J. Martínez-martínez - Academia.edu (original) (raw)
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A multidisciplinary approach has been applied to assess the hazard related to the eruptive fissur... more A multidisciplinary approach has been applied to assess the hazard related to the eruptive fissures on the slopes of the main volcanic edifices in Tenerife. We concentrated on the historical eruptions and on the whole framework of the NW rift. Tenerife is a complex volcanic area in which long-lived volcanic centers coexist with a number of different-age scoriae and spatter cones. Our work concentrated on the youngest portion of these cones, their age, and distribution. In order to achieve this goal, a detailed fieldwork has been carried out by means of a 1:5,000 scale cartography of volcanic deposits and structural feature. An area of about 10 sq km has been surveyed up to now. Archaeomagnetic method has been used to elucidate age relations of the main recent volcanic record (last 8.000 yrs BP) and a suitable field work was done. Chronostratigraphic characterization of 42 different localities (i.e. 590 archaeomagnetic measurements) was performed. In addition, petrochemical and rheological analyses are in progress. The present data set allows us to point out some important findings that depict a new scenario for the flank activity in Tenerife with strong implication for hazard assessment. At present, we are able to produce the complete cartographic and chronostratigraphic sequence of two famous and distinct areas, Fasnia-Siete Fuentes-Arafo and Chinyero volcanic fields, that probably represent the "general behavior" of the up to known monogenetic activity on Tenerife. Polygenetic cones, instead of monogenetic scoriae cones, seem to be the main character of the Holocene activity in Tenerife. No previous investigations have ever evidenced this aspect. In the Fasnia area we recognized the effect of at least two volcano-tectonic trends (one 120N oriented and the other one 50N) resulting in three different lava flows outpouring, in stratigraphic order, from two dismantled scoriae cones and from an eruptive fissure oriented 30N. Three orders of lava levees are well exposed in the southern portion of the lava field. Only the 1km long and last emitted lava flow belongs to the 1704-05 eruptive episode. Scoriae ramparts, pit craters and a small cover of lapilli fallout complete the deposits associated to 1704-05 eruption. The contemporaneous presence of big scoriae cones and small scoriae ramparts has been frequently recognized elsewhere in Tenerife and is a characteristic of the Chinyero volcanic field too, where the 1909 volcanics are mantling two previous scoriae cones.
The temporal evolution of fundamental flow conditions in the magma chamber plus conduit system–su... more The temporal evolution of fundamental flow conditions in the magma chamber plus conduit system–such as pressure, velocity, mass flow-rate, erupted mass, etc.–during sustained magmatic explosive eruptions was investigated. To this aim, simplified one-dimensional and isothermal models of magma chamber emptying and conduit flow were developed and coupled together. The chamber model assumed an homogeneous composition of magma and a vertical profile of water content. The chamber could have a cylindrical, elliptical or spherical rigid geometry. Inside the chamber, magma was assumed to be in hydrostatic equilibrium both before and during the eruption. Since the timescale of pressure variations at the conduit inlet–of the order of hours–is much longer than the travel time of magma in the conduit–of the order of a few minutes–the flow in the conduit was assumed as at steady-state. The one-dimensional mass and momentum balance equations were solved along a circular conduit with constant diameter assuming choked-flow conditions at the exit. Bubble nucleation was considered when the homogeneous flow pressure dropped below the nucleation pressure given the total water content and the solubility law. Above the nucleation level, bubbles and liquid magma were considered in mechanical equilibrium. The same equilibrium assumption was made above the fragmentation level between gas and pyroclasts. Due to the hydrostatic hypothesis, the integration of the density distribution in the chamber allowed to obtain the total mass in the chamber as a function of pressure at the chamber top and, through the conduit model, as a function of time. Simulation results pertaining to rhyolitic and basaltic magmas defined at the Volcanic Eruption Mechanism Modeling Workshops (Durham, NH, 2002; Nice, France, 2003) are presented. Important flow variables, such as pressure, density, velocity, shear stress in the chamber and conduit, are discussed as a function of time and magma chamber and conduit properties. Results indicate that vent variables react in different ways to the pressure variation of the chamber. Pressure, density and mass flow-rate show relative variations of the same order of magnitude as the conduit inlet pressure, whereas velocity is more constant in time. Sill-like chambers produce also significantly longer and more voluminous eruptions than dike-like chambers. Water content stratification in the chamber and the increase of chamber depth significantly reduce the eruption
A multidisciplinary approach has been applied to assess the hazard related to the eruptive fissur... more A multidisciplinary approach has been applied to assess the hazard related to the eruptive fissures on the slopes of the main volcanic edifices in Tenerife. We concentrated on the historical eruptions and on the whole framework of the NW rift. Tenerife is a complex volcanic area in which long-lived volcanic centers coexist with a number of different-age scoriae and spatter cones. Our work concentrated on the youngest portion of these cones, their age, and distribution. In order to achieve this goal, a detailed fieldwork has been carried out by means of a 1:5,000 scale cartography of volcanic deposits and structural feature. An area of about 10 sq km has been surveyed up to now. Archaeomagnetic method has been used to elucidate age relations of the main recent volcanic record (last 8.000 yrs BP) and a suitable field work was done. Chronostratigraphic characterization of 42 different localities (i.e. 590 archaeomagnetic measurements) was performed. In addition, petrochemical and rheological analyses are in progress. The present data set allows us to point out some important findings that depict a new scenario for the flank activity in Tenerife with strong implication for hazard assessment. At present, we are able to produce the complete cartographic and chronostratigraphic sequence of two famous and distinct areas, Fasnia-Siete Fuentes-Arafo and Chinyero volcanic fields, that probably represent the "general behavior" of the up to known monogenetic activity on Tenerife. Polygenetic cones, instead of monogenetic scoriae cones, seem to be the main character of the Holocene activity in Tenerife. No previous investigations have ever evidenced this aspect. In the Fasnia area we recognized the effect of at least two volcano-tectonic trends (one 120N oriented and the other one 50N) resulting in three different lava flows outpouring, in stratigraphic order, from two dismantled scoriae cones and from an eruptive fissure oriented 30N. Three orders of lava levees are well exposed in the southern portion of the lava field. Only the 1km long and last emitted lava flow belongs to the 1704-05 eruptive episode. Scoriae ramparts, pit craters and a small cover of lapilli fallout complete the deposits associated to 1704-05 eruption. The contemporaneous presence of big scoriae cones and small scoriae ramparts has been frequently recognized elsewhere in Tenerife and is a characteristic of the Chinyero volcanic field too, where the 1909 volcanics are mantling two previous scoriae cones.
The temporal evolution of fundamental flow conditions in the magma chamber plus conduit system–su... more The temporal evolution of fundamental flow conditions in the magma chamber plus conduit system–such as pressure, velocity, mass flow-rate, erupted mass, etc.–during sustained magmatic explosive eruptions was investigated. To this aim, simplified one-dimensional and isothermal models of magma chamber emptying and conduit flow were developed and coupled together. The chamber model assumed an homogeneous composition of magma and a vertical profile of water content. The chamber could have a cylindrical, elliptical or spherical rigid geometry. Inside the chamber, magma was assumed to be in hydrostatic equilibrium both before and during the eruption. Since the timescale of pressure variations at the conduit inlet–of the order of hours–is much longer than the travel time of magma in the conduit–of the order of a few minutes–the flow in the conduit was assumed as at steady-state. The one-dimensional mass and momentum balance equations were solved along a circular conduit with constant diameter assuming choked-flow conditions at the exit. Bubble nucleation was considered when the homogeneous flow pressure dropped below the nucleation pressure given the total water content and the solubility law. Above the nucleation level, bubbles and liquid magma were considered in mechanical equilibrium. The same equilibrium assumption was made above the fragmentation level between gas and pyroclasts. Due to the hydrostatic hypothesis, the integration of the density distribution in the chamber allowed to obtain the total mass in the chamber as a function of pressure at the chamber top and, through the conduit model, as a function of time. Simulation results pertaining to rhyolitic and basaltic magmas defined at the Volcanic Eruption Mechanism Modeling Workshops (Durham, NH, 2002; Nice, France, 2003) are presented. Important flow variables, such as pressure, density, velocity, shear stress in the chamber and conduit, are discussed as a function of time and magma chamber and conduit properties. Results indicate that vent variables react in different ways to the pressure variation of the chamber. Pressure, density and mass flow-rate show relative variations of the same order of magnitude as the conduit inlet pressure, whereas velocity is more constant in time. Sill-like chambers produce also significantly longer and more voluminous eruptions than dike-like chambers. Water content stratification in the chamber and the increase of chamber depth significantly reduce the eruption