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Pyroclastic flows sweep down the flanks of Mayon Volcano, Philippines, in 2018. A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) [1] is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of 100 km/h (30 m/s; 60 mph) but is capable of reaching speeds up to ...
Tephra is a generalized word for the various bits of debris launched out of a volcano during an eruption, regardless of their size. [4] Pyroclastic materials are generally categorized according to size: dust measures at <1/8 mm, ash is 1/8–2 mm, cinders are 2–64 mm, and bombs and blocks are both >64 mm. [5] Different hazards are associated with the different kinds of pyroclastic materials.
When an emulsion of volcanic gas and magma falls back to the ground, it can create a density current called a pyroclastic flow. The emulsion is somewhat fluidised by the gas, allowing it to spread. These can often climb over obstacles, and devastate human life. [2]
The most dangerous eruptive feature are the pyroclastic flows generated by material collapse, which move down the side of the mountain at extreme speeds [40] of up to 700 km (435 mi) per hour and with the ability to extend the reach of the eruption hundreds of kilometers. [43]
A wide variety of volcanic processes can produce tsunamis. This includes volcanic earthquakes, caldera collapse, explosive submarine eruptions, the effects of pyroclastic flows and lahars on water, base surges with accompanying shock waves, lava avalanching into the sea, air waves from explosive subaerial eruptions, avalanches of cold rock, and avalanches of hot material. [1]
A lahar travels down a river valley in Guatemala near the Santa Maria volcano, 1989. A lahar (/ ˈ l ɑː h ɑːr /, from Javanese: ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water.
The resulting deposits formed a fan-like pattern of overlapping sheets, tongues, and lobes. At least 17 separate pyroclastic flows occurred during the May 18 eruption, and their aggregate volume was about 0.05 cu mi (0.21 km 3). [9] The flow deposits were still at about 570 to 790 °F (300 to 420 °C) two weeks after they erupted. [9]
If it does happen, then material reaching the bottom of the convective thrust region can no longer be adequately supported by convection and will fall under gravity, forming a pyroclastic flow or surge which can travel down the slopes of a volcano at speeds of over 100–200 km/h (62–124 mph). Column collapse is one of the most common and ...