The dacitic dome intrudes and is in high angular unconformity with surrounding outward-dipping lavas of the crater of the El Zamorano volcano. With an altitude of 3360 m above sea level, this spine forms the highest point of the El Zamorano volcano, which is also the highest peak of the State of Querétaro.

Peña de Bernal dacite is interpreted in this work as a lava dome with intrusive contacts with the adjacent rocks. This interpretation is based upon the porphyritic texture and fabric of the rock and the dome-type foliation geometry (e.g., Duffield et al., 1995). A thermal aureole is not present in country rock surrounding the Peña de Bernal dome. The slight metamorphism of Las Trancas Formation was caused by the earlier regional tectonic event of the Laramide orogeny (Suter, 1987) and not by the intrusion of Peña de Bernal dome.

Peña de Bernal dacite dome is intensively fractured along its margins, but it is not brecciated. Shearing of Peña de Bernal dacite along the contact surface formed an intensively fractured rock along some margins, and vertical slickensides were also observed in some places due to this shearing. These observations, together with the highly crystallized nature of the rock, suggest a forceful intrusion of the dome when it was nearly solid (80% crystalline) and relatively cold (Fig. 16). This is confirmed by geothermometry obtained by microprobe analyses on minerals, which yielded temperatures as high as 711 °C and as low as 377 °C, following the hematite-magnetite buffer (Fig. 15). Therefore, the crystal-rich dacitic magma was almost totally crystallized and relatively cold when it was emplaced at or near surface.

Peña de Bernal dacite was formed through three simultaneous pulses, each constructing a plug; these are the Main plug forming Peña de Bernal peak, the Tortuga plug, and the Northern plug. The Tortuga and Northern plugs are much smaller than the main one. There is no evidence for magmas reaching a vent and flowing over the surface as lava coulees. Instead, Peña de Bernal dacite was emplaced as an endogenous lava dome; that is, the dome grew as a dacitic magma batch that was emplaced and cooled below but near surface.

In summary, Peña de Bernal dacite was apparently intruded as a spine dome, following Blake’s (1990) classification, and it probably has retained its peculiar steep peak shape since its formation. However, in the case of Peña de Bernal, the spine dome did not form inside a crater of a volcano, as normally occurs in similar cases, for instance, El Zamorano, Sangangüey, or Tequila volcanoes (Nelson and Carmichael, 1984; Wallace and Carmichael, 1994). Instead, Peña de Bernal was emplaced through a marine Mesozoic sedimentary sequence, and without any evident relationship with a preexisting volcano. There is no evidence that Peña de Bernal vented to the surface, either in volcanic deposits of appropriate age or in dikes that might have fed surface eruptions. The presence of Miocene, relatively intact volcanoes nearby, such as El Zamorano and San Martin, suggests that erosion would not have removed a volcanic edifice above Peña de Bernal dacite, if such had existed.

Peña de Bernal is a prominent igneous peak, and it emphasizes the particular geographic and cultural setting where it lies, at the entrance of UNESCO’s Sierra Gorda Biosphere Reserve, as part of UNESCO’s Intangible Cultural Heritage Patrimony List, and next to the Magic Town of Bernal. With a measured height of 433 m from its base, it is apparently the highest monolith in the world. Peña de Bernal includes three plugs with a general N40°E trend and covers an area of ∼4.8 km². We conclude that Peña de Bernal was emplaced as a spine-type, endogenous dome, at ca. 8.7 ± 0.2 Ma. Compositionally it corresponds to a dacitic lava with SiO₂ = 67 wt% and Na₂O + K₂O = 6 wt%. The rock is 80 vol% crystalline and only 20 vol% glass. It is porphyritic with plagioclase, pyroxene, hornblende, biotite, quartz, magnetite, ilmenite, apatite, and zircon. On the basis of temperatures as low as 377 °C, the nearly crystalline texture, and the fragile intrusion observations, such as slickensides at the contact with the country rock, it is concluded that Peña de Bernal rock was emplaced by forceful intrusion when nearly solid.

We thank the reviewers Don F. Parker, Fred W. McDowell, and Geosphere’s associate editor Megan Elwood Madden and science editor Carol Frost for their comments and recommendations that substantially improved the final version. We are grateful to Isaac A. Farraz-Montes and Azalea J. Ortiz-Rodríguez for their help with figures and digital elevation data analysis. We thank Compañía Minera Peña de Bernal, S.A. de C.V. for allowing the use of some data from the unpublished internal report Estudio geológico-estratigráfico y estructural, en el área del proyecto San Martín performed in 2006 by researchers of Universidad Autónoma de San Luis Potosí (UASLP). We thank Andrea Di Muro for arranging access to the microprobe of the Laboratoire de Pétrologie Modélisation des Matériaux et Processus, of Universite Pierre et Marie Curie, Paris 6, France. This work was partially supported by Consejo Nacional de Ciencia y Tecnología (CONACYT) and Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica–Universidad Nacional Autónoma de México (PAPIIT-UNAM) grants to the first author, numbers IN-114606, IN-106109, IN-112312, P-46005F and 33084T.