V51A-01
How ‘Monogenetic' is the Auckland Volcanic Field?
The Auckland Field is the youngest basaltic intraplate volcanic field in New Zealand; it is located about 350-400 km behind the present day active convergent plate boundary. The field contains about 50 recognised late Pleistocene to Holocene eruptive centres generated by the rise and eruption of very small volume (mainly less than 0.35 km3) batches of magma. The field covers approximately 100 km2 of the Auckland urban area and has been termed monogenetic, implying that individual centres erupt single magma batches during brief eruptive periods. Detailed studies of individual centres reveal significant compositional diversity. The following trends are recognised: 1). Single trends from early evolved to later less evolved compositions representing deep near source fractionation of a single magma batch generated in the garnet peridotite stability field (e.g. Crater Hill about 29 ka, 0.1 km3), this is demonstrably monogenetic behaviour. 2). Multiple compositional trends in magmas from a single eruption event signifying the sequential rise and fractionation of magma batches generated from different sources (3-8 percent melt of a garnet peridotite source at depths of about 80-50 km and 5-12 percent melt of spinel peridotite at depths about 50- 22km), for example Pupuke (about 250 ka, 0.1 km3) this is polygenetic behaviour. 3). Multiple compositional trends in temporarily discrete eruption events from the same centre (Rangitoto, 8 to 700 a, 2.3 km3) this is also polygenetic behaviour. The chemical diversity observed within these three volcanic centres, representing the life span of the Auckland Volcanic Field, questions how well we actually understand this very common type of global volcanism. The range of compositions observed in individual centres of the Auckland Volcanic Field reflects the interplay of melting and fractionation processes at different depths in the mantle and calls into question the use of the term monogenetic to describe them.
V51A-02 INVITED
Gas segregation and two-phase flow in basaltic explosive activity
Basaltic explosive activity is highly variable in intensity, ranging from less energetic fire fountaining and intermittent strombolian explosions, to more energetic ash-forming violent strombolian, subplinian and plinian activity. Moreover, unlike silicic volcanism, there is no direct relationship between explosivity and magma flux, due to the complex interplay between gas segregation and initial gas content of the magma, ascent rate, and gas segregation. Highly explosive activity is particularly common in mafic arc volcanoes, where magmas contain abundant water and higher gas fluxes are expected. Gas segregation and two-phase flow processes play a fundamental role in the explosive dynamics of basaltic magma. Passive degassing and bubble bursts are common in lava lakes or lava-filled vents, that is, in nearly static lava ponds. This style of activity indicates the rise of discrete bubbles through the low viscosity liquid. With an increase in the magma supply rate and initial water content, activity changes to that of contemporaneous lava emission and explosive activity, as is typical in many cinder cone eruptions. This paired activity illustrates preferential segregation of gas into the vertical conduit with respect to a lateral dyke system; the result is eruptive activity that is referred to as either transitional or violent strombolian. When magma rise rate exceeds values of the order of 104-5 kg/s, gas segregation is no longer possible and eruptive activity takes the form of sustained columns (subplinian to plinian activity). This summary illustrates the role of liquid and gas fluxes on the development of two-phase flow patterns in the conduit, which, in turn affects the eruption dynamics. For example, discrete explosions are generated when the pattern is periodic (characterized by regular temporal and spatial fluctuations), due to formation of gas slugs or void fraction waves, whereas strong fluctuations in the eruptive dynamics may be related to flow instability during transitions and in churn flow. Sustained flow, in turn, occurs when the flow regime has a regular geometry, such as in annular and homogenous bubbly flow. Using theoretical and experimental models, we explore the possible two-phase flow patterns (bubbly, slug, churn, annular and dispersed) that can develop in vertical conduits for different liquid and gas fluxes (Taitel et al., 1980; McQuillan and Whalley 1986; Lucas et al., 2005). We then discuss their expected stability in magmatic systems and their potential effects on the explosive eruption dynamics. Finally, we compare the theoretical results with natural examples.
V51A-03
Isopach map and Characteristics of the Ashfall Deposits From Parícutin Volcano, (México)
Parícutin volcano belongs to the Michoacán-Guanajuato volcanic field located in the central portion of the state of Michoacán, in western Mexico, approximately 50 km northwest of the town of Uruapan. Parícutin is the famous Mexican volcano born in a cornfield in 1943 erupting until 1952, and has been the object of many studies since its birth up to now. In 1946 Segerstrom & Kenneth (1950) initiated the first geological study of Parícutin Volcano. In their work, they published the only existing isopach map of the fall deposits. In the summer of 2005, fieldwork was carried at Parícutin volcano in which a new isopach map was generated. This new isopach map was build by direct measurement of total thicknesses of the in situ ashfall deposits (reworked material was not included) at trenches dug around the volcano. Also, sampling and description of Parícutin tephras, stratigraphy combined with modern processing and analytical tools (as compared with those existing by the time the first studies were done) were done. Forty-two trenches were excavated, and ground-penetrating-radar (GPR) profiles were also performed. In this work we present the differences resulting from the comparison between the Segerstrom & Kenneth (1950) isopach map and our map build half century later. This difference in time implies changes due to erosion and the availability of modern geophysical tools. We also present the results of detail granulometric analyses as well as a quantification of the volume of materials emitted during the eruption and volumes removed by erosion.
V51A-04
The 1943-1952 Eruption of Parícutin Volcano - the Pyroclastic Record
The 1943-1952 eruption of Parícutin volcano, Michoacan, is famous for the compositional evolution preserved in successive lava flows. The physical nature of the eruption is less well known, despite the remarkable observational record that was obtained during its nine years of activity. We have revisited both the tephra deposits and the observational accounts with the goal of reconstructing, from a modern perspective, the nature of the ‘cineritic' activity that serves as Walker's (1973) type example of a violent strombolian eruptive style. We have described sixteen tephra sections around the volcano. Only very proximal sections preserve the late (post-1946) andesitic tephra sequence and associated breadcrust bombs. The tephra sequence preserved in medial to distal sites varies more with direction than with distance, not surprising given the strong variations in prevailing wind directions in this region. (1) Southern sections preserve a complex basal lapilli sequence comprising thin alternating black (enriched in lithics and scoria) and tan (enriched in vesicular tephra) layers; analysis of the vesicular tephra shows it to have a more primitive composition than the earliest erupted lava. These basal layers are overlain by a coarse ash layer, consistent with observations of a persistent weak plume to the SW that accompanied early lava effusion and cone growth. The ash is overlain by a thick sequence of intercalated lapilli and ash. To the north, an ash bed forms the deposit base and is again overlain by alternating lapilli and ash; to the east, alternating beds of lapilli and ash mark the base of the section. We interpret this widespread lapilli and ash sequence to have been deposited during the ‘heavy cineritic phase' of activity that extended from mid-March through the summer of 1943. Lapilli are homogeneous basaltic andesite with the same bulk composition as lava erupted during this time period. Unique to eastern sections is a pronounced tan vesicular tephra in the middle of the sequence that may represent deposits associated with a change in activity in late1943 to continuous lava emission and strombolian explosions from a flank vent on the NE side (the Sapichu vent). The correspondence of the preserved tephra sequence with both contemporaneous accounts and the chemical stratigraphy provided by analysis of lava flows allows us to make some generalizations about the violent strombolian eruptive style. All tephra deposits, proximal to distal, are characterized by alternating layers of ash and lapilli, consistent with accounts of eruption styles varying from explosions (‘tubular' eruptions) to more prolonged ash-laden ‘blasts' to ‘gaseous' eruptions of white vapors (Luhr and Simkin 1993). Moreover, the protracted nature of the eruptive activity means that individual marker beds can be traced only along depositional axes, such that traditional methods of whole-deposit characterization are not applicable. Finally, from the hazards perspective, it is important to note that although both the average magma supply rate and tephra emissions decreased with time, the strength of individual explosions increased with the onset of Vulcanian activity during the last several months.
V51A-05 INVITED
Not so Simple Monogenetic Volcanism at Volcan El Jorullo
Volcan El Jorullo (1759 to 1774) was produced in one of two historical eruptions within the Michoacan- Guanajuato Volcanic field (MGVF) in the western Trans Mexican Volcanic Belt. Although direct observations of the eruption were sparse compared to the nearby 20th century Paricutin eruption, detailed geological, petrological and geochemical studies begun by Luhr and Carmichael (CMP 90, 1985) and continued by ourselves and others have uncovered a complex (and non-linear) sequence of melting, ascent, differentiation, assimilation and eruption. Collectively these betray a much larger spatial and temporal variability in magma storage and migration conditions than one might anticipate for a one-time volcanic event in a volcanic field fed sufficiently with magma to have produced nearly 1000 similar volcanic cones in the Holocene and Pleistocene. This presentation will summarize primarily ongoing geochemical work (mostly in the past decade) since the seminal Luhr and Carmichael study. We show that melts supplied to Jorullo were a mixture of those generated from an arc-fluid fluxed mantle and the lower crust and that magmas carry a signature of two distinct assimilation events (one deep, one probably shallow). Further, conditions of magma accumulation, differentiation, storage and transport were sufficiently heterogeneous to produce spatial and temporal variations in lava compositions erupted in at least 8 distinct effusive events that collectively formed the compound Jorullo flow field. We suggest that there are direct feedbacks between what, where and how various magma compositions were erupted at Jorullo and contrast these conditions to those at Paricutin, at which an apparently simpler sequence of events prevailed (e.g., McBirney et al CMP 95, 1987)
V51A-06
Use of GPR to Determine Thickness and Volcanic Stratigraphy: Case Study Parícutin Volcano (Mexico)
Parícutin is a monogenetic volcano born on February 20, 1943 in the middle of a cornfield in Michoacán, Mexico. During and after the end of its activity, several studies were carried out about the volcano and the surrounding area. However, additional detailed studies on its tephra deposits have not been performed since 1946 (Segerstrom, 1950). Recent studies have been carried out re-visiting this volcano and its deposits. Michoacán is a region where erosive processes are intense thus, the volume and distribution of tephra was expected to have changed accordingly. In parallel with direct studies observing the tephra sequences, Ground Penetrating Radar (GPR) surveys using 40, 70 and 200 MHz antennas were performed in close proximity to several selected stratigraphic sections observed at trenches. This allowed a precise calibration of propagation velocities and other acquisition parameters that were later used for surveying a wider area. This allowed us to map and laterally correlate the extent and thickness of several tephra units using a much faster GPR based method. We present several examples where we can confirm the dramatically tephra-thickness changes since Segerstrom's study sixty year ago. We also show that the use of GPR techniques can be very useful to improve the detail and resolution for tephra thickness mapping while at the same time making much more efficient field work efforts.
V51A-07 INVITED
Assessing Magmatic Processes and Hazards at two Basaltic Monogenetic Centers: Volcan Jorullo, Mexico, and Blue Lake Maar, Oregon
Although monogenetic basaltic volcanoes exhibit a wide variety of eruption styles, the origin of this diversity is poorly understood and often ignored when assessing volcanic hazards. To better understand magmatic processes and hazards associated with these eruptions, we have studied two monogenetic centers with differing behavior: Volcan Jorullo, a cinder cone in Mexico, and Blue Lake, a maar in the Oregon High Cascades. Although compositionally similar (medium-K basalt to basaltic andesite), their eruptive styles and products are quite different. Jorullo had violent strombolian eruptions that deposited alternating beds of ash and tephra, as well as lava flows. In contrast, Blue Lake exhibited initial phreatomagmatism that formed a 100m deep crater and produced surge deposits. This activity was followed by magmatic eruptions that produced deposits of tephra and bombs, but no lava flows. The diversity in eruptive style at these two centers reflects different magma ascent and crystallization processes, deduced using olivine-hosted melt inclusions. Jorullo melt inclusions trap variably degassed melts (0.5-5 wt% H2O; 0-1000 ppm CO2), with associated crystallization pressures that decrease from early (<4 kbars) to late (<100 bars) in the eruption. These data support the formation of a shallow storage region beneath the volcano that facilitated both crystallization and magma degassing, which is consistent with effusion of degassed lavas from the base of the cone throughout the eruption. In contrast, Blue Lake inclusions trap melts with a restricted range of volatiles (2.6-4 wt% H2O; 677-870 ppm CO2) corresponding to crystallization pressures of 2.2-3.2 kbars. This suggests that the magma feeding Blue Lake stalled in the upper crust and crystallized before ascending rapidly to the surface, without further crystallization of olivine or shallow storage. This is consistent with both the observed unstratified tephra deposits (indicating single rather than pulsatory eruptions) and the absence of lava flows. Our data suggest that in spite of similar compositions and volatile contents, these two volcanoes produced distinctive eruption styles. Although external water clearly played an important role in the eruption at Blue Lake, both volcanoes had explosive, magmatic volatile-driven eruptions. These eruptions clearly show that monogenetic centers are capable of a wide variety of eruptive styles and hazards, which may depend in large part on processes of magma ascent, degassing, and crystallization.
V51A-08
Revisiting Jorullo volcano (Mexico): monogenetic or polygenetic volcano?
Jorullo volcano is located near the volcanic front of the westernmost part of the Trans-Mexican Volcanic Belt, which is related to the subduction of the Cocos plate beneath the North American plate. This part of the TMVB is known as the Michoacán-Guanajuato Volcanic Field, a region where widespread monogenetic volcanism is present although polygenetic volcanism is also recognized (i. e. Tancítaro volcano; Ownby et al., 2006). Jorullo volcano was born in the middle of crop fields. During its birth several lava flows were emitted and several cones were constructed. The main cone is the Jorullo proper, but there is a smaller cone on the north (Volcán del Norte), and three smaller cones aligned N-S on the south (Unnamed cone, UC; Volcán de Enmedio, VE; and Volcán del Sur, VS). The cone of Jorullo volcano is made up of tephra and lava flows erupted from the crater. The three southern cones show very interesting histories not described previously. VE erupted highly vesiculated tephras including xenoliths from the granitic basement. VS is made of spatter and bombs. A very well preserved hummocky morphology reveals that VE and VS collapsed towards the west. After the collapses, phreatomagmatic activity took place at the UC blanketing VE, VS and the southern flank of the Jorullo cone with sticky surge deposits. The excellent study by Luhr and Carmichael (1985) indicates that during the course of the eruption, lavas evolved from primitive basalt to basaltic andesite, although explosive products show a reverse evolution pattern (Johnson et al., 2006). We mapped lava flows not described by the observers in the 18th century nor considered in previous geologic reports as part of the Jorullo lavas. These lavas are older, distributed to the west and south, and some of them resemble the lava flows from La Pilita volcano, a cone older than Jorullo (Luhr and Carmichael, 1985). These lava flows were not considered before because they were not extruded during the 1759-1774 eruption. Therefore, in spite of the long-standing idea of Jorullo being a monogenetic volcano, we hypothesize it as a stratovolcano in the making. The polygenetic nature of the volcano and the processes described here for Jorullo volcano (cone collapse, phreatomagmatic activity) are of great importance because of their implications for hazards assessment.
V51A-09 INVITED
Characteristics and Similarities of Lava Suites from Jorullo and La Pilita Volcanoes, Volcanic Front of the Westernmost Trans-Mexican Volcanic Belt
The monogenetic volcanic field of Michoacán-Guanajuato is at the westernmost part of the Trans-Mexican Volcanic Belt. This is the region where the influence of the Cocos plate starts under the subduction system beneath North America. Jorullo volcano had eruptive activity between 1759-1774 and La Pilita within the Holocene. The eruption of Jorullo and the products from La Pilita were first treated in the incredible work of Luhr and Carmichael (1985), however nothing else has been done on those lava flows since. We have visited the lava flows from Jorullo and La Pilita volcanoes in January 2007 and many new observations were made. Two flows were emitted from the north cone, the age of these lavas is debatable and apparently are older than Jorullo. Also a big older flow is running from the southern base of the main Jorullo cone. So far, this big flow had been ignored in the literature and we found that it is texturally very similar to the La Pilita flow. In this work we present geochemical and textural analyses of over 20 lava flows samples from Jorullo, and 3 lava samples from La Pilita. These data are then compared with samples from the large flow south of Jorullo to constrain his origin and chronology. The origin of this big flow maybe the proof that Jorullo is in fact a volcanic complex, and it was active before the 1959 eruption. This implies that Jorullo may not be a monogenetic volcano.
V51A-10
Mass Balance Calculation at Parícutin Volcano (Mexico): Erosion of Tephras 1946 - 2000
On February 20, 1943 Parícutin volcano was born in the Michoacán-Guanajuato Volcanic Field in western Mexico. The eruption lasted 9 years and during this period several studies were carried out, among them, tephra deposition and erosion studies (Segerstrom, 1946; Fries, 1946). Segesrtrom & Kenneth obtained an isopach map from which the total volume of tephra could be determined. Fries recalculated the volume of tephra based on planimetric measurements using the isopach map of Segerstrom (1946) who also did erosion studies at the same time when getting thickness measurements for the construction of the isopach map. In the 50's Segerstrom observed and recorded mass movements and re-deposition of ashes by water streams and compared the capacity of the ashes for infiltration with that of the previous soil. He obtained a total change in infiltration patterns and drainage. No further work was done ever since so, in this study we show preliminary results on the calculation of erosion rates of tephras as compared to the volume reported for 1946. We show results on mass loss and erosive processes, as well as cone degradation with time. The erosion activity has been favored by climatic factors that we discuss. Calculation of mass balances was possible by construction and subtracting of digital elevation models at scales: 1:10000, 1:20000, and 1:50000 for 1946, 1995 and 2000, respectively.