Lateral vent migration during phreatomagmatic and magmatic eruptions at Tecuitlapa Maar, east-central Mexico

Journal of Volcanology and Geothermal Research 181 (2009) 67–77

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j v o l g e o r e s

Lateral vent migration during phreatomagmatic and magmatic eruptions at Tecuitlapa Maar, east-central Mexico Michael H. Ort a,⁎, Gerardo Carrasco-Núñez b a b

Environmental Sciences and Geology, Northern Arizona University, Flagstaff, AZ 86011, USA Centro de Geociencias, Universidad Nacional Autónoma México, Juriquilla, Qro., 76001, Mexico

a r t i c l e

i n f o

Article history: Received 26 June 2008 Accepted 3 January 2009 Available online 18 January 2009 Keywords: maar vent migration phreatomagmatism Mexican volcanoes

a b s t r a c t Tecuitlapa Maar, located in the eastern Central Volcanic Belt of Mexico, is a late Quaternary, 100-m-deep, N 1-km-diameter crater with an alignment of scoria cones, building in height to the east, within it. Analysis of stratigraphic sections around the maar indicates that the phreatomagmatic eruption began in the eastern part of the crater, with interaction between the basaltic magma and liquefied tuffaceous sediments. The explosion locus gradually moved westward, as evidenced by bomb-sag trajectories, duneform axes, and facies changes, producing an elliptical crater. The eruption then dried out, and began to produce scoria/ spatter cones with nested craters that young in age and climb in height to the east along the same alignment exhibited by the phreatomagmatic eruption. The lithic lapilli and ash in the deposits throughout the eruption are from the 10–40-m-thick water-saturated tuffaceous sediments partially exposed in the maar-crater walls, rather than the underlying lavas and limestones. This implies that the vent migrated laterally rather than vertically during the eruption, and this lack of deepening of the explosion loci may be related to the high lithologic contrast between the non-consolidated sediments and the underlying fractured bedrock. Water movement in the bedrock would have been by fracture flow, whereas movement within the sediments was through liquefaction, failure and fluidization of the deposit. Low magma supply rates would allow collapse of the non-lithified walls of the dike, producing mingling and phreatomagmatic explosions, whereas the same magma supply rate in fractured bedrock may seal off water access to the magma. The alignment of the vents is parallel to regional structural trends, so was probably set in the underlying bedrock, and the distance that the vent migrated is likely related to the overall dike length. The lack of fluidized sediment coating the scoria-cone clasts, both in the cones and in the deposits in the walls of the maar, implies that the phreatomagmatic eruptions used up most of the liquefied sediment, which ended the phreatomagmatic portion of the eruption. The migration of the scoria-cone vent loci can be explained by an actual migration of the vents, or, because the evidence on their locations is based on surficial crater-overlap relations, by a fissure eruption that closed up from the west to the east, producing later activity at the taller eastern vents. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Maar volcanoes are created by the interaction of water, typically phreatic, and rising magma, and the resulting phreatomagmatic explosions create craters up to 1–2 km in diameter. The dynamics of such eruptions have been investigated through fieldwork (e.g. White, 1991; Abrams and Siebe 1994; Carrasco-Núñez et al., 2006; Lorenz and Kurszlaukis, 2007), grain size and shape (e.g. Wohletz, 1983; Wohletz et al., 1989; Dellino et al., 2001, 2004) and experiments (e.g. Wohletz and McQueen, 1984; Zimanowski et al., 1997; Büttner et al., 2002). One of the results of these studies is an understanding that the

⁎ Corresponding author. E-mail addresses: [email protected] (M.H. Ort), [email protected] (G. Carrasco-Núñez). 0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2009.01.003

supply of water to the magma–water interface is critical to the eruption, and many eruptions apparently ‘dry out’ over the course of time, possibly due to a decreasing supply of water (Lorenz, 1986; Németh et al., 2001). However, other eruptions appear to experience an increase in the access of water to hot magma during the eruption, or the water–magma ratio varies sporadically throughout the eruption (e.g. Gutmann, 2002; Carrasco-Núñez et al., 2004, 2007). Most basaltic magma rises as dikes through the crust, and different conditions may exist along the dike during an eruption, leading to phreatomagmatic behavior concentrating in one part. This paper discusses the migration of the locus of phreatomagmatic activity along the feeder dike, followed by a reversal of this migration during subsequent strombolian eruption, at Tecuitlapa volcano in east-central Mexico. Some phreatomagmatic eruptions may involve a more-or-less continuous interaction of water and magma in the eruption column (Mastin, 2007), but maar eruptions, with their thin repetitive deposits from surges and fallout, are likely characterized by punctuated events

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M.H. Ort, G. Carrasco-Núñez / Journal of Volcanology and Geothermal Research 181 (2009) 67–77

that occur many times during the eruption. These events are selfsimilar, as demonstrated by the similarity of textures from one bed to the next. Molten fuel-coolant interactions (MFCI; Zimanowski, 1998) are commonly thought to be the cause of the explosions that produce these deposits. These interactions produce punctuated explosions and discrete, repetitive deposits. The grain shapes and grain-size distributions of the deposits are consistent with those created in experiments (e.g. Wohletz, 1983; Büttner et al., 1999, 2002; Zimanowski et al., 2003; Dellino et al., 2004). MFCI is thought to occur through a four-step process (Morrissey et al., 2000). Coarse mixing occurs as the two liquids mingle with a thin vapor film separating them. During this stage, Kelvin–Helmholtz instabilities occur due to shear stresses while Rayleigh–Taylor instabilities form tiny diapirs because of differences in densities of the two liquids. This stage can last seconds to minutes and leads directly to the second stage, when the vapor film collapses over a period of microseconds and the two liquids hydraulically couple. Heat transfer increases by 1–2 orders of magnitude (Zimanowski, 1998; Büttner et al., 2002) while the melt is cooled and the water heated. The third stage, milliseconds in length, is a brittle response and fine fragmentation of the magma. Shock waves exceed the bulk modulus of the magma, causing fronts of brittle failure to move through it. Thermal granulation also causes a fine fragmentation of the magma. The water flashes to vapor during the fourth stage, leading to a dramatic expansion and explosion lasting milliseconds to minutes (Zimanowski, 1998). The particles created by the MFCI are deposited from fallout or surges. The MFCI processes, thus, are dependent on supply of both magma and water, and upon appropriate conditions in the vent area. At a single location, the supply of water or magma can vary during an eruption and different portions of the ascending magma body may have better conditions for MFCI at various times during the eruption, leading either to multiple explosion foci or to foci migration over the course of the eruption. Several mechanisms can be postulated for the variations in the explosivity of a phreatomagmatic eruption, including variations in the water and magma supplies, the efficiency of their interaction, physical characteristics of the magma (e.g. viscosity), and variations in the magmatic gas content (Houghton et al., 2000). If the magma and water supplies are changing, this could occur due to pulses of one or both liquids, but it could also relate to changes in the locus of the water– magma interaction. If the focus of water–magma interaction moves laterally or vertically along a dike, the country rock may be less broken up by explosions at the new site, which may affect the efficiency of the magma–water interactions. If water moves along fractures, more fracturing should allow greater water movement, and thus access of water to magma could increase during an eruption as the country rock (and magma) is shattered. Conversely, if the country rock is nonlithified, and water supply for the phreatomagmatism is through porous flow or liquefaction of water-saturated sediments, a new area along the dike may store considerably more water available for the interactions and explosions. Another variable is the purity of the MFCI coolant. White (1996) and White et al. (2008) describe some of the possible effects of impure coolants on the MFCI. Viscosity increases with particle concentration, and this can affect the convection of heat away from the magma, but also improves the ability of the magma and coolant to mix (because of more similar viscosities). Maximum explosivity is probably damped by impure coolants, though. The effects of impure coolants are still poorly characterized, and the liquefaction of the surrounding deposits may affect the structure of the vent in ways that enhance or diminish the effects of impure coolants on the MFCI process. In this paper, we present evidence from the latest Pleistocene Tecuitlapa Maar in east-central Mexico for the migration of the MFCI locus during the eruption. The maar erupted through unconsolidated sediments, which overlie limestone and igneous rocks. The vent migrated along a line that parallels regional faults and vent

alignments, suggesting that structural features controlled this migration. When the vent had migrated to its western end point, the eruption dried out and strombolian eruptions produced scoria cones along the vent alignment, with an eastward younging trend (thus reversing the order of vent migration during the phreatomagmatic portion of the eruption). We suggest this migration was controlled by favorable conditions for MFCI along the dike — the explosions were concentrated where the most liquefied sediment was available, leading to the liquefaction of the surrounding sediments and more available coolant for further explosions. As the liquefied sediments were exhausted in one area, nearby areas were liquefied and the eruption migrated along the dike toward where more coolant was available. 2. Geology of Tecuitlapa Maar Tecuitlapa Maar lies in the Serdán-Oriental Basin of east-central Mexico (Fig. 1A). This basin is the easternmost of the intermontane basins that make up the Trans-Mexican Volcanic Belt, and is bounded by the Citlaltéptl — Cofre de Perote volcanic range to the east, isolated Mesozoic marine sedimentary rocks to the south, Los Humeros Caldera to the north, and a Miocene volcanic ridge to the west. Numerous eruptions, with bimodal compositions of basalt and rhyolite, formed Pliocene to Holocene scoria cones, maars, and domes. Many of the scoria cones within the basin are aligned in E–W or ENE–WSW direction, similar to the dominant fault system for the central Trans-Mexican Volcanic Belt (Suter et al., 1992). About a dozen maar volcanoes are found in the Serdán-Oriental Basin, with compositions that range from basalt to rhyolite. Some of these contain modern lakes, but others are dry. The activity of the surrounding and intrabasinal volcanoes produced thick basin-filling deposits of a fluvial and eolian volcaniclastic unit informally termed the Toba Café. The Toba Café is dominantly silt-fine sand in grain size, but contains sandy fluvial beds with pebbles and cobbles derived from both within the basin and the surrounding stratovolcanoes. Some of the clasts have distinctive features that link them to particular volcanoes surrounding the basin, such as particular andesites from Citlalteptl and pumice from eruptions from Los Humeros caldera, whereas others are of unknown sources, although of rock types (e.g. basalt) that are well represented within and surrounding the basin. The sand grains are dominantly resistant crystals, such as quartz and feldspars, and rounded pieces of volcanic glass. This unconsolidated deposit varies in thickness from 10 to ~ 40 m where exposed within the basin. Within the Serdán-Oriental Basin, the Toba Café is typically water-saturated at shallow depths and, during the summer rainy season, the water table reaches the surface in many places, producing shallow salty ponds. This volcaniclastic unit is a potential aquifer for phreatomagmatism at many of the Pleistocene maars in the basin. At some Serdán-Oriental maars (e.g. Atexcac; Carrasco-Núñez et al., 2007), underlying fractured Cretaceous limestone and shale, as well as altered and fractured Tertiary volcanic rocks and intrusions, may also be the interactive aquifers. In map view, Tecuitlapa Maar is an elliptical basaltic maar about 1.3 km (E–W) × 1.0 km (N–S) and 100 m deep (Fig. 1B). The late Pleistocene maar cuts through Toba Café to about 20 m below the preeruptive surface and the eruption deposited about 50–70 m of surge and fallout deposits at the maar edge. The crater contains an east– west aligned series of scoria and spatter cones, up to about 70 m above the present maar floor, and a shallow (about 2–3 m deep at the time of this writing, and it is shallowing as regional groundwater is withdrawn) lake fills the western moat. Tecuitlapa deposits are well exposed along the interior rim of the maar, but the surrounding area is covered by post-Tecuitlapa Toba Café deposits and distal deposits have only been found along a stream northwest of the volcano. The deposits of the maar eruption form a ~ 70-m-high ring around the crater, and many beds can be traced around the maar,

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Fig. 1. A) Map of the eastern Mexican Volcanic Belt, showing location of Tecuitlapa Maar. B) Geologic map of Tecuitlapa Maar, including locations of the described stratigraphic sections.

although, as discussed below, they vary in thickness. Vegetation obscures the section in many places and cliffs make careful examination difficult in others. An embayment in the western wall of the maar is probably due to post-eruption slumping. The beds are thinner there than elsewhere, and the section is missing the upper fallout. If one extends the beds of the embayment upward along their same strike and dip, marker beds would be at the same height as in neighboring sections where the embayment meets the more regular ellipsoid shape for maar. Thus, post-eruption mass wasting has affected the shape of the maar, but retreat of the walls cannot be more than about 100 m in most places, given the location of the scoria cones within the maar. 3. Stratigraphy of Tecuitlapa Maar Three stratigraphic sections are described in this paper (Figs. 2–4) in order to characterize the deposits around the maar. These were chosen because they are the best-exposed sections of the deposits and because they are distributed around the crater so differences between the sections can be seen. Correlations between depositional units

were carried out using stratigraphic height, marker beds, and similarities in lithic and juvenile clasts. Some beds occur at all stratigraphic sections whereas others have a more limited distribution. Dune-form ridge axes were measured where possible. Typically three to five axes were measured in a location and their values averaged. Typical variance for the directions was about 10°, which is reasonable given the uncertainty in identifying the precise axis. Bomb-sag asymmetry was also measured in order to ascertain the direction from which the bomb impacted the tuff layer. Again, several were typically measured at a locality. The dune-form axis directions are interpreted as being roughly perpendicular to the flow direction. This assumption could not be rigorously tested, but the axis data are consistent with the bomb-sag asymmetry data everywhere they were obtained from the same set of beds. Detailed descriptions are contained in the stratigraphic columns, and general descriptions, first-order interpretations, and comparisons between the important characteristics of each section are presented here. The bases of the maar-deposit sections at Te-03 and Te-05 appear to be higher in elevation by a few meters than the base at Te-04, but it is difficult to quantify this without a detailed leveling survey.

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Fig. 2. Stratigraphic Section Te-04 of Tecuitlapa Maar inner rim. Tie-lines connect photographs to their place within the section. Location of this section is shown in Fig. 1.

3.1. Stratigraphic sections 3.1.1. Eastern wall (section Te-04; Fig. 2) The lowermost deposits of the maar occur along its eastern wall, overlying the Toba Café, and consist of a basal 30–36-cm-thick well-

sorted coarse vesicular scoria-lapilli fallout bed overlain by blocky juvenile and lithic lapilli tuffs and tuffs interbedded with blocky juvenile lapilli beds (Unit IA; Fig. 2A). Bomb sags and dune forms in the tuffs consistently indicate flow from a vent to the west/southwest. The coarse grain size and poor sorting and bedding of the deposits are

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Fig. 3. Stratigraphic Section Te-03 of Tecuitlapa Maar inner rim. Tie-lines connect photographs to their place within the section. Location of this section is shown in Fig. 1.

consistent with a nearby vent. Scoria-lapilli grain size varies continuously from the tops of one bed to the bases of the next, across the intervening tuff beds, implying that the tuffs were deposited by discrete surge pulses moving through a continuous fallout. The overlying unit IB is marked by accretionary lapilli fallout, tuff, and lapilli tuff beds (Fig. 2B), as well as by an absence of breccias. The accretionary lapilli beds are 1–10 cm thick and make up about 10% of

this unit. The rare large bombs formed bomb sags indicating a vent to the west. Unit IB and all overlying units are recognized on walls around the maar. Breccias and more abundant blocks (~ 1%, but some 30–50-cm beds contain 5–10% blocks) re-appear in unit IIA, which is dominantly tuff breccia and contains abundant large bomb sags. Dune-form axes and bomb sags indicate a vent to the west-southwest. Unit IIB (Fig. 2C) is generally better bedded and finer grained than

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Fig. 4. Stratigraphic Section Te-05 of Tecuitlapa Maar inner rim. Tie-lines connect photographs to their place within the section. Location of this section is shown in Fig. 1.

unit IIA, with abundant well-bedded tuff. The lower half of the unit is 40% laminated and cross-laminated tuff (with almost no lapilli component) and 60% accretionary lapilli beds (up to 25 cm thick). Dune-form axes are oriented about 340°, indicating a source area to the west-southwest. The upper half of this unit contains breccia and tuff breccia, but the tuff component remains very similar and retains

the same proportion of accretionary lapilli and laminated tuff beds. Overlying this is a heterogeneous unit (III) with interbedded tuff, tuff breccia, and breccia (Fig. 2D). Dune-form axes indicate vents to the west-southwest. Lithic fragments in the breccia unit are of all identified types, possibly indicating that this breccia is from an explosion through vent-clogging debris. Outsized blocks are large (to

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150 cm) in this unit, but are smaller than those found at the same stratigraphic height in the western walls of the maar (Te-03, to 250 cm, and Te-05, to 220 cm), and the average grain size is smaller here too. A drying-out sequence is evident in Unit IV (Fig. 2E), with the occurrence of more juvenile scoria lapilli with increased vesicularity and angularity. Few beds of true well-sorted scoria fallout occur though, perhaps indicating an up-wind location for this site at that point in the eruption, along with a long distance to the scoria cones during the early phases of scoria cone activity in the crater. 3.1.2. Southwestern wall (section Te-03; Fig. 3) This section is a transverse section along a road that traverses much of the western wall of the maar. Thus, the upper section is about 0.5 km laterally and 70 m vertically from the base of the section. Unit IA of Te-04 is not identified in this section nor in Te-05, either because it was not deposited this far from the vent or because the lateral facies changes are so great that it was not recognized. The section overlies Toba Café (Fig. 3A), and begins with unit IB, which is generally lithic-poor and fine-grained. Dune-form axes indicate flow from 110°, consistent with flow from the eastern end of crater. Bomb sags, created by juvenile and lithic clasts, are asymmetric, indicating a vent to the east (Fig. 3C), and accretionary lapilli layers are abundant (Fig. 3B). The lower contacts of tuffs in unit II are abruptly gradational, meaning they grade over b1 cm. Bomb sags become less asymmetric upward, whereas blocks become bigger, suggesting the vent is becoming closer. The unit IB-II sequence coarsens and bed thickness increases upward. Near the base of unit IB, breccia beds are 10–20 cm thick with 0.5–1 cm lapilli, whereas breccia beds in upper unit II are 1–2 m thick and lapilli are 1–25 cm in diameter, with outsized bombs to 80 cm. Unit III is similar to unit II but less well bedded, with more mixing of ash and lapilli, a characteristic consistent with less distance from the vent. Unit III contains the coarsest lithic blocks of section Te-03, but they are within tuff breccia rather than breccia. These tuff breccias are interpreted to be from nearby explosion loci, with insufficient travel distance to separate ash from lapilli. Bomb sags, many spectacular (Fig. 3D), in unit III are

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symmetric in east–west cross-section, pointing into the outcrop, so the vent lay to the north. Unit IV combines lithic and abundant juvenile fallout with surge layers. Bomb sags indicate a nearby northerly vent (plunging into the outcrop) and dune forms indicate flow from 30–40°, which is the western end of the maar crater. Unit IV represents a drying-out sequence, with more scoria, as well as dense juvenile clasts with fusiform shapes, upward. The scoria from the lower part of unit IV is blocky and likely from low-efficiency phreatomagmatic explosions. The scoria in this unit is similar to, but finer than, the scoria on the cones in the crater and may correlate with that portion of the eruption. 3.1.3. Northwestern wall (section Te-05; Fig. 4) This section begins with 30 cm of thin-bedded to laminated tuff. These layers may correlate with the early deposits (unit IA) in section Te-04, but would be very distal and/or upwind. The facies change is so great that this correlation is uncertain, and the deposits are grouped under unit IB. These deposits are overlain by tuff and lapilli tuff, which dominate (80%) in the lower 16 m of the section, with tuff breccia making up about 20% of the section (Fig. 4A). Lapilli are of many types, including blocky basalt, rare red scoria, andesite, arkose, and cored lapilli (Toba Café cores with basalt rims). Bombs reach 1 m in diameter at 10 m up in the section, and another bomb-rich layer occurs at 12 m (Fig. 4B). Overall, this is a monotonous section, possibly indicating a more distal vent location. The lower 6 m of unit II consists of clastsupported tuff breccia interbedded with laminated and dune-formed tuff. This is interpreted as fallout through an ash-rich atmosphere (possibly with some ash matrix washed in later) interbedded with surge deposits. Then 2 m of laminated to dune-formed tuff with accretionary lapilli fallout layers occur (Fig. 4C), interpreted as being from an ash-rich eruption producing fallout and surges. Juvenile and lithic bombs and blocks reach 1 m in diameter and produce deep sags. Overlying this is bedded lapilli-tuff breccia and lapilli tuff, with one 1.2-m-thick breccia (Fig. 4D). The tuff contains low-angle dunes and breccias are clast-supported. This is interpreted as a sequence of surges and lapilli fallout and ballistic blocks. Overlying this is another

Fig. 5. Panorama of Tecuitlapa Maar, looking to the east from the western rim. Late-stage scoria cones are clearly visible, building in height to the east. Lake has dropped in height over the past few decades.

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3.2. Comparison of facies characteristics

Table 1 Volume calculations for Tecuitlapa Maar

Eastern scoria cones Western scoria cones Total scoria-cone volume

Height

Radius

Volume

(m)

(m)

(× 106 m3)

70 50

250 350

4.6 6.4 11

Radius

Depth

(m)

(m)

Total crater volume 550 100 Crater volume below pre-existing surface 450 30 Full maar-ring volume Amount of juvenile material in phreatomagmatic deposits Total juvenile material (scoria-cone volume + juv. phreatomagmatic) Total juvenile material as DREa a

95 19.1 55.8 36.7 47.7 34.9

(0.8 × phreatomagmatic juvenile material) + (0.5 × total scoria-con volume).

2 m of thin-bedded tuff from surges, covered by 8 m of bedded moderately sorted lapilli-tuff breccia. The tuff breccia predominantly contains blocky dense basalt clasts but 20–30% are lithic lapilli. Unit III is marked by thicker beds in general, but is similar to unit II. It begins with a poorly sorted lapilli-tuff breccia, likely an explosion breccia from an approaching vent. Above this are other similar layers, interbedded with weakly to well-bedded tuff and lapilli tuff, with dune forms. Bomb sags reach 20 cm deep, with bombs to 15 cm. Toward the top of the unit, 5-cm-thick moderately well sorted blocky dense juvenile lapilli breccias occur, likely from strombolian activity. These become dominant in the overlying unit IV, where open framework lapilli-breccia beds, up to 50 cm thick, are common. These are interbedded with tuff and lapilli-tuff beds (Fig. 4E), which have dune-forms. Some ash laminae lace through the breccia layers, suggesting surges occurred under continuous fallout in the latter part of the eruption. No pure scoria fallout occurs in the section here, but scoria lapilli, some with a cauliflower texture (Fig. 4F), are abundant in the overlying soil. The uppermost beds of the sequence may have been eroded or tilled during cultivation.

The data from these three stratigraphic sections through the maar deposits are most easily interpreted as indicating a locus of explosions that moved from east to west in the maar over time. Thicknesses of beds and maximum grain sizes in the west generally increase, while those in the east decrease, up-section. Dune forms and asymmetric bomb sags indicate a westward migration in vent locations up-section. The facies changes also reflect an approaching (receding) vent in the west (east), as the degree of sorting in the deposits decreases as the explosion locus nears. 3.3. Scoria and spatter cones in the maar crater The Tecuitlapa crater contains a series of scoria and spatter cones with an east–west alignment, although individual cones have roughly circular craters (Figs. 1, 5). They increase in height and overall volume from west to east, and the overlap of crater borders, with later vents cutting through the rims of earlier vents, indicates a consistently eastward younging direction. The earliest cones formed low-rimmed scoria rings, but later, more easterly cones are tall and conical in shape. Two small (50–100 m in diameter) tuff rings and one scoria cone form a lineation parallel to but 200–300 m south of the scoria-cone line. This latter scoria cone produced 1–5 to 2.2-m-diameter spindle bombs deposited within a few tens of meters of the vent. The landforms of these vents blend with those of the neighboring aligned scoria cones, and may indicate a similar timing for them. This alignment of the scoria cones is similar to those seen at some other scoria eruptions where venting is concentrated at localities along a fissure (e.g. La Palma, White and Schmincke, 1999; Tarawera, Houghton et al., 2004; Keating et al., 2007). 3.4. Age of the eruption No isotopic dates have been reported previously for the Tecuitlapa volcanic products. Inferences about the age of the monogenetic

Fig. 6. Components in Tecuitlapa Section Te-03, combined for the −4 to −1 phi grain sizes. Aggregate samples were separated into those that were identifiably tuff (called ‘tuff’) and those of uncertain origin (called ‘aggregates’).

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volcanism of the Serdán-Oriental basin were made by Negendank et al. (1985), based on regional stratigraphic and geomorphologic considerations. They suggested that most of the scoria cones, and possibly also the maar volcanoes, are younger than 30 ka. The younglooking morphology exhibited by the easternmost Tecuitlapa scoria cone is in accordance with that assumption. We attempted to date the Tecuitlapa products using the 39Ar/40Ar method at the Geochronology Lab of New Mexico Bureau of Mines and Mineral Resources. As the lava is basaltic, only groundmass concentrate could be used for dating. A sample of fresh scoria from the youngest (easternmost) scoria cone produced a date of 0.0 + /−0.02 Ma. We interpret this as a very recent age that the method cannot resolve and suggest that the Tecuitlapa eruption was likely more recent than about 20 ka. No datable carbon fragments have been found within the sequence. 3.5. Volumes The original base of the crater is not exposed (it is now covered by water, lake sediments, slump deposits, and the scoria cones themselves), so it is not possible to measure the original crater volume, nor is it possible to precisely determine the amount of magma involved in the phreatomagmatic eruptions without better control on bed thicknesses and composition beyond the crater walls. However, the scoria cones are roughly 11 × 106 m3 in volume (Table 1). The current crater is 95 × 106 m. If we assume that the original crater was 30 m below the pre-existing ground surface (which is marked by the top of the Toba Café in the maar walls), the amount that was excavated is 19 × 106 m. Just using the current morphology of the maar cone, which has a volume of 56 × 106 m3 above the pre-existing ground level, and subtracting the amount of Toba Café represented by the volume excavated, we obtain a rough volume of 37 × 106 m3 for juvenile material involved in the phreatomagmatic portion of the eruption, and a volume of 48 × 106 m3 for juvenile material in the overall eruption. This is not dense rock equivalent, and it is not clear what factor should be applied to arrive at that value, as the deposits vary from open-framework to dense, and the fragments are dense to highly vesiculated. We suspect that vesiculation and pore space represent something less than 20% of the total juvenile volume for the phreatomagmatic eruption, but likely represent a value close to 50% of the scoria cone volume. Applying those correction values to the volumes produces a total DRE volume of 35 × 106 m3 for the eruption. As these estimates do not include any deposits beyond the maar ring and assume all the lithic material is incorporated in the maar-ring deposits, they are minimum values, but indicate a small-scale eruption, on the order of the magma volume of small scoria-cone eruptions such as those of Crater Flat (Valentine et al., 2006). The violence of the eruption, compared to that of a small scoria-cone eruption, emphasizes the importance of predicting which basaltic eruptions will be phreatomagmatic. 3.6. Componentry Clasts were described in the field. The clasts larger than −4 phi in size are dominantly volcanic in composition. All have some rounded edges, suggesting they are stream cobbles in the Toba Café rather than pieces of bedrock broken off by explosions. Component analysis was carried out on the −4, −3, −2, and −1 phi fractions of the deposits (Fig. 6). The proportion of juvenile component in the deposits fluctuates through the section. A sudden decrease is observed in the uppermost part of unit II of section Te03, and a marked increase in the upper part of unit IV, as activity switched to a more strombolian style. The lithic fragments found in the different beds within the complete sequence are diverse and include volcanic (basalt, porphyritic andesite, aphanitic andesite, rhyolite, altered and oxidized lava, grey pumice, accretionary lapilli, and tuff) and non-volcanic (sedimentary

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clasts, aggregate, and metamorphic) clasts. The most abundant of these are basalt and altered/oxidized clasts. The altered clasts are most abundant in units I (upper part) and II, while the proportion of the basalt clasts is highest at the lower part of unit I and in unit IV. The proportion of the other components is much lower and less variable, but the aphanitic andesite is most common in unit III, where sedimentary and scarce metamorphic clasts are also relatively abundant. 4. Discussion The Tecuitlapa eruption produced a crater about 1 km wide and N70 m deep, partially filled by scoria cones. The eruption shows strong evidence of a drying-out trend at the end, but the first two-thirds of the magma came out under fluctuating conditions with abundant water–magma interaction. The evidence for this water–magma interaction is clear throughout each of the three described sections, but the evidence for proximity to vent varies between them. The early eruptions occurred closer to the eastern end of the maar, and then moved toward the west during the eruption. The change appears to have been gradual, rather than in large jumps, as the changes in facies and flow direction indicators are gradational. 4.1. Vent migration with time What controls this vent migration with time? Some consistent change caused this, as the vent migration was in a steady direction. The rock type of the lithic fragments did not change — no large clasts of fragmented bedrock (angular clasts rather than rounded stream pebbles and cobbles) were brought up in the eruption and the same types of lithic and juvenile ash occur throughout the sequence (Fig. 6). Therefore, the same country rock was involved throughout the eruption and the level of explosions did not descend to the underlying lavas or limestones, as occurred at nearby Atexcac maar (CarrascoNúñez et al., 2007). Instead, the explosions remained within the Toba Café, which occurs as rounded silicic shards and crystals and basalt to rhyolite lava and pumice lapilli throughout the eruption sequence. Cored bombs and juvenile fragments with mud inclusions (fluidal clasts of Toba Café material) are good indicators that the magma/water interaction occurred at this level. The magma type did not change, and no consistent differences in vesicularity of juvenile material occur within the sequence. The explosion locus did not migrate downward, as has been observed at many maars (Lorenz, 1986; Nemeth et al., 2001), but instead moved laterally. The coolant source, fluidized Toba Café, was continuous laterally, but the large contrast in rock characteristics downward may have limited migration in that direction. Coolant access to the magma would have been via fracture in the limestone and Tertiary andesite (e.g. Nemeth et al., 2001) and may have been limited. Low magma discharge, as suggested for Tecuitlapa, could result in sealing off fractures in the bedrock and thus limit water access to the magma, whereas low discharge in non-consolidated sediments would allow collapse of the sediment into the dike, resulting in intimate mingling and the potential for phreatomagmatic explosions. Such high lithological contrasts may limit the depth of fuel-coolant interactions, as other examples of such high contrasts are commonly associated with a limited depth of excavation (e.g. Ukinrek Maar, Self et al., 1980; Hopi Buttes, Hooten and Ort, 2002). Auer et al. (2007) found that weak, water-rich substrates collapse into the vent area, maintaining a constant supply of water in the vent area and keeping the explosion locus shallow. They found no migration of the vent and suggested that this was because of the constant supply of water. The unconsolidated sediments in the Balaton Highland appear to be much thicker than those of the Serdán-Oriental Basin, which are in the range of 10–40 m. At Tecuitlapa, the supply of liquefied sediment may not have been sufficient to keep phreatomagmatism

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active at a single locus along the dike. Thus, the influence of the structural fabric of the underlying consolidated rocks was greater, and the vent migrated along the dike. The dike that fed the eruption has an orientation parallel to the regional ENE structural trend, which is apparent in the bedrock throughout the region (Carrasco-Núñez et al., 2006). Characteristic alignments, with the same orientation, of scoria cones occur to the east (Ajojuca) and the west (at the summit area of Cerro Brujas shieldlike lava cone) of the Tecuitlapa crater. The Toba Café, however, has no apparent structural fabric, with only roughly horizontal bedding to create any anisotropies. Thus, the linearity of the eruption was set in the underlying bedrock, and carries through the Toba Café because of the relative thinness and isotropy of that deposit. The low volume of the eruption is consistent with relatively low magma flux. Valentine et al. (2006) calculate fluxes of N1–3 m3/s for lava-flow eruptions of similar sizes at Crater Flat. 4.2. MFCI with an impure coolant The phreatomagmatism at Tecuitlapa likely occurred between the basaltic magma and an impure coolant (White, 1996). The Toba Café is generally fine grained (silt to fine sand) and water movement via permeability would be too slow to provide much water. The lack of induration of the deposit results in no significant fractures within the Toba Café and thus, fracture flow was not a source of water for the phreatomagmatism either. The grain size and non-indurated nature of the deposit are conducive to liquefaction, however. Liquefied sediment as the coolant is also consistent with the great abundance of Toba Café grains in the ash fraction and rare grains of Toba Café within juvenile lava clasts. We suggest that explosions happened where magma met liquefied sediment and produced MFCI. As interpreted for the 1949 eruption of La Palma (White and Schmincke, 1999), the dike would have experienced collapsing walls as it ascended. These collapses would have produced water-rich slurries for MFCI, while extension caused by the dike intrusion would also enhance water/slurry flow toward the dike. Each explosion and consequent seismicity produced more liquefied sediment to interact with magma ascending along the dike, but as this slurry was used up in one location, the locus of explosions moved toward where the best conditions for MFCI existed. As such a slurry is fairly mobile, the lateral vent movement would have been gradual. Magma may have continued to rise along the dike in places where little MFCI was occurring, ponding in the crater, or the dike may have frozen in those areas, as stagnant magma froze ascent (e.g. Houghton et al., 2004). Below we argue that the eruption used up most of the liquefied sediment, leaving a mostly dry crater, rather than drowning the crater in slurry. 4.3. Scoria cone eruptions Another question that arises is why the eruption stopped progressing to the west and switched to an eastward, drying migration trend. The Toba Café continues to the west, apparently unchanged in character, so the dike did not run out of coolant for MFCI. The end of the westward vent progression is more likely related to the length of the dike — the locus of explosions had reached the end of the dike. The orientation of the dike was largely set in fractures in the lavas and limestones underlying the Toba Café and its length is probably related to the size of its mantle source area, which in turn is related to the erupted volume (Valentine and Perry, 2006, 2007). The low volume of the eruption is comparable to those of Pleistocene scoria cones in Crater Flat, Nevada (Valentine et al., 2006), and the Crater Flat dike lengths are similar to the length of the vent lineation at Tecuitlapa. If the dike came out of the rocks into the non-indurated Toba Café at a certain length, it was unlikely to lengthen much over the remaining short distance to the surface. The length of the eruption

fissure was controlled by factors that lay beneath the influence of the phreatomagmatic explosions, so the dike, and hence the vent, could not migrate farther to the west. The scoria cones were formed when the dike no longer had access to water at depths appropriate for MFCI. No evidence for phreatomagmatism occurs in the scoria cones, although two small explosion pits occur within the maar crater, but significantly off the scoria cone alignment. This implies that the crater was not filled by liquefied Toba Café. Such a slurry would have provided a ready coolant for MFCI, or at least have coated many clasts and formed a significant part of the scoria-cone componentry, both in the cones and the strata exposed in the maar wall. The scoria clasts are angular and show no signs of brittle fragmentation typical of phreatomagmatism. The cause of the apparently steady progression of the scoria-cone vents back to the east is difficult to ascertain. At least two interpretations are possible for the data. The first, and most obvious, is that the vents actually migrated to the east in a steady progression, with the eastern vents having significantly longer activity periods. A second possible interpretation of the data relies on the observation that the knowledge of the timing of activity of individual vents is based upon the overlapping vent craters, which is related to the end of activity at each crater. This allows the possibility that eruption was broadly simultaneous along a fissure that extended the full length of the scoria-cone lineation (or in smaller sections at any given time), but that the vents gradually closed from the west while eruption continued in the east. This model would explain the increasing heights of the scoria cones eastward along the lineation. If the first interpretation is true, a possible reason for the migration can be speculated. The dike would have been rising through chaotic and unstable vent breccias. Collapses of this material could stop the magma ascent in a location, leading it to ascend slightly farther along the dike, leading to an eastward migration. With the second interpretation, the progressive shutting down of the eruption needs explanation. The maarrim deposits indicate that the eruption began in the east. The eastern sector of the maar may represent the portion of the dike with the easiest ascent path. This could also cause the eastern vent to both remain active the longest and be the most voluminous. One way to check whether the hypothesized length of the dike is reasonable is to back-calculate the size of the magmatic source area using the dike length. Using the method of Valentine and Perry (2006, 2007), we can estimate the length scale of the source area. The length of the dike at its intersection with the surface was about 600 m. The length at depth is probably not more than 2–3 times that value, based on experiments that show that dikes have concave downward upper surfaces and nearvertical sides (Menand and Tait, 2002). This produces a mantle sourcearea length scale of 1.2–1.8 km. This small area is reasonable given the small volume of magma −1% melting of a 1-km3 area in the mantle would produce three times the minimum estimated volume for the Tecuitlapa eruption. Given the uncertainties in the calculations of magma volume and lengthening of dikes with depth, this suggests that the estimated dike length is at least reasonable for the eruption. 5. Conclusions The Tecuitlapa Maar eruption involved fuel-coolant interaction between an impure coolant (fluidized Toba Café sediments) and basaltic magma along a 500-m-long (minimum) dike. The locus of explosions migrated westward until the coolant ran out, at which point scoria cones formed along the dike. These scoria cones may reflect an eastward migration of the scoria cone vents or possibly an eastward migration of shutting off of a fissure vent. The explosions stayed within the water-saturated 10–40-m-thick Toba Café volcaniclastic deposits, with no migration downward into the underlying lavas or limestones. Downward migration of the explosion locus is a common feature at many maars, but it may be that such migration requires a continuous source of water to carry the explosions and

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consequent fracturing downward. A high lithological contrast may also hinder this downward migration. Acknowledgements Funding was provided by grants PAPIIT IN104401 and IN107907. Marisol Cano and Javier Hernández, helped with part of the componentry analysis and Juan Vázquez and Bartolo Rodríguez helped with sample preparation. Mike Kelly scanned many of the photographs. Reviews by K. Németh and an anonymous reviewer are gratefully acknowledged. References Abrams, M., Siebe, C., 1994. Cerro Xalapaxco: an unusual tuff cone with multiple explosion craters, in central Mexico (Puebla). J. Volcanol. Geotherm. Res. 63, 183–199. Auer, A., Martin, U., Németh, K., 2007. The Fekete-hegy (Balaton Highland Hungary) “soft-substrate” and “hard-substrate” maar volcanoes in an aligned volcanic complex — implications for vent geometry, subsurface stratigraphy and the palaeoenvironmental setting. J. Volcanol. Geotherm. Res. 159, 225–245. Büttner, R., Dellino, P., Zimanowski, B., 1999. Identifying modes of magma/water interaction from the surface features of ash particles. Nature 401, 688–690. Büttner, R., Dellino, P., La Volpe, L., Lorenz, V., Zimanowski, B., 2002. Thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from Molten Fuel Coolant Interaction experiments. J. Geophys. Res. 107 (B11), 2277–2290. Carrasco-Núñez, G., Dávila, P., Puente, R., 2004. Hawaiian-Strombolian precursory activity of phreatomagmatic maar-forming eruptions, case studies of central Mexico. IAVCEI General Assembly, Pucón, Chile. Abstracts CD. Carrasco-Núñez, G., Díaz-Castellón, R., Siebert, L., Hubbard, B., Sheridan, M.F., Rodríguez, S.R., 2006. Multiple edifice-collapse events in the Eastern Mexican Volcanic Belt: the role of sloping substrate and implications for hazard assessment. In: Tibaldi, A., Lagmay, A. (Eds.), The Effects of Basement Structural and Stratigraphic Heritages on Volcano Behaviour. J. Volcanol. Geotherm. Res., vol. 158, pp. 151–176. Carrasco-Núñez, G., Ort, M.H., Romero, C., 2007. Evolution and hydrologic conditions of a maar volcano (Atexcac crater, eastern Mexico). J. Volcanol. Geotherm. Res. 159, 179–197. Dellino, P., Isaia, R., La Volpe, L., Orsi, G., 2001. Statistical analysis of textural data from complex pyroclastic sequences: implications for fragmentation processes of the Agnano Monte Spina Tephra (4.1 ka), Phlegraean Fields, southern Italy. Bull. Volcanol. 63 (7), 443–461. Dellino, P., Isaia, R., La Volpe, L., Orsi, G., 2004. Interaction between particles transported by fallout and surge in the deposits of the Agnana-Monte Spina eruption (Campi Flegrei, southern Italy). J. Volcanol. Geotherm. Res. 133, 193–210. Gutmann, J.T., 2002. Strombolian and effusive activity as precursors to phreatomagmatism; eruptive sequence at maars of the Pinacate volcanic field, Sonora, Mexico. J. Volcanol. Geotherm. Res. 113, 345–356. Hooten, J.A., Ort, M.H., 2002. Peperite as a record of early stage phreatomagmatic fragmentation processes: an example from the Hopi Buttes volcanic field, Navajo Nation, Arizona, USA. J. Volcanol. Geotherm. Res. 114, 95–106. Houghton, B.F., Wilson, C.J.N., Smith, R.T., Gilbert, J.S., 2000. Phreatoplinian eruptions. In: Sigurdsson, H. (Ed.), Encyclopedia of Volcanoes. Academic Press, San Diego, pp. 513–525. Houghton, B.F., Wilson, C.J.N., Del Carlo, P., Coltelli, M., Sable, J.E., Carey, R., 2004. The influence of conduit processes on changes in style of basaltic Plinian eruptions: Tarawera 1886 and Etna 122 BC. J. Volcanol. Geotherm. Res. 137, 1–14. Keating, G.N., Valentine, G.A., Krier, D.J., Perry, F.V., 2007. Shallow plumbing systems for small-volume basaltic volcanoes. Bull. Volcanol. doi:10.1007/s00445-0070154-1.

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