Will subglacial rhyolite eruptions be explosive or intrusive? Some insights from analytical models.

Simple analytical models of subglacial eruptions are presented, which simulate evolving subglacial cavities and volcanic edifices during rhyolitic eruptions beneath temperate glaciers. They show that the relative sizes of cavity and edifice may strongly influence the eruption mechanisms. Intrusive eruptions will occur if the edifice fills the cavity, with rising magma quenched within the edifice and slow melting of ice. Explosive magma-water interaction may occur if a water- or steam- filled gap develops above the edifice. Meltwater is assumed to drain away continuously, but any gap above the edifice will be filled by meltwater or steam. Ductile roof closure will occur if the glacier weight exceeds the cavity pressure and is modelled here using Nye�s law. The results show that the effusion rate is an important control on the eruption style, with explosive eruptions favoured by large effusion rates. The models are used to explain contrasting eruption mechanisms during various Quaternary subglacial rhyolite eruptions at Torfajökull, Iceland. Although the models are simplistic, they are first attempts to unravel the complex feedbacks between subglacial eruption mechanisms and glacier response that can lead to a variety of eruptive scenarios and associated hazards

Volcano-ice interactions at Prestahnukur, Iceland : rhyolite eruption during the last interglacial-glacial transition.

Prestahnukur is a 570m high rhyolite glaciovolcanic edifice in Iceland's Western Rift Zone with a volume of 0.6 km3. Uniform whole rock, mineral and glass compositions suggest that Prestahnukur was constructed during the eruption of one magma batch. Ar-Ar dating gives an age of 89+/-24 ka, which implies eruption during the transition (Oxygen Isotope substages 5d to 5a) between the Eemian interglacial and the Weichselian glacial period. Prestahnukur is unique among published accounts of rhyolite tuyas because a base of magmatically-fragmented tephra appears to be absent. Instead, basal exposures consist of glassy lava lobes and coarse hyaloclastite, above which are single and multiple lava sheets with matrix-supported basal breccias and hyaloclastite upper carapaces. Steepening ramp structures at sheet termini are interpreted as ice-contact features. Interactions between erupting magma and water/ice have affected all lithologies. A preliminary model for the construction of Prestahnukur involves an effusive subglacial eruption between 2 and 19 years duration which never became emergent, into an ice sheet over 700m thick. If 700m of ice had built up during this interglacial-glacial transition, this would corroborate models arguing for the swift accumulation of land-based ice in rapid response to global cooling

HARVESTING VALUE FROM ENTREPRENEURIAL SUCCESS

This article discusses ways for entrepreneurs to gain liquidity from their businesses, either with or without a sale of the business. In today's financial arena there is a wide variety of methods and financing vehicles that can enable private companies to harvest liquidity to meet their own needs for growth, the consumption requirements of their founders, or the challenges of tax and estate planning. For companies with limited growth opportunities but fairly stable cash flows, the alternatives range from orderly liquidation to highly leveraged transfers of ownership such as those accomplished by leveraged buyouts, ESOPs, and mezzanine finance. For companies with abundant growth opportunities, value is typically maximized through sale to a strategic buyer or an initial public offering of equity (although a new hybrid called the "private IPO" has recently emerged that looks more like an LBO than an IPO). 2000 Morgan Stanley.

Multi-model assessment of stratospheric ozone return dates and ozone recovery in CCMVal-2 models

International audienceProjections of stratospheric ozone from a suite of chemistry-climate models (CCMs) have been analyzed. In addition to a reference simulation where anthropogenic halogenated ozone depleting substances (ODSs) and greenhouse gases (GHGs) vary with time, sensitivity simulations with either ODSs or GHGs concentrations fixed at 1960 levels were performed to disaggregate the drivers of projected ozone changes. These simulations were also used to assess the two distinct milestones of ozone returning to historical values (ozone return dates) and ozone no longer being influenced by ODSs (full ozone recovery). These two milestones are different. The date of ozone returning to historical values does not indicate complete recovery from ODSs in most cases, because GHG induced changes accelerate or decelerate ozone changes in many regions. In the upper stratosphere where GHG induced stratospheric cooling increases ozone, full ozone recovery has not likely occurred by 2100 while ozone returns to its 1980 or even 1960 levels well before (~2025 and 2040, respectively). In contrast, in the tropical lower stratosphere ozone decreases continuously from 1960 to 2100 due to projected increases in tropical upwelling, while by around 2040 it is already very likely that full recovery from the effects of ODSs has occurred, although ODS concentrations are still elevated by this date. In the lower midlatitude stratosphere the evolution differs from that in the tropics, and rather than a steady decrease of ozone, first a decrease of ozone is simulated between 1960 and 2000, which is then followed by a steady increase throughout the 21st century