Period Of Repose Research Articles

Insights into the explosive eruption history of the Campanian volcanoes prior to the Campanian Ignimbrite eruption

The Campanian Volcanic Zone (CVZ) comprises multiple active volcanoes and includes the highly productive Campi Flegrei and Ischia caldera systems. These caldera volcanoes have produced probably the largest eruptions in Europe in the past 200 ka, such as the Monte Epomeo Green Tuff (MEGT; Ischia) at ca. 56 ka and the Campanian Ignimbrite (CI; Campi Flegrei) at ca. 40 ka, which form widespread isochrons across the Mediterranean region. These closely-spaced volcanic centres erupt phonolitic to trachytic glass compositions that are similar, and thus it can be challenging to correlate tephra deposits to specific volcanic sources. Here we present a detailed tephrostratigraphy for pre-CI eruption activity using the units preserved within a sequence at the coastal Acquamorta outcrop, on the western side of the CI caldera rim. Both the MEGT and CI units are present in the section, and they bracket twelve eruption units that were logged and sampled. New major and trace element glass chemistry data have been acquired for these Acquamorta tephra deposits. Three eruption deposits from Ischia and nine from Campi Flegrei are identified, which helps constrain the tempo of volcanic activity of these centres between the large caldera-forming eruptions. The three Ischia tephra deposits between the MEGT and the CI are indistinguishable based on both major and trace element glass chemistry and cannot be correlated to a specific or known eruption in this interval, such as the Schiappone tephra. The compositional variations between the Campi Flegrei eruptions reveal temporal shifts in the composition of the tephra deposits that reflect changes in the magmatic system prior to the CI eruption. These deposits indicate that there were at least nine eruptions at Campi Flegrei within 16 ka of the enormous CI eruption, and suggest that there was no significant period of repose before the caldera generating eruption.

Minimum Detectable Mass and Volume Fluxes During Magmatic Recharge at High Prominence Volcanoes: An Application to Erciyes Dağ Volcano (Turkey)

Magma reservoir recharge is widely recognised as a precursor of eruptive activity. However, the causative relationships between reservoir rejuvenation and surface observables such as gravitational potential field changes and ground deformation are still poorly understood. At intermediate and silicic intra-plate volcanoes where crustal mechanical heterogeneity combined with high-prominence are expected to fundamentally affect the crustal stress and strain relationship, protracted period of repose and absence of monitoring data raise questions about the detectability of magma recharge. Here we report results from integrated geodetic forward modelling of ground displacements and gravity changes from reservoir recharge at Erciyes Dağ, a large prominence (∼2,800 m), yet poorly studied, stratovolcano of the Central Anatolian Volcanic Province in Turkey. The most recent eruption at ∼7000 BC, close proximity to the Kayseri Metropolitan Area and absence of dedicated volcano monitoring set a precedent to explore stealth magmatic processes at the volcano. Using finite element analysis we systematically explore the influence of subsurface mechanical heterogeneities and topography on surface deformation and gravity changes from magmatic recharge of Erciyes Dağ’s reservoir. We show that whilst crustal heterogeneity amplifies ground displacements and gravity variations, the volcano’s substantial prominence has the opposite effect. For generic reservoir pressure and density changes of 10 MPa and 10 kg m−3 predicted vertical displacements vary by a factor of 5 while residual gravity changes vary by a factor of 12 between models ignoring topography or mechanical heterogeneity and those that do not. We deduce reservoir volume and mass changes of order 10–3 km3 and 1010 kg, respectively, at the detectability limit of conventional surveying techniques at the volcano. Though dependent on model assumptions, all results indicate that magma recharge at Erciyes Dağ may go undetected at fluxes 1) sufficient to maintain an active reservoir containing eruptable magma and 2) similar to those reported for intermediate/silicic volcanoes with repose times of 100–1,000s of years (e.g., Parinacota) and persistently active mafic volcanoes such as Mt. Etna and Stromboli. Our findings may be utilised to inform integrated geodetic and gravimetric monitoring at Erciyes Dağ and other large prominence silicic volcanoes and could provide early insights into reservoir rejuvenation with implications for the development of disaster risk reduction initiatives.

Precursors to intra-eruptive reposes during the 1998–2019 lava dome-building eruption at Volcán de Colima, México

The 1998–2019 lava dome-building eruption at Volcan de Colima, Mexico, involved two eruptive cycles of activity during which andesitic lava domes were extruded. The first cycle of extrusion lasted from November 1998 to June 2011 and was followed by a period of intra-eruptive repose from June 2011 to January 2013. The second cycle lasted from January 2013 to March 2017. The second intra-eruptive repose began from March 2017, and continues through the time of writing (April 2019). The reposes were preceded by interruptions in the endogenous regime of growth of the lava domes by extrusion of exogenous shear lobes (before repose 1) and by collapse of the lava dome (before repose 2). We conclude that interruptions in endogenous lava dome growth may herald the onset of intra-eruptive repose periods at this volcano. The nature of such precursors is potentially associated with the process of batch supply from deep magma storage into the shallow system feeding the eruption. During the two cycles of eruption at Volcan de Colima, the equal volumes of products (about 75 × 106 m3) were erupted with a difference in the mean magma discharge rates (approximately 5 × 106 m3 and 18 × 106 m3 per year for cycles 1 and 2 respectively). It seems that a cycle was controlled by a volumetric limit of 75 × 106 m3; once this volume has been erupted, then the cycle ends irrespective of discharge rate. At the same time, a difference in the mean magma discharge rates may be a reason for differences in the type of interruptions.

Months between rejuvenation and volcanic eruption at Yellowstone caldera, Wyoming

Research Article| August 01, 2015 Months between rejuvenation and volcanic eruption at Yellowstone caldera, Wyoming Christy B. Till; Christy B. Till * 1School of Earth and Space Exploration, Arizona State University, 781 Terrace Mall, Tempe, Arizona 85287, USA2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA *E-mail: christy.till@asu.edu Search for other works by this author on: GSW Google Scholar Jorge A. Vazquez; Jorge A. Vazquez 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Jeremy W. Boyce Jeremy W. Boyce 3Department of Earth, Planetary and Space Sciences, University of California–Los Angeles, 595 Charles Young Drive East, Los Angeles, California 90095, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Christy B. Till * 1School of Earth and Space Exploration, Arizona State University, 781 Terrace Mall, Tempe, Arizona 85287, USA2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Jorge A. Vazquez 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Jeremy W. Boyce 3Department of Earth, Planetary and Space Sciences, University of California–Los Angeles, 595 Charles Young Drive East, Los Angeles, California 90095, USA *E-mail: christy.till@asu.edu Publisher: Geological Society of America Received: 07 Apr 2015 Revision Received: 26 May 2015 Accepted: 28 May 2015 First Online: 09 Mar 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (8): 695–698. https://doi.org/10.1130/G36862.1 Article history Received: 07 Apr 2015 Revision Received: 26 May 2015 Accepted: 28 May 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Christy B. Till, Jorge A. Vazquez, Jeremy W. Boyce; Months between rejuvenation and volcanic eruption at Yellowstone caldera, Wyoming. Geology 2015;; 43 (8): 695–698. doi: https://doi.org/10.1130/G36862.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Rejuvenation of previously intruded silicic magma is an important process leading to effusive rhyolite, which is the most common product of volcanism at calderas with protracted histories of eruption and unrest such as Yellowstone caldera (Wyoming), Long Valley caldera (California), and Valles caldera (New Mexico) in the United States. Although orders of magnitude smaller in volume than rare caldera-forming supereruptions, these relatively frequent effusions of rhyolite are comparable to the largest eruptions of the 20th century, and pose a considerable volcanic hazard. However, the physical pathway from rejuvenation to eruption of silicic magma is unclear, particularly because the time between reheating of a subvolcanic intrusion and eruption is poorly quantified. This study uses nanometer-scale trace element diffusion in sanidine crystals to reveal that rejuvenation of a near-solidus or subsolidus silicic intrusion occurred in ∼10 mo or less following a protracted period (220 k.y.) of volcanic repose, and resulted in effusion of ∼3 km3 of high-silica rhyolite lava at the onset of Yellowstone’s last volcanic interval. The future renewal of effusive silicic volcanism at Yellowstone will likely require a comparable energetic intrusion of magma that remelts the shallow subvolcanic reservoir and generates eruptible rhyolite on month to annual time scales. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Months between rejuvenation and volcanic eruption at Yellowstone caldera, Wyoming

Rejuvenation of previously intruded silicic magma is an important process leading to effusive rhyolite, which is the most common product of volcanism at calderas with protracted histories of eruption and unrest such as Yellowstone caldera (Wyoming), Long Valley caldera (California), and Valles caldera (New Mexico) in the United States. Although orders of magnitude smaller in volume than rare caldera-forming supereruptions, these relatively frequent effusions of rhyolite are comparable to the largest eruptions of the 20th century, and pose a considerable volcanic hazard. However, the physical pathway from rejuvenation to eruption of silicic magma is unclear, particularly because the time between reheating of a subvolcanic intrusion and eruption is poorly quantified. This study uses nanometer-scale trace element diffusion in sanidine crystals to reveal that rejuvenation of a near-solidus or subsolidus silicic intrusion occurred in ∼10 mo or less following a protracted period (220 k.y.) of volcanic repose, and resulted in effusion of ∼3 km3 of high-silica rhyolite lava at the onset of Yellowstone’s last volcanic interval. The future renewal of effusive silicic volcanism at Yellowstone will likely require a comparable energetic intrusion of magma that remelts the shallow subvolcanic reservoir and generates eruptible rhyolite on month to annual time scales.

Lying in wait: deep and shallow evolution of dacite beneath Volcán de Santa María, Guatemala

Abstract The Plinian eruption in October 1902 of 8.5 km 3 of dacitic pumice and minor basaltic andesite scoria and ash at Volcán de Santa María, Guatemala violently interrupted a 25 kyr period of repose that had followed ∼75 kyr of cone-growth via extrusion of 8 km 3 of basaltic andesite lava. Two-oxide and pyroxene thermometry reveal an oxidized (Ni-NiO+2 log units) and thermally-zoned magma body in which basaltic andesite with 54 wt% SiO 2 at 1020 °C and dacite with 65 wt% SiO 2 at 870 °C coexisted. Plagioclase in dacite pumice and basaltic andesite scoria shows remarkably similar zoning characterized by repeated excursions toward high anorthite and increases in Mg, Fe, and Sr associated with resorption surfaces along which dacitic to rhyolitic melt inclusions are trapped. The melt inclusions increase slightly in K 2 O as SiO 2 increases from 69 to 77 wt%, whereas H 2 O contents between 5.2 and 1.4 wt% drop with increasing K 2 O. These observations suggest that crystallization of the plagioclase, and evolution of a high-silica rhyolitic residual melt, occurred mainly in the conduit as the compositionally-zoned magma body decompressed and degassed from >180 MPa, or >5 km depth, toward the surface. The similarity of plagioclase composition, zoning, and melt inclusion compositions in pumice and scoria suggests that crystals which grew initially in the cooler dacite, were exchanged between dacitic and basaltic andesite magma as the two magmas mingled and partially mixed en route to the surface. Since 1922>1 km 3 of dacitic magma similar to the 1902 pumice has erupted effusively to form the Santiaguito dome complex in the 1902 eruption crater. Trace element and Sr–Nd–Pb–O and U–Th isotope data indicate that cone-forming basaltic andesite lavas record processes operating in the deep crust in which wallrock heating sufficient to induce partial melting and assimilation involved several pulses of recharging mantle-derived basalt over at least 50 kyr. A fundamental shift in process coincides with the termination of cone-building at 25 ka: the 1902 dacite reflects >40% fractional crystallization of plagioclase+amphibole+clinopyroxene+magnetite from ∼20 km 3 of basaltic andesite magma left-over following cone-building that cooled slowly without assimilating additional crust. Small contrasts in Sr–Nd–Pb ratios, a modest contrast in δ 18 O(WR), and a large difference in the ( 238 U/ 230 Th) activity ratio between the 1902 scoria and dacite indicate that these two magmas are not consanguineous, rather this basaltic andesite is likely a recent arrival in the system. A glass–whole rock–magnetite–amphibole 238 U– 230 Th isochron of 9.5±2.5 ka for a 1972 Santiaguito dacite lava suggests that deeper, occluded portions of the silicic magma body, not erupted in 1902, incubated in the crust for at least 10 kyr prior to the 1902 eruption. Basaltic andesite inclusions in the Santiaguito dacite lava domes are interpreted to be modified remnants of the cone-forming magma parental to the 1902 dacite. Supplementary material: Electron probe analyses of glass standards, and SIMS data from standards and melt inclusions for the hydrogen measurements are available at http://www.geolsoc.org.uk/SUP18606

Insights into the 2011-2012 submarine eruption off the coast of El Hierro (Canary Islands, Spain) from statistical analyses of earthquake activity

SUMMARY The purpose of this work is to gain insights into the 2011–2012 eruption of El Hierro (Canary Islands) by mapping the evolution of the seismic b-value. The El Hierro seismic sequence offers a rather unique opportunity to investigate the process of reawakening of an oceanic intraplate volcano after a long period of repose. The 2011–2012 eruption is a submarine volcanic event that took place about 2 km off of the southern coast of El Hierro. The eruption was accompanied by an intense seismic swarm and surface manifestations of activity. The earthquake catalogue during the period of unrest includes over 12 000 events, the largest with magnitude 4.6. The seismic sequence can be grouped into three distinct phases, which correspond to well-separated spatial clusters and distinct earthquake regimes. The estimated b-value is of 1.18 ± 0.03, and a magnitude of completeness of 1.3, for the entire catalogue. B is very close to 1.0, which indicates completeness of the earthquake catalogue with only minor departures from the linearity of Gutenberg–Richter frequency–magnitude distribution. The most straightforward interpretation of this result is that the seismic swarm reached its final stages, and no additional large magnitude events should be anticipated, similarly to what one would expect for non-volcanic earthquake sequences. The results, dividing the activity in different phases, illustrate remarkable differences in the estimate of b-value during the early and late stages of the eruption. The early pre-eruptive activity was characterized by a b-value of 2.25. In contrast, the b-value was 1.25 during the eruptive phase. Based on our analyses, and the results of other studies, we propose a scenario that may account for the observations reported in this work. We infer that the earthquakes that occurred in the first phase reflect magma migration from the upper mantle to crustal depths. The area where magma initially intruded into the crust, because of its transitional nature is characterized by high fracturing, thus favours anomalously high b-values. The larger magnitude earthquakes recorded in the second phase may reflect relaxation around the magma reservoir that had fed the eruption and, thus, lower b-values.

Tracking Open-system Differentiation during Growth of Santa María Volcano, Guatemala

Prior to the ad 1902 Plinian eruption of 8 km3 of dacite and subsequent growth of the >1 km3 Santiaguito dacite dome complex, Santa María volcano grew into an 8 km3 composite cone over ∼75 kyr in four phases (at 103–72, 72, 60–46, and 35–25 ka). The 1902 eruption occurred after an ∼25 kyr period of repose in growth of the composite cone. To provide context for processes that ultimately led to the 1902 eruption, we present geochemical and isotopic (Sr, Nd, Pb, U-series) data from lavas of the composite cone for which ages are constrained by 40Ar/39Ar dating. The four cone-building phases comprise basaltic to basaltic-andesite lava (51·4–56·1% SiO2) whose major- and trace-element compositions suggest that crystallization was important in differentiation. Relative to other Central American arc volcanoes, these lavas also have large 238U excesses and high 207Pb/204Pb ratios that imply melting of a mantle wedge modified to an unusual extent by fluid from subducted crust and sediment of the Cocos plate. Major- and trace-element and isotopic variations over time imply that mafic recharge and magma mixing were prevalent during early phases of cone-building, whereas assimilation processes were more dominant during the latest stage of cone growth. Indeed, some early erupted basalts have lower 143Nd/144Nd and higher 87Sr/86Sr ratios than more SiO2-rich basaltic andesites that erupted during the final phase of cone-building. These features point to an assimilant that is not typical continental crust and instead may be more like mid-ocean ridge basalt with respect to major- and trace-element composition and Sr, Nd, Pb, and U–Th isotope ratios. Energy-constrained modeling of a parental basalt that undergoes crystal fractionation, assimilation and periodic recharge with basalt in the lower crust can reproduce lava compositions erupted during phases I–III and the early part of phase IV. Modeling further indicates that assimilation within the lower crust of partially melted garnet-amphibolite metabasalt, without basaltic recharge, may produce the youngest cone-forming lavas in phase IV. These models link the 8 km3 of cone growth over 75 kyr to the mass flux of magma into the crust. Our findings suggest an along-arc magma flux into the lower crust beneath Santa María of >20 km3 km−1 Myr−1, which is higher than anticipated in recent numerical–thermal approaches to basalt–crust interaction. Consequently, the thermal incubation period needed to produce hybrid basaltic-andesite magma may be only a few tens of thousand years.

Relating magma composition to eruption variability at andesitic volcanoes: A case study from Mount Taranaki, New Zealand

Acquiring accurate eruption records and understanding the volcanic processes behind eruption periodicity are important in the development of realistic hazard assessments and volcanic emergency planning. Here, we use a detailed study of the Holocene ( 0.5 km 3 ) eruptions appear to be statistically predictable because they tend to occur just prior to a period of repose, and they erupt the most strongly evolved magmas. The fundamental properties of magma-volcano systems identified here offer a paradigm for constraining the time scales and nature of magmatic processes, in addition to providing a foundation for more robust probabilistic time-varying hazard forecasts.

Rule 10b-5 and the Statute of Limitations: Actual Knowledge Not Inquiry Notice. An Amicus Brief Filed at the US Supreme Court in Merck v. Reynolds

After more than a half century of judicial monopoly, Congress in 2002 adopted Section 1658(b), thereby creating a statute of limitations and repose for fraud actions brought under Section 10(b) and Rule 10b-5. The five year period of repose represents an outside cut-off that commences at the time of the violation. The two year limitations period begins upon “discovery of the facts constituting” the fraud. See 28 U.S.C. § 1658(b). Under the self-evident meaning of this language, the limitations period begins only when plaintiffs discover - that is, actually know - that they have been defrauded. The legislative history confirms this actual-knowledge standard. The limitations period was not meant to be used to investigate whether a fraud occurred, but to address the “obstacles” that exist “after the fraud is discovered,” including the need to unravel the complexities of the violation and address the logistical concerns in bringing a claim, particularly the need to marshal the facts sufficient to meet the heightened pleading standards set out in the Private Securities Litigation Reform Act of 1995, Pub. L. No. 104-67, 109 Stat. 737 (PSLRA). See S. Rep. No. 146, 107th Cong., 2d Sess. 9 (May 6, 2002). This Court in Lampf adopted an actual-knowledge, not an inquiry-notice, standard for determining the accrual date of the one year limitations period. This was apparent from this Court’s express reliance on Section 9(e) of the Exchange Act, 15 U.S.C. § 78i(e), with its actual knowledge standard, as the appropriate model, and the explicit rejection of Section 13 of the 1933 Act, with its inquiry notice standard. Lampf’s reliance on an actual knowledge standard was also apparent in this Court’s treatment of equitable tolling. Tolling had allowed courts to ensure fairness by using equity to delay the accrual date where plaintiffs were unaware of the fraud through no fault of their own. Lampf concluded, however, that equitable tolling would henceforth be “unnecessary,” an approach that could only be understood as a direct consequence of an accrual standard that depended upon plaintiffs’ actual knowledge of the fraud. With plaintiffs aware of having been defrauded, a tolling doctrine designed to provide additional time to uncover the violation had become superfluous or “unnecessary.” The actual-knowledge standard does not interfere with the goals of finality and the elimination of stale claims. Those concerns are addressed through the adoption of a five year statute of repose. As for the assertion that the actual knowledge standard somehow allows plaintiffs to delay the onset of the limitations period through inactivity, this concern is misplaced. First, the modern realities of class action securities fraud suits against public companies provide considerable practical incentive to investigate the mere suspicion of a violation. Any delay in investigation could result in the expiration of the period of repose or the failure to meet the heightened pleading standards contained in the PSLRA. The possibility of deliberate inactivity is also belied by the competitive reality of class action securities litigation. SOX sought to increase the role of institutional investors, those plaintiffs with the resources available to conduct investigations. In addition, law firms that delay investigating the possibility of fraud could find themselves at a competitive disadvantage. The first firm to become aware of the fraud and discern the applicable class will be in a position to identify possible lead plaintiffs, the investors with the largest financial interest in the relief sought, increasing the potential likelihood that it will be designated lead counsel. See 15 U.S.C. § 78u-4. Second, a legal standard that determines actual knowledge through reference to the entire class will essentially eliminate even the possibility of the strategic use of inactivity. In the context of class actions against public companies for securities fraud, actual knowledge is most appropriately considered through resort to the information known to the class as a whole. To the extent known to the class, plaintiffs will be unable to rely on their own unawareness to delay the onset of the limitations period. This also provides plaintiffs with an additional incentive to investigate even the possibility of fraud in order discover facts known to the market. This use of the inquiry notice standard on the one hand permits the elimination of potentially meritorious claims and on the other hand encourages the filing of meritless claims. Meritless claims do more than waste resources. They open plaintiffs and their counsel to the possibility of sanctions under Rule 11. See 15 U.S.C. § 78u-4(c)(making mandatory consideration of sanctions under Rule 11 upon final adjudication of a securities fraud action).

Study regarding the in vitro propagation in the Lilium orientale.

The modelling of the in vitro morphogenetic process with a view to the propagation to horticultural species of commercial interest is one of the main preoccupations of the present vegetal biotechnologies. The involvement of the external phytohormones in the management of the caulogenic and rhizogenic processes represents a recognized modality for inducing and performing the in vitro propagation at various plants species [2]. The in vitro formation of minibulbs by means pf biotechnological methods, at species of the Liliaceae family, offers the advantage to obtain flower bulbs after just four years of culture instead of six – seven years necessary by means of recognized horticultural methods [13]. Consequently, this study presents both the settlement of viable aseptic cultures and the influence of cytokinins BA and TDZ in the modelling of the in vitro caulogenesis starting from explants, of the stem disk fragment type or of the flaked leaf type, sampled from flower bulbs of Lilium orientale found in the period of vegetative repose (dormancy). This study also presents the influence of auxins NAA and IBA in managing the adventive rhizogenesis at the level of the minibulbs obtained in vitro. The Lilium orientale varieties studied were selected for their commercial value as cut flowers (Lilium orientale var. stargazer and Lilium orientale var. devotion), or as pot plants (Lilium orientale var. prestige and Lilium orientale var. boulevard) – [8].

Alaska's Pavlof Volcano Ends 11‐Year Repose

After an 11‐year period of repose, Pavlof volcano on the Alaska Peninsula (Figure 1) began an episode of Strombolian eruption lasting 31 days, from 14 August to 13 September 2007.The eruption began abruptly on 14 August after a minor increase in seismicity the previous day. Nearly continuous lava fountaining, explosions, and lahars caused by minor disruption of the ice and snow cover on the volcano characterized the eruption. The eruption also produced diffuse ash plumes that reached 5–6 kilometers above sea level, but the plumes were too small and did not extend high enough to affect local or regional air travel. Melting of snow and ice on the upper part of the edifice by hot debris avalanches and lava resulted in numerous lahars that entered the sea and inundated a 2×106 square meter area on the volcanos southern slope.

The Pro-collusive Effect of Increasing the Repose Period for Price Fixing Agreements

Intuitively, extending the period of repose for price fixing agreements enhances the effectiveness of competition policy enforcement. This paper proofs this intuition wrong. As extending the repose period reduces cartel members' defection payoff while it leaves unaltered expected compliance profits, it induces cartels to be more stable internally.

The Pro-collusive Effect of Increasing the Repose Period for Price Fixing Agreements

Abstract Intuitively, extending the period of repose for price fixing agreements enhances the effectiveness of competition policy enforcement. This paper proves this intuition wrong. As extending the repose period reduces cartel members' defection payoff while it leaves unaltered expected compliance profits, it induces cartels to be more stable internally.

The reawakening of Alaska's Augustine volcano

Augustine volcano, in south central Alaska, ended a 20‐year period of repose on 11 January 2006 with 13 explosive eruptions in 20 days. Explosive activity shifted to a quieter effusion of lava in early February, forming a new summit lava dome and two short, blocky lava flows by late March (Figure 1).The eruption was heralded by eight months of increasing seismicity, deformation, gas emission, and small phreatic eruptions, the latter consisting of explosions of steam and debris caused by heating and expansion of groundwater due to an underlying heat source.

Tectonic evolution of the central-eastern sector of Trans Mexican Volcanic Belt and its influence on the eruptive history of the Nevado de Toluca volcano (Mexico)

The Nevado de Toluca is an andesitic to dacitic stratovolcano of Late Pliocene–Holocene age located within the central and eastern sectors of the Trans Mexican Volcanic Belt. Morphostructural analysis, aerial photograph and satellite image interpretation, structural analysis and geological fieldwork were methods used to investigate the relationship between the evolution of the volcano and the tectonic framework of its basement. The study revealed that the area of Nevado de Toluca is affected by three main fault systems that intersect close to the volcanic edifice. These are from oldest to youngest, the Taxco–Querétaro, San Antonio and Tenango fault systems. The NNW–SSE Taxco–Querétaro fault system was active in the area since Early Miocene, and is characterized by right-lateral transtensive movement. Its reactivation during Early to Middle Pleistocene was responsible for the emplacement of andesitic to dacitic lava flows and domes of La Cieneguilla Supersynthem. The NE–SW San Antonio fault system was active during Late Pliocene, before the reactivation of the Taxco–Querétaro fault system, and is characterized by extensional left-lateral oblique-slip kinematics. The youngest is the E–W Tenango fault system that has been active since Late Pleistocene. This fault system is characterized by transtensive left-lateral strike-slip movement, and partly coeval with the youngest eruptive phase, the Nevado Supersynthem, which formed the present summit cone of the Nevado de Toluca volcano. The stress re-orientation from the Taxco–Querétaro to the Tenango fault system during Late Pleistocene is responsible for the ∼ 1 Ma hiatus in the magmatic activity between 1.15 Ma and 42 ka. After this period of repose, the eruptive style drastically changed from effusive to explosive with the emission of dacitic products. The methodology presented here furnish new data that can be used to better assess the complex structural evolution of this sector of the Trans Mexican Volcanic Belt and to understand the influence of changes in the tectonic setting on the growth of volcanic edifices.

Reproductive characteristics of free-livingCalomys venustus (Rodentia, Muridae)

Calomys venustus showed a cyclic seasonal pattern of reproductive activity characterized by a period of repose and a variable length of the period of sexual activity between August—September to May—June. The study was done from 1983 to 1999. During this time the population density increased from October—November to the end of autumn and sharply decreased in winter. Three peaks in rate of pregnancy were recognized in the breeding period: spring, summer and late summer. The average litter size was different among the three peaks in rate of pregnancy. The reproductive pattern had a peak at the beginning of the spring produced by overwintering animals, and another peak in summer-autumn caused by the reproductive activity of young born in the same period in which they breed. The spring cohort exhibited the greatest breeding contribution suggesting that this cohort is principally responsible for the yearly peak in abundance.

Chromatin structure and the state of human organism

The state of chromatin in human buccal epithelium cell nuclei upon the influence of sport trainings was investigated. Chromatin state was evaluated in interphase buccal cell nuclei after orcein staining. The heterochromatin granule quantity (HGQ) was estimated in 30 nuclei per sample, and for every donor the mean HGQ value per 30 cells was determined. Donors of masculine sex, aged from 18 to 48 years performed training walks and samples of buccal epithelium were collected. Sportive charges induced the process of chromatin condensation in cell nuclei. After the period of repose (24-48 h) the HGQ decreased to control level therefore the process of chromatin decondensation was observed. The state of chromatin changes in connection with circadian rhythm. Chromatin became more condensed at nighttime and less condensed in the morning. Hormones such as adrenaline, noradrenaline, and hydrocortisone in vitro induced the increase of HGQ.

Size-related reproductive biology of the infaunal bivalve Abra alba (Bivalvia) in Kinsale Harbour (south coast of Ireland)

The reproductive behaviour of 248 specimens comprising four size classes of the infaunal bivalve Abra alba (Wood) was investigated between September 1994 and November 1995 in Kinsale Harbour. Small males returned a very small proportion of sexually mature and spawning animals. By contrast, no spawning females were found in this size class, suggesting that males develop and mature at a smaller size than females. Mean gonad maturity indices indicated that, except for the smallest size class, A. alba exhibits two spawning events in 1 year, i.e. a lesser spring and a major autumn spawning event. The spring event was well synchronised between males and females, while the autumn spawning was more broadly synchronised between the sexes. Only one spawning event was recorded for very large males and females, also occurring in spring. Spawning stages were recorded for both sexes over all size classes between February and November, indicating only a short period of sexual repose, if any. Oocyte diameters did not show a discrete size range from developing to spawning stages. All stages were seen to contain varying proportions of smaller, less mature oocytes, suggesting that large A. alba females do not develop and spawn en bloc, but release their gametes periodically. Maturity indices showed only a broad similarity with off-bottom temperatures.

Evolution of a complex isolated dome system, Cerro Pizarro, central M�xico

Cerro Pizarro is an isolated rhyolitic dome in the intermontane Serdan-Oriental basin, located in the eastern Trans-Mexican Volcanic Belt. Cerro Pizarro erupted ~1.1 km3 of magma at about 220 ka. Activity of Cerro Pizarro started with vent-clearing explosions at some depth; the resultant deposits contain clasts of local basement rocks, including Cretaceous limestone, ~0.46-Ma welded tuff, and basaltic lava. Subsequent explosive eruptions during earliest dome growth produced an alternating sequence of surge and fallout layers from an inferred small dome. As the dome grew both vertically and laterally, it developed an external glassy carapace due to rapid chilling. Instability of the dome during emplacement caused the partial gravitational collapse of its flanks producing various block-and-ash-flow deposits. After a brief period of repose, re-injection of magma caused formation of a cryptodome with pronounced deformation of the vitrophyric dome and the underlying units to orientations as steep as near vertical. This stage began apparently as a gas-poor eruption and no explosive phases accompanied the emplacement of the cryptodome. Soon after emplacement of the cryptodome, however, the western flank of the edifice catastrophically collapsed, causing a debris avalanche. A hiatus in eruptive activity was marked by erosion of the cone and emplacement of ignimbrite derived from a caldera to the north of Cerro Pizarro. The final growth of the dome growth produced its present shape; this growth was accompanied by multiple eruptions producing surge and fallout deposits that mantle the topography around Cerro Pizarro. The evolution of the Cerro Pizarro dome holds aspects in common with classic dome models and with larger stratovolcano systems. We suggest that models that predict a simple evolution for domes fail to account for possibilities in evolutionary paths. Specifically, the formation of a cryptodome in the early stages of dome formation may be far more common than generally recognized. Likewise, sector collapse of a dome, although apparently rare, is a potential hazard that must be recognized and for which planning must be done.