China
Africa
Explore chapters and articles related to this topic
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
After losing its momentum, a rising eruption column may be overcome by gravity and collapse directly back to Earth. Clouds of hot gas (up to 1000 °C; 1800 °F), ash, and rock can rush down a volcano’s side at up to 700 kilometers/hour (450 miles per hour) in a pyroclastic flow or surge. Figure 7.11 shows one example, and the top layer of ash in Figure 7.24, which is unstratified, probably formed from a pyroclastic flow too. Flows and surges are some of the deadliest volcanic events, but fortunately, today there is usually ample warning before they occur. They are sometimes called nuée ardentes (French for “glowing clouds”), a term first used to describe the deadly 1902 eruption of Mount Pelée on Martinique because, in the dark, the pyroclastic flows glowed red. The distinction between flows and surges is a hazy one based on the ratio of gaseous to solid material. Flows contain more solids and, due to their density, often follow and are confined to river valleys. Surges, however, are more gaseous than flows and, consequently, less dense. They rise to cross ridges and hills. Upon settling and cooling, pyroclastic material from both flows and surges typically fuses to produce tuffs called ignimbrites, testimonials to violent eruptions of the past.
Radar Monitoring of Volcanic Activities
Published in Ramesh P. Singh, Darius Bartlett, Natural Hazards, 2018
The products of volcanic eruptions also vary widely, giving rise to a large range of associated hazards (Myers et al. 2008). Explosive eruptions produce ballistic ejecta (solid and molten rock fragments) that can impact the surface up to several kilometres away from the vent. Smaller fragments are carried upward in eruption columns that sometimes reach the stratosphere, forming eruption clouds that pose a serious hazard to aircraft. Large eruption clouds can extend hundreds to thousands of kilometres downwind, resulting in ash fall over large areas. Heavy ash fall can collapse buildings, and even minor amounts can cause significant damage and disruption to everyday life. Volcanic gases in high concentrations can be deadly. In lower concentrations, they contribute to health problems and acid rain, which causes corrosion and harms vegetation. Lava flows and domes extruded during mostly non-explosive eruptions can inundate property and infrastructure, and create flood hazards by damming streams or rivers. Pyroclastic flows – high-speed avalanches of hot pumice, ash, rock fragments and gas – can move at speeds in excess of 100 km/h and destroy everything in their path. In some cases, gravitational collapse of an unstable volcanic edifice results in a devastating debris avalanche; the most famous example is the 1980 debris avalanche at Mount St. Helens, which extended more than 20 km down the North Fork Toutle River Valley. Debris flows and lahars (volcanic mudflows) triggered by eruptions inundate valleys for distances approaching 100 km, causing long-term ecological impacts and increased flood hazards.
Volcanic activity
Published in F.G. Bell, Geological Hazards, 1999
Pyroclastic flows are hot, dry masses of clastic volcanic material that move over the ground surface. Most pyroclastic flows consist of a dense basal flow, the pyroclastic flow proper, one or more pyroclastic surges and clouds of ash. Two major types of pyroclastic flow can be recognized. Pumiceous pyroclastic flows are concentrated mixtures of hot to incandescent pumice, mainly of ash and lapilli size. Ashflow tuffs and ignimbrites are associated with these flows. Individual flows vary in length from less than 1 up to 200 km, covering areas of up to 20 000 km2 with volumes from less than 0.001 to over 1000 km3.
Lower Cambrian volcanism in the Hawker Group and the Billy Creek Formation, Arrowie Basin, Flinders Ranges, South Australia
Published in Australian Journal of Earth Sciences, 2023
The pale green tuffs contain an abundance of well-preserved glass shards of various shapes, including complete bubbles and ‘Y’ shapes, textures diagnostic of explosive volcanism. There are also weakly developed welding and compaction textures suggestive of an ignimbrite. The tuff is interpreted to be a subaqueous pyroclastic flow deposit or ash-flow tuff (Fisher & Schmincke, 1984; McPhie et al., 1993; Ross & Smith, 1961). The thicker Big Green Tuff bed may have been deposited from an initially subaerial pyroclastic flow that was focussed into a restricted marine depositional site by run-out through a valley (e.g. McPhie et al., 1993) and would suggest a relatively proximal entry into the marine environment. The welding in the chilled basal layer and largely uncontaminated cream-coloured layers indicate that at least part of the tuff was deposited while hot. The local abundance of fossil fragments and fine carbonate, and the mud balls (Figure 11a) indicates some strong turbulence during emplacement. There has been much controversy about the existence and possible emplacement mode of submarine welded ash-flow tuffs. There are a few examples where such pyroclastics have been described (e.g. Fritz & Stillman, 1996; Kokelaar & Königer, 2000) and in these cases, the ash-flow tuffs were deposited close to the shoreline in relatively shallow water (<50 m). This may indicate a proximal source for the Mernmerna Formation tuffs, rather than a distal source as air-fall pyroclastics.
A review of lahars; past deposits, historic events and present-day simulations from Mt. Ruapehu and Mt. Taranaki, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2021
Jonathan Procter, Anke Zernack, Stuart Mead, Michael Morgan, Shane Cronin
Volcanic mass-flow deposits are typically mapped and distinguished as: pyroclastic density current deposits (comprising pyroclastic surge deposits, pumiceous ash-rich pyroclastic flow deposits/ignimbrites and block-and-ash flow deposits), lahar deposits (including debris-flow and hyperconcentrated-flow deposits) or debris-avalanche deposits. Volcanic mass flows are dominantly gravity-driven (although sometimes initially accelerated by volcanic explosions) and involve rock materials as their primary solid component along with either water or gas as the fluidising component. Sediment-water mixture flows are the most common type and they span a wide range of volumes, discharges, velocities, compositions, bulk rheologies, and flow hydraulics (Pierson 1998). However, the nomenclature used to distinguish different types of volcanic mass-flows is often misleading (Pierson and Costa 1987) and variations are mainly a result of the interdisciplinary study of volcanic mass-flow processes.
Tephrochronology in Aotearoa New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2021
Jenni L. Hopkins, David J. Lowe, Joanna L. Horrocks
Aotearoa New Zealand (ANZ hereafter) has an extensive history of explosive volcanic activity with relatively localised basaltic and more widely-dispersed andesitic events punctuated by powerful rhyolitic events occurring from at least 24 Ma to the present in North Island (Houghton et al. 1995; Wilson et al. 1995; Briggs et al. 2005; Lowe et al. 2008a; Wilson et al. 2009; Wilson and Rowland 2016; Barker et al. 2021; Pittari et al. 2021). The most recent explosive activity since ∼4 Ma has occurred at centres in the southern Coromandel Volcanic Zone (CVZ) and Taupō Volcanic Zone (TVZ), with the locus of activity since ∼2 Ma in the TVZ and at Taranaki Maunga (previously known as Mt Taranaki or Mt Egmont) and Mayor Island (Tuhua Volcanic Centre) (Figure 1). (Other centres of active volcanism in central North Island are described by Pittari et al. 2021; broad overviews of volcanism in ANZ are provided by Hayward 2017; Shane 2017; Mortimer and Scott 2020.) The eruptions have been responsible for blanketing the landscape surrounding the volcanoes with both pyroclastic-flow and widespread tephra-fall deposits, the distributions of which are governed by a number of factors including eruption volume, mass eruption rate and temperature, eruption column height, particle sizes, strength and direction of wind at the time of the eruption, and radius of the umbrella cloud (e.g. Alloway et al. 2013; Cashman and Rust 2016; Barker et al. 2019; Constantinescu et al. 2021).