Paricutin Research Articles

Deciphering the sources and processes feeding young monogenetic volcanoes from the Michoacán Guanajuato Volcanic Field (Mexico): A study case of El Astillero and El Pedregal

El Astillero and El Pedregal monogenetic volcanoes formed ∼500–700 CE in the Tancítaro region in the southern part of the Michoacán-Guanajuato volcanic field, only 25 km to the SW of the historic Paricutin volcano. The ∼6-year-long eruption was characterized by a change from explosive to effusive activity, accompanied by a shift in the location of the active vents. Initial activity was Strombolian-explosive and first formed the El Astillero cone before turning effusive with the emission of several lava flows. Then, a new vent located 2 km to the ENE opened and produced the purely effusive (non-explosive) El Pedregal lava flow field. As the eruption progressed, the bulk magma composition (major and trace elements) changed from basaltic andesite to andesite (SiO2 = 52–59 wt%), which is also reflected in a successive change in the petrography of the erupted tephras and lavas. However, the El Pedregal lava sequence shows small Mg# reversals followed by a marked final reversal to more mafic compositions. Likewise, 87Sr/86Sr (0.70388–0.70403), 143Nd/144Nd (0.512836–0.512742), 206Pb/204Pb (18.632–18.671), 207Pb/204Pb (15.583–15.598), 208Pb/204Pb (38.376–38.450), 176Hf/177Hf (0.28301–0.28290), and 187Os/188Os (0.1258–0.1865) isotope ratios changed systematically as the eruption progressed, and record a final shift to a distinct isotopic signature. The spatio-temporal proximity of both vents and the petrographic and geochemical characteristics of their magmas suggest a comagmatic evolution that can be explained by a combination of variable degrees of magma recharge, magma mixing, and fractional crystallization of subduction-modified mantle melts. A similar combination of magmatic processes for the genesis and evolution of these magmas has also been proposed for other young monogenetic volcanoes in the Michoacán-Guanajuato volcanic field (e.g., Paricutin, Jorullo, and the Tacámbaro cluster). Accordingly, primitive magmas in the Trans-Mexican Volcanic Belt are subduction-modified mantle melts that evolve largely by crystal fractionation and pass through the crust without significant assimilation.

Repeated seismic swarms near Paricutin volcano: precursors to the birth of a new monogenetic volcano in the Michoacán-Guanajuato volcanic field, México?

The birth of a new monogenetic volcano is difficult to forecast with precision, both in space and time. Nevertheless, seismic activity can alert of the imminence of such an eruption because it usually occurs as small-magnitude earthquake swarms that can last for a few weeks to months prior to an eruption. These swarms are usually related to magma that becomes stalled in the Earth’s crust for variable periods of time before its eventual eruption at the surface. For several reasons, volcanic seismic swarms have rarely been recorded with seismometers before the birth of a new monogenetic volcano. Over the past 25 years, six distinct seismic swarms (in 1997, 1999, 2000, 2006, 2020, and 2021) were detected between Tancítaro and Paricutin volcanoes, in the southwestern part of México’s Michoacán-Guanajuato volcanic field. They are believed to represent repeated attempts of magma to reach the surface hinting that in this region magma might become stalled for some time, so as to not reach the surface in a single ascent event from its source in the mantle. To better understand the magma’s migration path through the crust, we re-located with greater precision some of these seismic swarms by using the same methodology and velocity model to the entire data set. Our results show that these swarms originated within a small area beneath the NE flank of Tancítaro at depths of between 15 and 8 km below sea level (bsl). Apparently, magma is trying to reach the surface within the same conduit network at these crustal depths, but stalls when reaching a depth of ~ 8 km bsl. It is crucial to study these swarms because they might be precursors to a new eruption in this part of the Michoacán-Guanajuato volcanic field. This monogenetic field has been very active, producing several dozen eruptions during the Holocene, the last two Jorullo (1759–1774) and Paricutin (1943–1952). Furthermore, the Tancítaro area displays one of the highest densities of Holocene volcanoes within the entire field, making it a probable candidate location for the birth of a future monogenetic volcano. For these reasons, a permanent seismic network should be installed as soon as possible.

Conifer establishment after the eruption of the Paricutin volcano in central Mexico

Volcanic eruptions have large effects on forest ecosystems and create new substrates, triggering primary succession processes. The Paricutín volcano, born in central-western Mexico, erupted between 1943 and 1952. After the cessation of the eruptive activity, plant colonization began on the lava flows and tephra deposits, including the conifer species that dominate the surrounding mature forests. This study aims to reconstruct the history of the establishment of conifer trees on the substrates created by the Paricutín eruption. 16 sampling plots were established along three transects with northern, southern, and south-western aspects, located every 250 ​m from the preserved forest to the volcanic cone. Increment cores from 400 conifer trees were extracted and their age was determined by cross-dating annual tree rings. The order of the species colonization and the tree establishment, abundance, and dominance patterns were characterized. Also, the influence of the distance from the mature forests and the inter-annual climatic conditions on the temporal tree establishment pattern was evaluated. Eight pine and one fir species have been established since 1970, only 18 years after the end of the eruptive period. However, tree establishment increased by 12.9% annually after 1995, with the youngest tree in our sample getting established in 2015. We did not find a well-defined temporal and spatial pattern of species arrival, which suggests that colonization occurred randomly, although the four pine species that were established early became the most abundant and dominant. Tree establishment was not influenced by the distance from the mature forest, and wet inter-annual conditions did not enhance pulses of tree recruitment, exhibiting a continuous tree establishment pattern. Conifer species have shown a great capacity for colonizing volcanic substrates created by the Paricutín eruption, which suggests that tropical montane conifers can regenerate rapidly under high-magnitude disturbances. These findings support the use of these forest species for ecological restoration.

Reconstruction of gully erosion based on exposed tree roots in a recent landform of Paricutin Volcano, Mexico

AbstractGullies in volcanic deposits are among the first signs of erosion that can be observed during or immediately after the volcanic activity and reflective of unstable conditions of newly formed deposits. Gully formation and erosion pulses may well continue to occur long after the initial landform was created, mostly as a result of torrential rainfall and/or land‐use changes. Yet, proper understanding of erosion dynamics and its relations with environmental drivers remains challenging as long‐term observations are, especially in volcanic terrain. Here we applied dendrogeomorphic approaches to (i) analyse both microscopic and macroscopic disturbances in tree roots exposed by gully erosion in a young volcanic landform of central Mexico and to (ii) quantify the evolution of erosion in the volcanic deposits of Paricutin Volcano. To this end we analysed 154 cross‐sections of 30 living, exposed roots of Pinus pseudoestrobus Lindl., of which 25 were used for anatomical and 129 for macroscopic analysis. Wood anatomical – or microscopic – disturbances were mostly in the form of strong growth reductions (~50%) in lumen area of earlywood tracheids, substantial increases in ring widths (mostly stemming from eccentric growth) and the increased formation of latewood cells. We observe major erosion pulses in 2010, 2012, 2015, 2016, 2017 and 2018, triggered primarily by torrential rainfall (with >100 mm 24 h−1 for the maximum intensity event: standard precipitation index values ≥ 2). Along the gully, the lower sector formed between 2009 and 2017, whereas the central sector exhibited important phases of exposure from 2012 to 2018. In the upper, most recent sector of the gully, erosion started only in 2017. Erosion rates in the lower and upper sectors are smaller with a channel widening rate of 6 to 16 cm yr−1. Around the main knickpoint found in the central part of the gully, retreat rates are up to 60 cm yr−1 and gully incision is 30 cm yr−1. The methods applied in this research are novel and can be applied to other volcanic environments with intense gully erosion, so as to provide baseline data for land‐use and environmental management plans.

The historical case of Paricutin volcano (Michoacán, México): challenges of simulating lava flows on a gentle slope during a long-lasting eruption

Simulating lava flows on a gentle slope is complex since they can propagate in a wide range of directions. It is an even greater challenge to define lava flow trajectories when an eruption lasts over several years and flows cool down, changing the surrounding topography. In this study, we test Q-LavHA, an open source plug-in that simulates lava flow inundation calculating its probability, and using Paricutin’s eruption (1943–1952) in central Mexico as a case study. We have appropriately calibrated the Q-LavHA plug-in for the Paricutin case study, which provides insights on how to better model lava flows in gentle terrain. From this work, we observe that each phase is characterized by a unique set of parameters requiring a careful calibration and that low-relief topographies require special consideration. Our findings could be useful for real-time hazard evaluation in future volcanic scenarios in the Michoacán–Guanajuato volcanic field and elsewhere, where new monogenetic eruptions similar to Paricutin can be expected.

An integrated magnetic survey on lava flows associated to the Paricutin volcano (Western Mexico)

We report a detailed aeromagnetic, field magnetometry and paleomagnetic survey from the most recent volcano on the American continent that emerged in February of 1943. Paleomagnetic measurements were performed to determine the characteristic remanent magnetization directions, considering that the Geomagnetic Field observed in nearby existing observatories at the time of extrusions is well known for the time of eruption of this volcano. The paleomagnetic records of the Paricutin flows showed lower values compared to the Teoloyucan Geomagnetic Observatory and those estimated with the International Geomagnetic Reference Field model (IGRF). Also, an aeromagnetic data modeling provided by the Mexican Geological Service together with field magnetometry along two profiles in perpendicular directions were carried out. The main objective was to model two levels of the internal structure, one that has to do with deep sources (not exclusively associated with the volcano itself), and another one of a more superficial area directly related to recent eruptions and the immediate geological basement.

Crystals reveal magma convection and melt transport in dyke-fed eruptions

The processes and ranges of intensive variables that control magma transport and dyke propagation through the crust are poorly understood. Here we show that textural and compositional data of olivine crystals (Mg/Fe, Ni and P) from the tephra of the first months of Paricutin volcano monogenetic eruption (Mexico, 1943–1952) record fast growth and large temperature and oxygen fugacity gradients. We interpret that these gradients are due to convective magma transport in a propagating dyke to the Earth’s surface in less than a few days. The shortest time we have obtained is 0.1 day, and more than 50% of the calculated timescales are < 2 days for the earliest erupted tephra, which implies magma ascent rates of about 0.1 and 1 m s−1. The olivine zoning patterns change with the eruptive stratigraphy, and record a transition towards a more steady magma flow before the transition from explosive to effusive dynamics. Our results can inform numerical and experimental analogue models of dyke propagation, and thus facilitate a better understanding of the seismicity and other precursors of dyke-fed eruptions.

The ~ AD 500–700 (Late Classic) El Astillero and El Pedregal volcanoes (Michoacán, Mexico): a new monogenetic cluster in the making?

The recent identification of Holocene volcanic clusters in small areas within the Michoacan Guanajuato Volcanic Field (MGVF) opens several questions regarding future volcanic hazard assessments in this region. Documenting vent alignments and eruption recurrence intervals within clusters will provide parameters necessary for making temporal and spatial hazard evaluations. Here, we present a possible new case of a small cluster consisting of only two monogenetic volcanoes, El Astillero and El Pedregal located in the ~ 4400-km2 Tancitaro-Nueva Italia region in the southwestern part of the MGVF, only 25 km to the south of Paricutin volcano. We determined from paleomagnetic and radiocarbon dating that El Astillero and El Pedregal most likely erupted one after the other between AD 500 and 700 (within the Late Classic period of Mesoamerican archeology). While the eruptions were likely separated by a short period of time, the exact length is difficult to ascertain. After the ~ 6 years of total estimated eruption duration of the two volcanoes, both together occupied an area of 14.7 km2 and emitted a dense rock equivalent (DRE) volume of magma of ~ 0.5 km3. Notable characteristics of the eruptions include a switch from the explosive activity exclusive of El Astillero (Strombolian) to effusive activity early after the initiation of the El Astillero eruption, a shift in the active vents, and a progressive change in the bulk magma composition from basaltic andesite to andesite throughout the duration of the eruption. This activity first formed the El Astillero scoria cone and tephra deposits followed by its lava field and ended with the emplacement of the El Pedregal viscous lavas. The discovery of pre-Hispanic pottery sherds and obsidian artifacts underneath the El Astillero tephra fallout unambiguously attests to human activities in the area before the eruption. Judging by their eruptive style, the eruptions probably had a limited impact on the small area affected and the surrounding human activities, but the hazard for this area remains since El Astillero and El Pedregal could represent the initial stages of a new cluster that is still in the making. If so, another eruption should be expected in this area again.

A re-interpretation of the petrogenesis of Paricutin volcano: Distinguishing crustal contamination from mantle heterogeneity

Paricutin volcano is the youngest and most studied monogenetic cinder cone in the Michoacán-Guanajuato volcanic field (Mexico), with an excellent historical record of its nine years (February 1943 to March 1952) of eruptive activity. The eruption produced lavas and tephras that range in composition from basalt to andesite. We have conducted new major and trace element and isotopic studies (whole rock Sr-Nd-Pb-Os) of the Paricutin lavas and tephras spanning the entire duration of the eruption, together with xenoliths found in early erupted lavas and bombs, and crustal samples representative of the Paricutin basement. This work contributes to our understanding of the potential roles of mantle source heterogeneity, subduction-related metasomatism, and crustal assimilation in the petrogenesis of arc magmas; moreover this study underscores the complexity of magma generation and evolution in monogenetic volcanoes. Although Paricutin has been traditionally considered as the classical example of magma evolution by fractional crystallization and crustal contamination, our multi-isotopic study has revealed that Paricutin compositional variations are inconsistent with significant crustal assimilation. Alternatively, we suggest that Paricutin's geochemical evolution can be explained by a combination of variable degrees of fractional crystallization of magmas produced by melting heterogeneous mantle beneath the TMVB that has been metasomatized by subduction components including sediment- and oceanic crust-derived fluids.

Comparative Analysis of Geomorphosites in Volcanoes of Costa Rica, Mexico, and Spain

This study is a comparative analysis of geomorphosites in Poas volcano in Costa Rica, Paricutin volcano in Mexico, and Teide-Pico Viejo volcanoes in Spain. The main objective of the paper is to compare the geomorphosites of these volcanoes using a geomorphosites evaluation to define which volcano is better for touristic activity. In this study, the geomorphosites evaluation methodology of Serrano and Gonzalez (Geomorphologie: Relief, Processus, Environment 3:197–208, 2005) was used. The study shows that these volcanoes have many different geomorphosites with geological and cultural importance for the population and that these characteristics make touristic development possible. One of the conclusions of this paper is that these volcanoes are representatives of the nations of Costa Rica, Mexico, and Spain, with interesting cultural and geological characteristics and unique landscapes. The methodology used gives another perspective to study the topographic setting and presents ideas to make new touristic sites available and attractive in other locations.

Compositional and volumetric development of a monogenetic lava flow field: The historical case of Paricutin (Michoacán, Mexico)

Paricutin volcano is the youngest and most studied monogenetic volcano in the Michoacán-Guanajuato volcanic field (Mexico), with an excellent historical record of its nine years (February 1943 to March 1952) of eruptive activity. This eruption offered a unique opportunity to observe the birth of a new volcano and document its entire eruption. Geologists surveyed all of the eruptive phases in progress, providing maps depicting the volcano's sequential growth. We have combined all of those previous results and present a new methodological approach, which utilizes state of the art GIS mapping tools to outline and identify the 23 different eruptive phases as originally defined by Luhr and Simkin (1993). Using these detailed lava flow distribution maps, the volume of each of the flows was estimated with the aid of pre- and post-eruption digital elevation models. Our procedure yielded a total lava flow volume ranging between 1.59 and 1.68km3 DRE, which is larger than previous estimates based on simpler methods. In addition, compositional data allowed us to estimate magma effusion rates and to determine variations in the relative proportions of the different magma compositions issued during the eruption. These results represent the first comprehensive documentation of the combined chemical, temporal, and volumetric evolution of the Paricutin lava field and provide key constraints for petrological interpretations of the nature of the magmatic plumbing system that fed the eruption.

Magma intrusion near Volcan Tancítaro: Evidence from seismic analysis

Between May and June 2006, an earthquake swarm occurred near Volcan Tancítaro in Mexico, which was recorded by a temporary seismic deployment known as the MARS network. We located ∼1000 events from this seismic swarm. Previous earthquake swarms in the area were reported in the years 1997, 1999 and 2000. We relocate and analyze the evolution and properties of the 2006 earthquake swarm, employing a waveform cross-correlation-based phase repicking technique. Hypocenters from 911 events were located and divided into eighteen families having a correlation coefficient at or above 0.75. 90% of the earthquakes provide at least sixteen phase picks. We used the single-event location code Hypo71 and the P-wave velocity model used by the Jalisco Seismic and Accelerometer Network to improve hypocenters based on the correlation-adjusted phase arrival times. We relocated 121 earthquakes, which show clearly two clusters, between 9–10km and 3–4km depth respectively. The average location error estimates are <1km epicentrally, and <2km in depth, for the largest event in each cluster. Depths of seismicity migrate upward from 16 to 3.5km and exhibit a NE-SW trend. The swarm first migrated toward Paricutin Volcano but by mid-June began propagating back toward Volcán Tancítaro. In addition to its persistence, noteworthy aspects of this swarm include a quasi-exponential increase in the rate of activity within the first 15days; a b-value of 1.47; a jug-shaped hypocenter distribution; a shoaling rate of ∼5km/month within the deeper cluster, and a composite focal mechanism solution indicating largely reverse faulting. These features of the swarm suggest a magmatic source elevating the crustal strain beneath Volcan Tancítaro.

RECONSTRUCCIÓN DE SEQUÍAS FUERTES EN EL PARQUE NACIONAL "PICO DE TANCÍTARO", MICHOACÁN

Pico de Tancitaro National Park is the most important source of water for the collection and recharge cycle of the aquifers located in western region of the state of Michoacan. Understanding the precipitation variability throughout time is crucial. Nonetheless, there is a lack of extensive and quality series on climatic data. For this reason, this work established the following objectives: 1) to reconstruct the precipitation variability for Pico de Tancitaro National Park based on the Abies religiosa growth rings, and 2) to analyze the drought frequency during the XXth century and the first decade of the XXIst century. Thus, some A. religiosa samples were taken and dated using standardized dendrochronological techniques. The quality of the dating as well as the quality of its climatic response were verified with the COFECHA software. These series presented a significant correlation (r=0.473; P < 0.01), which indicates their potential to generate dendroclimatic series. The precipitation of the last 127 years (1884-2010) was reconstructed. The severest droughts were reconstructed for the 1890-1896, 1948-1956, 1967-1971, 1982-1989, and 1998-2004 periods. It was also found that the Paricutin volcano eruption (1943) had a significant effect on regional climate.

An Investigation into the Nature of the Magmatic Plumbing System at Paricutin Volcano, Mexico

compositional change from basaltic andesite to andesite magmas in the waning stages of the eruption that is consistent with progressive crustal assimilation within this latest magma batch. These data demonstrate that the petrogenetic evolution of magmas at Paricutin is more complex than simple progressive assimilation and fractional crystallization and requires the presence of three compositionally distinct magma batches at shallow levels.

Dike emplacement near Parícutin volcano, Mexico in 2006

A major seismic swarm occurred near Paricutin volcano between the end of May and early July 2006. More than 700 earthquakes with magnitude (M L ) exceeding 2.4 were located. Paricutin, located in the Michoacan–Guanajuato volcanic field in western Mexico, is well known as the site of the 1943 eruption in which a new 400 m cinder cone was constructed in what had been farmland. The 2006 swarm exhibits all of the characteristics typically associated with swarms of volcanic origins. The earthquake rate showed the typical ramp up and ramp down over the course of several days. Magnitudes were evenly distributed in time with a notably high b-value of 2.45. The earthquake locations cluster around a northeast-striking trend extending approximately 6 km. Over the first two weeks, hypocenters migrated steadily a few hundred meters per day, rising from 9 to 5 km depth and moving northeast about 5 km. On approximately June 7, the ascent of hypocenters stalled. For the next three weeks, hypocenters held their depth while migrating laterally back to the southwest. Focal mechanisms during the first part of the swarm reflected the increased stress caused by dike inflation. Following June 7, the stress orientation changed and became more consistent with the inflation of horizontal sill-like structures. Though only limited information is available from the seismic swarm preceding the 1943 eruption, several features, including the swarm duration and magnitude relationships, were comparable to those of the 2006 episode. The strong indicators of a magmatic origin to the 2006 swarm suggest that at this location there are few, if any, traditional seismic discriminants that could be used to distinguish which seismic swarms and dike emplacement events might culminate in eruption.

Effects of Organic Matter Added to Sand Deposits of Volcanic Origin on Recruitment of Seedlings

Vegetational development in areas covered by volcanic ash from the Paricutin Volcano shows striking differences among types of plant cover at the time of the eruption. Forests affected by deposition of ash recovered soon after the eruption (1943–1952), but areas lacking vegetation at the time of the eruption today lack a closed canopy. We tested the effect of adding organic matter, straw, and a combination of organic matter and straw on recruitment of seedlings in a sand deposit formed in agricultural fields covered by ash from the Paricutin Volcano in Michoacán, Mexico. Number of seedlings varied significantly among treatments. At the end of the rainy season, plots with organic matter showed the highest number of seedlings (154 ± 20 seedlings/m2) followed by plots with organic matter and straw (82 ± 7 seedlings/m2), plots with straw (68 ± 14 seedlings/m2), and control plots (21 ± 13 seedlings/m2). In contrast, plots with organic matter and straw accumulated significantly more biomass (61.7 ± 10.8 g/m2) than organic-matter plots (14.5 ± 3.1 g/m2) or mulched plots (9.9 ± 3.1 g/m2). Clustering of experimental plots indicated that species composition of seedlings differed between plots with organic matter and straw, and plots with straw or organic matter. Our results suggest that the nature of organic matter that accumulates in soil can have a significant effect on composition of the seedling community, and support Eggler's hypothesis that lack of organic matter prevents development of vegetation in sandy substrates of volcanic origin.

Response of Pinus pseudostrobus Lindl. to fertile growing medium and tephra-layer depth under greenhouse conditions

Major disturbances that remove vegetation cover create conditions that differ greatly from those in adjacent forests. In volcanic areas, tephra deposition can be a major disturbance that eliminates vegetation cover and creates a barrier for plant establishment. The eruption of the Paricutin volcano, in Michoacan, Mexico, from 1943 to 1952, created large tephra deposits adjacent to forest stands. Many of these deposits still lack native vegetation and understanding of the limiting factors for establishment of native tree species is necessary to restore these areas. Pinus pseudostrobus early growth in response to the amount of fertile growing medium on top of a variable-depth tephra layer was evaluated in a greenhouse experiment. Increasing amounts of fertile growing medium increased height of 28-week-old plants (no growing medium = 5.5 cm tall ±1.8 cm, 3 cm of growing medium = 21.2 ± 4.6 cm and 6 cm of growing medium 24.8 ± 4.4 cm). Other variables showed the same trend (diameter, number of fascicles, belowground biomass and aboveground biomass). Our results suggest that P. pseudostrobus establishment in tephra covered areas will depend on the accumulation of a fertile layer of more than 3 cm in thickness.

VEGETATION OF TEPHRA DEPOSITS 50 YEARS AFTER THE END OF THE ERUPTION OF THE PARICUTÍN VOLCANO, MEXICO

Abstract Volcanic eruptions offer unique opportunities for the study of successional processes. The Paricutin Volcano eruption that started in 1943 and lasted for 9 years created different conditions for primary and secondary succession. Bare areas at the time of the eruption (mainly agricultural fields) offer an opportunity to study the effect of major natural disturbances in sites previously affected by human activities. One representative area, the Mesa de Cutzato, was surveyed to determine vegetation species composition and structure 50 years after the volcanic eruption ceased. Our results showed that few species have established and that total plant cover remained low (< 10%) and is patchy. Four species dominated these patches: Eupatorium glabratum (23% relative cover), Senecio stoechadiformis (14%), Senecio salignus (12%) and Muhlenbergia minutissima (6%). These species are either early successional or disturbance-tolerant species. No late successional species, particularly tree species, were found....

Effect of tephra depth on vegetation development in areas affected by volcanism

Plant establishment on sites affected by major volcanic disturbances is limited by several factors, such as lack of suitable microsites for germination and establishment in sites affected by tephra from volcanic eruptions. Even after long periods of time, tephra deposited over un-vegetated areas (agricultural fields and other barren areas) lack closed vegetation cover and in many cases late successional species. To assess limiting factors for plant establishment, a field survey in a tephra deposit from the Paricutin volcano eruption (19°30′42.4′′ N, 102°12′03.0′′) and greenhouse experiments were carried out. The field survey determined the relationship between tephra depth and vegetation distribution. Greenhouse experiments determined the effect of tephra depth on establishment and growth of two dominant species in the tephra deposit surveyed, Eupatorium glabratum and Lupinus elegans. Our results suggest that size and spatial distribution of vegetation patches is related to tephra depth in the field (77% of the vegetation patches were on tephra 38.8 cm deep or less and only 2% on tephra of more than 46.8 cm). Under greenhouse conditions, Eupatorium glabratum and Lupinus elegans height sharply decreased as depth of the tephra layer increased. Lupinus elegans plants growing in tephra less than 30 cm deep had a mean weight of 10.56 g (±0.53 g) compared with 3.11 g (±0.46 g) for plants growing in tephra more than 30 cm deep. Our results suggest that tephra depth is a limiting factor for canopy development in barren areas affected by tephra deposition.

Palaeomagnetic, rock-magnetic and microscopy studies of historic lava flows from the Paricutin volcano, Mexico: implications for the deflection of palaeomagnetic directions

SUMMARY We report the results of a detailed rock-magnetic study of basaltic andesites for one of the best documented cases of the first occurrence of a new volcano, the 1943‐1952 eruption of Paricutin in central Mexico. 102 oriented standard palaeomagnetic cores corresponding to 12 sites were collected from seven different lava effusion episodes, which cover the interval between 1943 September‐December and 1946 March‐August. Thermomagnetic investigations reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution that probably occurred during the initial flow cooling. Unblocking temperature and coercivity, point to ‘small’ pseudo-single domain magnetic grains for these (titano)magnetites. Thermal and alternating field demagnetization indicates that the lavas are characterized by univectorial or two-component magnetizations. Site-mean declinations and inclinations range from 331.9 ◦ to 32.3 ◦ and from 17.6 ◦ to 51.3 ◦ , respectively. The overall mean direction for 12 sites is: Dec = 1.8 ◦ , Inc = 37.5 ◦ , k = 26 and α 95 = 8.7 ◦ . 23 samples were pre-selected for Thellier palaeointensity experiments because of their stable remanent magnetization and relatively low within site dispersion. Thellier palaeointensity experiments have yielded widely dispersed palaeointensity values from the historically observed geomagnetic intensity. Although, individual specimens yielded technically high quality palaeointensity results, the values obtained are significantly different from the expected value of approximately 45 µT. The palaeomagnetic record is dominantly characterized by shallower than expected sitemean inclinations as compared with data from the Teoloyucan Geomagnetic Observatory and estimates based on the International Geomagnetic Reference Field 1945 and 1950 models. Several alternative interpretations are examined, but no single factor resulting in abnormal shallow inclinations can be isolated. Direct field observations suggest that shallow inclinations and angular scatter can be associated with lava emplacement, in terms of internal viscous deformation and/or movement of parts of lava after cooling below the blocking temperatures during lava advance. Other factors analysed that could contribute to the shallow inclinations and angular scatter are high magnetic refraction effects and outcrop-scale to regional magnetic field anomalies.