Event trees are useful frameworks for discussing probabilities of possible outcomes of volcanic unrest. Each branch of the tree leads from a necessary prior event to a more specific outcome, e.g., from an eruption to a pyroclastic flow. Where volcanic processes are poorly understood, probability estimates might be purely empirical -utilizing observations of past and current activity and an assumption that the future will mimic the past or follow a present trend. If processes are better understood, probabilities might be estimated from a theoretical model, either subjectively or by numerical simulations. Use of Bayes' theorem aids in the estimation of how fresh unrest raises (or lowers) the probabilities of eruptions. Use of event trees during volcanic crises can help volcanologists to critically review their analysis of hazard, and help officials and individuals to compare volcanic risks with more familiar risks. Trees also emphasize the inherently probabilistic nature of volcano forecasts, with multiple possible outcomes.

Dome growth at the Soufriere Hills volcano (1996 to 1998) was frequently accompanied by repetitive cycles of earthquakes, ground deformation, degassing, and explosive eruptions. The cycles reflected unsteady conduit flow of volatile-charged magma resulting from gas exsolution, rheological stiffening, and pressurization. The cycles, over hours to days, initiated when degassed stiff magma retarded flow in the upper conduit. Conduit pressure built with gas exsolution, causing shallow seismicity and edifice inflation. Magma and gas were then expelled and the edifice deflated. The repeat time-scale is controlled by magma ascent rates, degassing, and microlite crystallization kinetics. Cyclic behavior allows short-term forecasting of timing, and of eruption style related to explosivity potential.

Dacite tephras produced by the 1991 pre-climactic eruptive sequence at Mt. Pinatubo display extreme heterogeneity in vesicularity, ranging in clast density from 700 to 2580 kg m -3 . Observations of the 13 surge-producing blasts that preceded the climactic plinian event include radar-defined estimates of column heights and seismically defined eruptive and intra-eruptive durations. A comparison of the characteristics of erupted material, including microlite textures, chemical compositions, and H 2 O contents, with eruptive parameters suggests that devolatilization-induced crystallization of the magma occurred to a varying extent prior to at least nine of the explosive events. Although volatile loss progressed to the same approximate level in all of the clasts analyzed (weight percent H 2 Op1.26-1.73), microlite crystallization was extremely variable (0-22%). We infer that syn-eruptive volatile exsolution from magma in the conduit and intra-eruptive separation of the gas phase was facilitated by the development of permeability within magma residing in the conduit. Correlation of maximum microlite crystallinity with repose interval duration (28-262 min) suggests that crystallization occurred primarily intra-eruptively, in response to the reduction in dissolved H 2 O content that occurred during the preceding event. Detailed textural characterization, including determination of three-dimensional shapes and crystal size distributions (CSD), was conducted on a subset of clasts in order to determine rates of crystal nucleation and growth using repose interval as the time available for crystallization. Shape and size analysis suggests that crystallization proceeded in response to lessening degrees of feldspar supersaturation as repose interval durations increased. We thus propose that during repose intervals, a plug of highly viscous magma formed due to the collapse of vesicular magma that had exsolved volatiles during the previous explosive event. If plug thickness grew proportionally to the square root of time, and if magma pressurization increased during the eruptive sequence, the frequency of eruptive pulses may have been modulated by degassing of magma within the conduit. Dense clasts in surge deposits probably represent plug material entrained by each subsequent explosive event.

No abstract

Abstract. Telemetered high-resolution tiltmeters were installed in Montserrat in summer of 1995, in December 1996, and in May 1997. The 1995 installations, several km from the Soufriere Hills vent, were too distant to yield useful data. However, the 1996 and 1997 installations on the crater rim revealed 6-14 h inflation cycles caused by magma pressurization at shallow depths (< 0.6 km below the base of dome). The tilt data correlated with seismicity, explosions, and pyroclastic flow activity, and were used to forecast times of increased volcanic hazard to protect scientific field workers and the general public.

Abstract.The

No abstract

Petrographic studies show that hematite is present in the Moenkopi Formation in at least five and possibly six forms: (1) microcrystalline hematite, (2) crystals of specular hematite, (3) polycrystalline and monocrystalline grains, (4) grains of partly hematitized ilmenite, (5) grains composed of primary ilmenite‐hematite intergrowths, and (6) ultrafine pigment. The microcrystalline hematite and crystals of specular hematite are unequivocally authigenic. They form cement in interstitial and secondary voids, and they have replaced detrital iron‐bearing silicate minerals. Furthermore, microcrystalline hematite is superimposed on other authigenic cementing minerals such as potassium feldspar, calcite, dolomite, and quartz, and in some cases it has replaced authigenic pyrite. In addition, both microcrystalline and specularite crystals are common daughter products of intrastratally altered biotite grains. Thermodynamic considerations coupled with studies of hematite‐magnetite relationships in modern sediments indicate that most of the hematite in the polycrystalline grains, and probably the monocrystalline grains as well, was formed authigenically by post‐depositional replacement of detrital grains of magnetite. The ilmenite probably has similarly altered in situ to hematite. The only hematite of unquestionable detrital origin in the red beds is the hematite in the ilmenite‐hematite intergrowths (‘tiger striped’ grains) and that in monocrystalline detrital grains containing rutile exsolution platelets, both of which are products of high‐temperature processes. With the exception of the ultrafine pigment, each of the above forms is coarser grained than the superparamagnetic threshold for hematite, and therefore each contributes components of remanent magnetism to the rocks. Inasmuch as most of the hematite varieties represent authigenic products of intrastratal alterations that require considerable geologic time, we conclude that the red bed remanence is largely chemical remanent magnetization (CRM) acquired over long time intervals. The pigment in the Moenkopi red beds consists partly of authigenic ultrafine red iron oxide and partly of translucent microcrystalline hematite. The ultrafine red iron oxide may or may not be hematite, but even if it is, the grain size probably lies below the paramagnetic threshold for hematite. Much of the pigment, therefore, may not contribute greatly to the remanent magnetism in the rocks.