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Volcanoes and Their Products
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Pyroclastic material, material ejected into the air during an eruption, is quite variable, being composed of clasts of different compositions that range from fine ash to large bombs and blocks. So, once it reaches the ground, the material becomes tephra that can contain clasts of different sizes (Table 7.3). The clasts themselves may be crystals, crystal fragments, glass fragments, or rock fragments. Furthermore, although some tephra and, consequently, some pyroclastic rocks are composed entirely of pyroclastic material, others are not. Tephra or rocks may contain pieces of local country rock or sediments that mixed with the pyroclastic material or may contain mineral or rock fragments formed after the initial volcanic event. Additionally, weathering or erosion may alter or modify pyroclastic material, producing reworked tephra, or pyroclastic rocks formed from reworked tephra. So, pyroclastic rocks are diverse in both origin and properties.
Hydrogeology of Groundwater Reservoirs
Published in Zekâi Şen, for Scientists and Engineers, 2017
Permanently flowing water often exists in the gravels below the surface of a large wadi. In areas of considerable annual fluctuations in the level of the groundwater table, wells may have only seasonal value and continuous irrigation is not possible. Such is the case in the western part of the Kingdom of Saudi Arabia, where alluvial filled wadis run more or less perpendicularly to the Red Sea (Al-Sayari and Zoti, 1978). In general, groundwater in the upstream parts of wadis is less saline than that in the downstream. A significant portion of the surface water that collects in the wadi at times of substantial rainfall eventually infiltrates the ground. With modern drilling and pumping methods, much of this water can be utilized for agriculture. The alluvial fill of plains and valleys, consisting of gravel, sand, and clay of volcanic origin, may present important groundwater possibilities, but these deposits are as diversified and variable as those in any region. Some coarse- to medium-grained pyroclastic deposits have a very high porosity.
Volcanic activity
Published in F.G. Bell, Geological Hazards, 1999
Pyroclastic flows formed primarily of scoriaceous or lithic volcanic debris are known as hot avalanches, glowing avalanches, nuées ardentes or block and ashflows. They generally affect a narrow sector of a volcano, perhaps only a single valley. Pyroclastic flows may be classified according to the ratio of gas to water they contain on the one hand and temperature on the other (Table 2.3). Maximum temperatures of pyroclastic flow material soon after it has been deposited range from 350 to 550°C. Hence they are hot enough to kill anything in their path. Because of their high mobility (up to 160 km h−1 on the steeper slopes of volcanoes) they constitute a great potential danger to many populated areas. Other hazards associated with pyroclastic flows, apart from incineration, include burial, impact damage and asphyxiation.
Warrumbungle Volcano: facies architecture and evolution of a complex shield volcano
Published in Australian Journal of Earth Sciences, 2021
K. F. Bull, A. L. Troedson, S. Bodorkos, P. L. Blevin, M. C. Bruce, K. Waltenberg
Pyroclastic facies are volcaniclastic facies that predominantly consist of fragments or ‘pyroclasts’ derived directly from an explosive eruption (e.g. pumice, scoria, crystals or coherent dome fragments) that have not been significantly redeposited. In the map area, these deposits include scoria lapilli ash breccia, trachyte pumice lapilli ash breccia, lapilli breccia and tuff. Estimating primary thicknesses and volumes of pyroclastic deposits based on present dimensions is difficult due to the ephemeral nature of any non-welded deposits. Pyroclastic facies also tend to be localised during emplacement that is controlled by conditions such as vent geometry or wind. On Warrumbungle Volcano, the existing deposits are one to up to 10 or more metres thick, locally, but they form a minor proportion of the volume of the volcano.