John Stix

Paleomagnetic chronology, fluvial processes and tectonic implications of the Siwalik deposits near Chinji village, Pakistan.

A 2800-m section of Siwalik strata containing the stratotypes for both the Chinji and Nagri formations has been dated by magnetic polarity stratigraphy, and the observed polarity zonation has securely been correlated with the Chron 17-7 segment of the time scale. The base of the section is the base of the Kamlial formation, which occurs near the top of Chron 17 (18.3 m.y.). The Kamlial-Chinji formation boundary occurs in the middle of Chron 15 (14.3 m.y.), the Chinji-Nagri boundary near the bottom of Chron 10 (10.8 m.y.), and the Nagri-Dhok Pathan boundary at the Chron 8-9 boundary (8.5 m.y.). The Siwalik deposits near Chinji Village consist of four distinct classes of fluvial cycles, each with a characteristic periodicity: a first order of

The chemical and isotopic composition of fumarolic gases and spring discharges from Galeras Volcano, Colombia

10^{7} years, a second order of 10^{6} years, a third order of 10^{4}-10^{5} years

The relationship between degassing and ground deformation at Soufriere Hills Volcano, Montserrat

, and a fourth order of

Scaling effects on vesicle shape, size and heterogeneity of lavas from Mount Etna

10^{0}-10^{4} years

A model of degassing at Galeras Volcano, Colombia, 1988-1993

. The river system responsible for Siwalik sedimentation flowed from west to east and had properties much like the modern Ganges River system. Sedimentation rates in the Chinji Village area increased gradually through time, going from 0.12 mm/yr in the Lower Siwaliks to 0.30 mm/yr in the Middle Siwaliks. At about 11 m.y. blue-green hornblendes suddenly appear in abundance in channel sands. The appearance of these hornblendes, accompanied with accelerated sedimentation, is the effect of uplift in the source area, specifically the Nanga Parbat region. The modern Chinji Village area is no longer a depositional site, but instead is involved in overthrusting, uplift, and erosion. As the Indian Plate has drifted north during the past 18 m.y., the Chinji Village area has been gradually transformed from a subdued karst topography into a major depositional center for Himalayan sediments and finally back into a sediment source area to complete a sedimentary cycle that has spanned some 20 m.y.

SO2 fluxes from Galeras Volcano, Colombia, 1989-1995: Progressive degassing and conduit obstruction of a Decade Volcano

Abstract A series of six vulcanian eruptions at Galeras volcano, Colombia, occurred during 1992–1993. These eruptions followed the emplacement of a lava dome in late 1991, which was accompanied by cooling, crystallizing and partial solidification of the column of magma plugging the conduit. Five of the eruptions were preceded by episodes of monochromatic long-period seismicity, suggesting cyclic pressurization of the magma. The quiescent intervals between eruptions generally decreased with each successive eruption. We propose a model of gas release from magma into confined, stationary pore spaces by 5–10% crystallization of anhydrous minerals, causing repeated overpressurization of the magmatic system. Using initial lithostatic pressures of 20–100 MPa (800–3900 m depth), gas buildup by crystallization indicates overpressures of 3–17 MPa, which is the approximate fracture criterion for a magma chamber. Our Galeras data suggest two models of pressurization: (1) gas exsolution may have occurred at similar depths, but with progressively smaller overpressures being developed before each successive eruption. According to this model, the comparatively long quiescent periods between the first eruptions resulted in relatively large amounts of gas exsolution from the magma and accumulation in pore spaces, whereas the shorter quiescent periods for the later eruptions exsolved less gas. The decrease in overpressure with time may have resulted from progressive fracturing of partly solidified magma during the eruptions, which progressively decreased the tensile strength and increased the permeability of the magma. (2) Since overpressures are smaller at shallower depths for a given amount of crystallization, the first eruptions of 1992–1993 were caused by gas exsolution at shallow depths, while the last eruptions, having shorter quiescent intervals, resulted from a greater amount of gas exsolution at greater depths. In this case, the rate of gas exsolution varied as a function of depth. The two scenarios for pressurization may be combined into a more general model involving edifice weakening and the development of a shallow hydrothermal system from continued degassing of the magma plug. As weakening of the edifice formed an extensive system of fractures, the shallow hydrothermal system deepened due to meteoric water input and magmatic degassing. With time, a greater number of fractures were filled and partly sealed by hydrothermal minerals at progressively deeper levels. As a result, the source of the overpressure also deepened with time.